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Introduction

Okey dokey, we’re back on track again. We kinda took a big detour there for a while after Part 3 of this series titled How Components Work Together. We got into a discussion on astable multivibrators, then we answered some questions on our last project — the LED blinker circuit — and we gained a deep understanding of that circuit in our last discussion. These discussions actually set us up pretty well and created its own little Astable Multivibrator Subseries.

Just so we have things straight and you’re not confused on the order of things, here’s the sequence of this series (and its subseries) and their titles up to this point:

Circuit Component Super Series

• Part 1: What is a Component?
• Part 2: Understanding Circuit Symbols and Schematics
• Part 3: How Components Work Together in Real Circuits

Astable Multivibrator Subseries (After Part 3)

1. What Exactly is an Astable Multivibrator?
2. Questions Answered on the Blinker Circuit
3. Gaining a Deeper Understanding of the LED Blinker Circuit

Later, we’ll progressively work through more topics and ideas of components, further advancing these ideas into a robot project to really get you to understand how they work together to create something awesome and useful.

The following is how this 12 Part Super Series on Components will progress, in order:

1. What Is a Component?
2. Understanding Circuit Symbols and Schematics
3. How Components Work Together in Real Circuits
4. Putting Components to Work (⬅️ YOU ARE HERE)
5. Introducing Microcontrollers
6. Smart Circuits Meet AI
7. Teaching Robots to Move
8. How Sensors, Data, and AI Create Perception
9. Human–Robot Interaction
10. Decision Making and Autonomy in Robots
11. Robotic Navigation
12. Distributed Robotics, Swarms, and Multi-Agent Intelligence

This is Part 4 of the Circuit Component Super Series

In this project, we will build a circuit that reacts to the real world. Instead of blinking on its own, this circuit senses temperature and makes a decision — just like real electronics inside robots, computers, and machines.

If you’ve completed the LED blinker circuit from Part 3, congratulations 🎉 — you’ve already built a dynamic circuit. Now it’s time to level up.

In Part 4, we’ll build a temperature-controlled fan using:

• A thermistor to sense heat
• A 555 timer as a decision-making circuit
• A MOSFET to safely drive a fan

This will be your first true sensor-to-logic-to-actuator system.

What The 555 Temperature-Controlled Fan Controller Does

The 555 Temperature-Controlled Fan Controller, once finished, will be a circuit that cools when the temperature rises with the use of a 12VDC fan and a potentiometer. The following is what it’ll do:

• A DC fan turns ON when the circuit gets warm, and turns OFF when it cools down
• The fan stays OFF when it’s cool
• As temperature rises past a set point, the fan turns ON automatically
• When things cool back down, the fan turns OFF cleanly
• A potentiometer lets you adjust the temperature threshold
• It uses a 555 timer as a Schmitt-trigger comparator (so it doesn’t “chatter” on/off around the threshold)
• A MOSFET handles the fan current safely

This project includes the exact “real circuit” skills beginners need:

• Reading a schematic with functional blocks
• Using a sensor divider
• Understanding a threshold decision
• Driving a higher-power load with a transistor/MOSFET stage
• Adding protection parts (like a flyback diode and decoupling capacitors)

No microcontroller. No code. Just clever electronics.

🧠 Learning Goals for This Project

There are several learning goals and outcomes we wish to accomplish out of this part of the Circuit Component Super Series that’ll help guide us in our understanding of other topics and projects we encounter later in this series — and elsewhere throughout this site — that are more advanced.

Here, we wish to learn:

• How a thermistor and resistor becomes a temperature-to-voltage sensor
• How the 555 can act like a comparator with hysteresis
• Why power loads shouldn’t be driven directly from IC pins
• How to layout “quiet” analog sensing, “noisy” load current paths, and proper decoupling on a breadboard

If all goes well, we’ll learn that temperature changes can change the voltage across a thermistor and that the 555 can compare those changes to its internal threshold, which in turn will output a clean flip to have a MOSFET switch a fan on — the circuit doing all this on its own without the use of a microcontroller!

So, let’s get started and do (and learn) some really cool things! First things first, let’s start with the parts and tools list of items we’ll need for this project, next.

Parts List

ITEM	QUANTITY
830-Point Solderless Breadboard	1
Breadboard Jumper Wires	Various Colors and Sizes
NE555 timer (or LM555/TLC555)	1
10k NTC thermistor	1
Logic-level N-MOSFET (recommended: IRLZ44N, IRLZ34N, FQP30N06L; anything logic-level TO-220 is fine)	1
12VDC fan	1
1N5819 Schottky diode (flyback diode)	1
10k potentiometer	1
100Ω resistor (current limit)	1
100kΩ resistor (pull-down)	1
1MΩ resistor (hysteresis)	1
10nF (0.01µF) capacitor (decoupling)	1
100nF (0.1µF) capacitor (decoupling)	1
10µF capacitor (decoupling)	1

Tools List

ITEM	QUANTITY/TYPE
12V Power Supply	Benchtop Power Supply
Multimeter	(Optional, but recommended) I used a real cheapo off-brand XL830L multimeter to take voltage measurements during the build process to continuously check my work.
Heat Gun or Hair Dryer	I use a Yaogong YG8586 rework station that came with a heat gun, but you can use a simple heat gun or even a hair dryer!
Tweezers	I used an ESD-11 straight tip tweezer for this project to handle components more easily. I keep a variety on hand at the workbench. They’re very handy.
Oscilloscope	(Optional) Later, I’ll show how to take some measurements of our circuit using an oscilloscope. I use a Hantek DSO5102P oscilloscope.

📝 A Quick Note About Breadboards

My breadboard configuration may be different from yours. Different brands of breadboards may configure the labeling of the lettered columns and numbered rows differently.

For example, the breadboard that I’m using for this project has the lettered columns read backwards (from J to A), when I orient my breadboard to have the numbered rows read in sequence from top-to-bottom (1 thru. 64). Others may have the lettered columns read forwards (from A to J), when you orient the breadboard to have the numbered rows read in sequence from top-to-bottom (1 thru. 64).

Just be aware of this possible confusion if your breadboard is different than the one I’m using here, because throughout the project I’ll give point locations on the breadboard that I used to place components which might be differently located on your breadboard due to label orientation differences.

🗺️ Schematic for the 555 Temperature-Controlled Fan Controller

A schematic shows a visual representation of the components we’ll use for our project and how each are to be connected in the circuit we’ll build.

How-To Make the 555 Temperature-Controlled Fan Controller Circuit

Before we get to making our circuit on the breadboard, let’s take a look at our schematic diagram above. Don’t let all those symbols get you anxious, we’ll walk through it all step-by-step. A good way to get to grips with our circuit diagram is to think of it as being in three major sections:

1. The power supply section
2. The power-enable-stable section
3. And the sensor section

Now that we have our schematic, let’s create a build plan and start with the easy part first — the power supply section.

🍞 Breadboard Build Plan

Just so you know, I proceeded with this breadboard build like I do with many others— with no particular planning involved. I followed my schematic diagram as I wanted the circuit to be, did the best I could to follow the diagram, and placed my components as best as possible on my breadboard.

This is a prototype circuit. What you see is the result of the first time of my set up of this circuit — so it’s not pretty. That’s the way I roll — take it or leave it 😎.

In that being said, I know that the placement of components and the jumper wires in the following may seem to be all over the place, but in my defense, I was trying to do my best. There’s quite often always a better way of doing things, and in this case I’m 100

Now, let’s move on to the power supply section.

1.) Power Supply Section

Power Rails

To be able to connect the 12V power supply to the breadboard, grab a couple of jumper wires to add to the power rails of your breadboard — it doesn’t matter what color jumper wires you use, as long as you stay fairly consistent of your color choices throughout the build (if possible). Doing so may help alleviate confusion for yourself later. Otherwise, use whatever floats your boat.

I happened to have a red and brown jumper wire available to use and the following is how I connected them to the breadboard for power:

• A red jumper wire was used to place on the upper point on the right-side positive (+) power rail.
• A brown jumper wire was used to place on the upper point on the left-side negative (-) power rail, as seen in the image below.

Now, we need to continue the path of power distribution throughout the entire 830-point breadboard. To do that, we need to continue both the positive and negative supply paths all the way down each power rail on both sides of the breadboard to be able to have other components access the power supply from anywhere on the breadboard later in the build process.

Let’s start with the distribution of the positive supply on both sides of the breadboard:

• Starting from the red jumper wire we placed on the right-side positive (+) power rail from before, I placed a small orange jumper wire across the center gap of vertical points for the positive power rail of the breadboard. This will continue the path of the upper points down to the lower points of the right-side positive power rail of the breadboard. If you need a reference as to how the points on a breadboard are connected, refer to our article on breadboard basics here.
• Next, let’s continue the path for the positive power supply from the right-side of the breadboard to the left-side by placing one end of a longer orange jumper wire at the lower point on the right-side positive (+) power rail. Then, place the other end of that same longer orange jumper wire at the lower point on the left-side positive (+) power rail of the breadboard, as seen in the image below.
• To complete the positive supply path, I placed a small orange jumper wire across the center gap of vertical points for the left-side positive power rail of the breadboard, as seen in the image below. This will continue the path of the lower points up to the upper points of the left-side positive power rail of the breadboard. This completes the distribution of the positive supply path throughout both sides of the breadboard.

Next, we need to distribute the negative supply on both sides of the breadboard:

• Starting from the brown jumper wire we placed on the left-side negative (-) power rail from before, I placed a small green jumper wire across the center gap of vertical points for the negative power rail of the breadboard. This will continue the path of the upper points down to the lower points of the left-side negative power rail of the breadboard.
• Next, let’s continue the path for the negative power supply from the left-side of the breadboard to the right-side by placing one end of a longer green jumper wire at the lower point on the left-side negative (-) power rail. Then, place the other end of that same longer green jumper wire at the lower point on the right-side negative (-) power rail of the breadboard, as seen in the image below.
• To complete the negative supply path, I placed a small green jumper wire across the center gap of vertical points for the right-side negative power rail of the breadboard, as seen in the image below. This will continue the path of the lower points up to the upper points of the right-side negative power rail of the breadboard. This completes the distribution of the negative supply path throughout both sides of the breadboard.

Adding a Bulk Capacitor

We will now place a bulk capacitor (also called a reservoir capacitor) across where the incoming power supply enters the breadboard. A bulk capacitor is used to help keep the circuit’s voltage stable and clean.

In this project, the bulk capacitor sits between +12V and GND.

• First — keeping with the same wiring color scheme — I placed one end of a red jumper wire at the point next to where we placed a red jumper wire on the upper point on the right-side positive (+) power rail for where the incoming +12V power supply will connect to. I placed the other end of this new red jumper wire at the point row 4, column A on the breadboard.
• Second, I placed one end of a brown jumper wire at the point next to where we placed a brown jumper wire on the upper point on the left-side negative (-) power rail for where the power supply ground (negative supply) will connect to. I placed the other end of this new brown jumper wire at the point row 4, column J on the breadboard.
• Taking a 10µF electrolytic (polarized) capacitor, I placed its cathode lead at point row 4, column E, and placed its anode lead at point row 4, column F — bridging the connection across (parallel) the incoming power supply we made on the breadboard.

We place the capacitor where the power supply first enters the breadboard (or as close as practical) so it can absorb sudden current demands and smooth out voltage dips caused by switching loads, such as the fan we’ll be using in our circuit.

Without this capacitor, brief current surges from the fan could momentarily disturb the supply voltage seen by the 555 timer.

Now that we’re done with setting up the breadboard to accept power from our power supply and distribute it throughout the board, let’s continue creating our circuit and set up the power-enable-stable section of the 555, next.

2.) Power-Enable-Stable Section

Looking at the schematic diagram provided above, we’re considering the “power-enable-stable” section as the part of the circuit that includes the 555 timer and the connections to its legs or leads (pins 1 thru. 8). Let’s start with placing the 555 timer on the breadboard, then we’ll make our power connections, followed by other pertinent connections.

555 Timer Placement

The 555 timer (NE555P) that we’re using for this project has eight legs. Looking closely at the 555 timer, you may notice a small dimple on the body of the chip. If you orient that small dimple to where it’s in the upper left-hand corner to your perspective, this notifies you that the leg (lead) next to that dimple is pin 1 of the 555.

Starting from pin 1 and going down the left-side, the pin count continues; pin 2, pin3, and pin 4, as seen in the image below. Moving across from pin 4, to the right-side of the chip, we start at pin 5 and work our way up in counting; pin 6, pin 7, and pin 8, as seen in the image below.

Taking our 555 timer and orienting it so that the small dimple (pin 1) is in the left-hand corner of the chip to our view, place it on the breadboard to where its pins straddle across the center gap of the breadboard — this isolates the left-side pins from the right-side pins on the board.

According to my placement of the 555 on my own breadboard, the pin locations are as follows:

• Pin 1: placed at point row 11, column F on the breadboard
• Pin 2: placed at point row 12, column F
• Pin 3: placed at point row 13, column F
• Pin 4: placed at point row 14, column F
• Pin 5: placed at point row 14, column E on the breadboard
• Pin 6: placed at point row 13, column E
• Pin 7: placed at point row 12, column E
• Pin 8: placed at point row 11, column E

Connections for Power and Adding a Decoupling Capacitor for the 555

Let’s set up the 555 for power and place a decoupling capacitor between the V_CC and GND pins, close to the 555 timer. Let’s start with making the power connections to pins 8 and 1, first:

• Pin 1 (GND): I took a blue jumper wire and placed one end at point row 11, column I on the breadboard — inline with pin 1 or the ground (GND) pin of the 555. I placed the other end of the jumper wire into a point on the left-side negative power rail.
• Pin 8 (V_CC): I took a white jumper wire and placed one end at point row 11, column B on the breadboard — inline with pin 8 or the power supply (V_CC) pin of the 555. I placed the other end of the jumper wire into a point on the right-side positive power rail.

Now, let’s place a decoupling capacitor between V_CC and GND, close to the 555 timer. We do this to keep the circuit’s voltage stable and clean, especially when parts of the circuit switch on and off.

The decoupling capacitor acts like a small local energy reservoir:

• If the supply voltage momentarily dips, then the capacitor supplies current
• If a voltage spike appears, then the capacitor absorbs it

This happens extremely fast — much faster than the power supply or long wires can respond. A 100nF (0.1µF) ceramic capacitor is ideal because:

• It responds very quickly to fast voltage changes
• It’s effective at filtering high-frequency noise
• It works best when placed physically close to the IC’s power pins

That’s why we’ll mount it near pin 8 (V_CC) and pin 1 (GND) of the 555:

• Take a 0.1µF (100nF) ceramic capacitor and place its leads across the gap of the breadboard, close to the 555. I placed my capacitor leads at the points row 9, columns F and E.
• To complete the connection of each lead of the 0.1µF capacitor to the pins of the 555, I took two small orange jumper wires:
  • I placed one small orange jumper wire lead at point row 9, column G — inline with the capacitor’s lead. I then placed the other lead of the orange jumper wire at point row 11, column H — inline with pin 1 (GND) of the 555.
  • I placed the second small orange jumper wire lead at point row 9, column D — inline with the capacitor’s lead. I then placed the other lead of the orange jumper wire at point row 11, column C — inline with pin 8 (V_CC) of the 555.

Next we’ll move on to keeping the RESET pin of the 555 always enabled or ON.

Wiring the 555 “Always-On” Power Pins

Before the 555 timer can make any decisions, it must be placed into a known, stable operating state. Several pins on the 555 are not part of the sensing or output logic at all — their job is simply to power the device and keep it enabled.

These are often referred to as the “always-on” pins, because they are wired the same way in many different 555 applications. The 555 timer requires a power source to operate:

• Pin 8 (V_CC) supplies power to the internal circuitry
• Pin 1 (GND) provides the return path for current

Together, these two pins power the entire device. Without them, nothing else in the circuit can function. The RESET pin (pin 4) allows the 555 to be externally disabled. If this pin is pulled low, the output is forced off regardless of the input conditions. In this project, we want the 555 to run continuously, so:

• Pin 4 (RESET): is tied directly to V_CC (+12V). I took a red jumper wire long enough to be placed from pin 4 to pin 8, which will set pin 4 HIGH. My red jumper wire placement goes from point row 14, column G (inline with pin 4), to point row 11, column D (inline with pin 8).

This ensures the timer is always enabled and free to respond to the sensor voltage.

CONTROL Pin: Stabilizing the Internal Thresholds

The CONTROL pin (pin 5) provides access to the internal reference voltages used by the 555’s comparators. In most applications, we don’t need to adjust these thresholds. Instead, we stabilize them by:

• Connecting a small 0.01µF (10 nF) capacitor from pin 5 to ground

This capacitor helps filter out noise and prevents false triggering caused by supply fluctuations.

Even though I left a link to a ceramic capacitor kit that has a 10nF capacitance value, I used a 10nF polyester film capacitor (2A103J), because I couldn’t find a ceramic capacitor of that value at the time of making my circuit. It didn’t seem to matter which type to use for this project, so either type should work fine — we’re just tinkering here.

The way I made my connections for the CONTROL pin was as follows:

• Pin 5 (CONT) and Pin 1 (GND): I took a 10nF polyester film capacitor and placed one of its leads at point row 11, column G (inline with pin 1) and placed the other lead of the capacitor at point row 14, column D (inline with pin 5).

Pins 1, 4, 5, and 8 of the 555 are wired to keep the device powered, enabled, and stable before any sensing or output connections are added. With the 555 powered and enabled, we can now connect the sensor node and configure the timer to respond to temperature changes.

3.) Sensor Section

🔑 The “Temperature Sensor Node” (This Is the Key)

Now let’s move on to make the connections we need to create what’s called the “temperature sensor node” of our 555 temperature-controlled circuit. Before we look at individual wire connections, it helps to step back and think in terms of nodes, rather than wires.

A node is simply a point in a circuit where multiple components connect and share the same voltage. In this project, the most important node is the sensor node, often labeled SENSE (see the color blocked schematic above).

The purpose of the SENSE node is to convert temperature into a voltage that the 555 timer can evaluate and make a decision about. Everything connected to the SENSE node plays one of two roles:

• Some parts create the voltage (the thermistor and potentiometer)
• Other parts observe or influence that voltage (the 555 input pins and the hysteresis resistor)

Once we understand what the SENSE node is responsible for, the individual wire connections become much easier to follow.

Now that we understand what the sensor node does and why it matters, we can look at how it is created using a thermistor and a potentiometer.

Just as a side note for the potentiometer for this project, I had to use a single-turn potentiometer, which is quite large, but that’s all I had at the time of making my circuit — I couldn’t find a trimmer potentiometer of that size, although I know I have one… somewhere. You can use whatever type between the two you want or have.

• Take your 10k potentiometer and place it somewhere on your breadboard where its terminals are separate from each other and are not going to be connected to anything else through the points on the breadboard. I placed my potentiometer leads in the points on column D, rows 20, 22, and 24.
• Next, I took my 10k NTC thermistor and placed its leads somewhere on the breadboard near the potentiometer to prepare a connection from the potentiometer to one of the leads of the thermistor. I also made sure to place one of the leads of the thermistor into a point along the negative power rail.
  • I placed one of my thermistor’s leads at point row 22, column J, then placed its other lead into one of the points in the adjacent negative power rail.

Creating the Sensor Node – Connections to the Potentiometer

We’re going to use the potentiometer as a variable resistor for simplicity:

• We need to tie the wiper of the potentiometer (usually the center lead) directly to the temperature sensor node. To start, I took a blue jumper wire and placed one end at the point row 22, column C (inline with the wiper or center lead of the potentiometer), and placed the other end of the jumper wire at point row 17, column C.
• Next, I took a yellow jumper wire and placed it at point row 22, column E (also inline with the wiper of the potentiometer), and placed the other end of this jumper wire at point row 22, column G, across the dividing center gap of the breadboard (inline with the lead of the thermistor) — this row of points tied together is the location of our sensor node on the breadboard (row 22, columns F thru. J).
• For the outer leads of the potentiometer, I connected one outer lead to a point on the neighboring negative supply rail using a brown jumper wire.
  • I connected one end of the brown jumper wire to point row 20, column A (inline with the potentiometer’s outer lead), and placed the other end of the jumper wire to a neighboring point on the negative supply rail.
• I connected the other outer lead of the potentiometer to a point on the neighboring positive supply rail using a yellow jumper wire.
  • I connected one end of the yellow jumper wire to point row 24, column A (inline with the potentiometer’s outer lead), and placed the other end of the jumper wire to a neighboring point on the positive supply rail.

Continuing from the established sensor node we just made, we’ll continue the connection to the node from pins 2 and 6 of the 555 — this is where we turn the 555 into a Schmitt-trigger comparator.

Turn the 555 into a Schmitt-trigger comparator

In this project, the 555 timer is not used to generate timing pulses. Instead, it is configured to act as a Schmitt-trigger comparator — a circuit that switches cleanly between ON and OFF based on an input voltage.

Inside the 555 are two comparators and a latch with built-in reference levels at approximately:

• 1/3 of the supply voltage, and
• 2/3 of the supply voltage.

By tying the TRIGGER pin (pin 2) and THRESHOLD pin (pin 6) together, we force both comparators to observe the same voltage — the sensor voltage from the SENSE node.

• When the SENSE voltage falls below ~1/3 V_CC, the 555 output switches ON
• When the SENSE voltage rises above ~2/3 V_CC, the 555 output switches OFF
• Between these two levels, the output remains latched in its previous state

This creates a built-in hysteresis window, which prevents rapid on/off switching when the input voltage changes slowly. We do this because the temperature changes gradually, not instantly.

Using the 555 as a Schmitt trigger ensures the fan:

• turns on decisively,
• stays on while the temperature is near the threshold,
• and turns off only after the system has cooled sufficiently.

The result is smooth, stable behavior without chatter. So, what I did was the following:

• Taking a red jumper wire long enough to connect from pin 2 to pin 6 of the 555, I placed one end of the jumper at point row 12, column H (inline with pin 2), and placed the other end of the jumper at point row 13, column C (inline with pin 6), connecting the two pins.
• We need to continue this connection between pins 2 and 6 to the sensor node we established earlier. I took a blue jumper wire and placed one end of it at point row 13, column A (inline with pin 6), and placed the other end of the jumper wire at point row 17, column A — inline with the blue jumper wire connected to the wiper (center pin) of the potentiometer we set earlier.

By tying pins 2 and 6 together, the 555 becomes a voltage-decision device rather than a timing device. With the 555 now configured as a Schmitt-trigger comparator, the next step is to connect the sensor voltage to this input and add hysteresis for stability.

Add Hysteresis (Prevents Fan “Chatter”)

Temperature changes slowly and often fluctuates slightly around a threshold. If the circuit switched ON and OFF at a single voltage point, the fan could rapidly toggle as the temperature hovered near that value. To prevent this, we add hysteresis.

A high-value resistor (typically hundreds of kilohms to a few megohms) is connected from the 555 output (pin 3) back to the sensor node (SENSE).

• I took a 1MΩ resistor (R2) and placed one of its leads at point row 13, column J (inline with pin 3), and placed the other lead at point row 16, column J.
• I continued the connection by placing one end of a blue jumper wire at point row 16, column F (inline with the 1MΩ resistor leg), and the other end of the jumper wire at point row 17, column D, across the center gap of the breadboard — inline with the blue jumper wire leading to the wiper (center pin) of the potentiometer we set earlier at row 17.
• From row 17, we made a path to the wiper of the potentiometer through to the yellow jumper wire we set on row 22, that leads directly to our sensor node.

This resistor (R2) feeds a small portion of the output voltage back into the input. Because of this feedback:

• The voltage required to turn the fan ON is different from the voltage required to turn it OFF
• The circuit now has two switching points instead of one

This creates a stable “dead zone” where small temperature changes do not affect the fan state.

The reason why this all matters is because with hysteresis:

• The fan turns on decisively
• It stays on until the system cools sufficiently
• It avoids rapid on/off cycling (chatter)

The result is smoother operation and less wear on components. Hysteresis adds stability by ensuring the circuit makes confident decisions instead of reacting to every tiny change.

With hysteresis in place, the circuit is now stable and ready to safely drive the fan through a MOSFET. Let’s install our MOSFET and fan onto the breadboard in the next steps.

Wire the MOSFET driver (low-side switch)

The output of the 555 timer is not designed to power a motor (fan) directly. While it can source or sink small currents, a fan requires more current than the 555 can safely provide. To handle this, we use a MOSFET as a low-side switch.

Even though it’s recommended to use one of the MOSFETs listed in the parts list above, I had to use a NTE2987 logic-level MOSFET, because that’s all I had at the time of making my circuit. It’s still a logic-level TO-220 package MOSFET, as recommended. I got it way back in the day when Fry’s Electronics was still around — I miss that place 🙁.

Anyway, in a low-side switching arrangement:

• The load (the fan) is connected to +12V
• The MOSFET sits between the load and ground
• The MOSFET controls whether current can flow through the load

When the MOSFET turns ON, it completes the path to ground and the fan turns ON and spins. When it turns OFF, the path is broken and the fan stops.

The reason why we use a logic-level N-channel MOSFET is because it requires very little gate current and can handle much higher load current than the 555. The MOSFET basically acts like an electronically controlled switch. The 555 simply drives the gate of the MOSFET, while the MOSFET does the heavy lifting.

I took my NTE2987 MOSFET and placed it on the breadboard, as follows:

• Gate pin: placed at point row 17, column H
• Drain pin: placed at point row 18, column H
• Source pin: placed at point row 19, column H

Now that the MOSFET is firmly in place on the breadboard, let’s tie the source lead to ground (GND), as shown in the schematic diagram provided above:

• I took a brown jumper wire and placed one of its ends into the point row 19, column J (inline with the source pin of the MOSFET), and placed the other end of the jumper wire into a neighboring point on the left-side negative power rail of the breadboard.

We’ll add two other components to this part of the circuit for reliability. Together, they’ll ensure predictable and safe operation. These two components are:

• A gate resistor (R1): limits sudden current spikes into the MOSFET gate
• A gate pull-down resistor (R3): ensures the MOSFET stays OFF when the circuit powers up

Starting with the gate resistor (R1), as shown in the schematic diagram provided above:

• I took a 100Ω resistor and placed one of its leads at point row 13, column G (inline with pin 3 of the 555 and the lead of the 1MΩ resistor (R2) we placed on the breadboard earlier), and placed the other lead of R1 at point row 17, column G (inline with the gate of the MOSFET).

Now, moving on to the gate pull-down resistor (R3), as shown in the schematic diagram provided above:

• I took a 100kΩ resistor and placed one of its leads at point row 22, column I (inline with the thermistor’s lead at the established sensor node), and placed the other lead of R3 at point row 17, column I (inline with the gate of the MOSFET).

With the MOSFET driver in place, the circuit can now control the fan reliably without stressing the 555 timer.

Adding the Fan to the Circuit

With all this talk about a fan, we’ve yet to place it within our circuit on the breadboard. I’m using a 12VDC brushless fan (AFB0712HB) that I think I might have scavenged from an old computer from long ago or I might have received it in one of those 3 pound grab-bags you can buy from Jameco that comes with all sorts of goodies in it — I don’t remember.

The fan that I’m using has three wires:

• A red wire for power
• A blue wire for signal
• A black wire for ground

We’re just going to use the red and black wire for basic powering of the fan. I have a connector at the end of my fan wires that I could have cut off, but chose to leave it and use jumper wires to shove in its terminals.

Let’s walk through the process of connecting the fan to our circuit on the breadboard:

• I took a long yellow jumper wire and stuck one of its ends into the female connector of the fan where the red wire of the fan is located. Then, I placed the other end of the jumper wire into a point on the left-side positive power rail of the breadboard.
• Next, I took a long green jumper wire and stuck one of its ends into the female connector of the fan where the black wire of the fan is located. Then, I placed the other end of the jumper wire into the point row 33, column J.
• To continue the connection from the fan’s black wire – to the green jumper wire – to the drain pin of the MOSFET, I took another long green jumper wire and placed one of its ends at point row 33, column I (inline with the green jumper wire coming from the black wire of the fan we just placed). Then I placed the other end of this new green jumper wire at point row 18, column J (inline with the drain pin of the MOSFET).

Adding the Flyback Diode across the Fan

A fan is an inductive load, which means it stores energy in a magnetic field while it is running.

When the MOSFET suddenly turns the fan OFF, that stored energy has to go somewhere. If it doesn’t have a safe path, it can create a high-voltage spike that may damage the MOSFET or interfere with the rest of the circuit.

Adding a flyback diode provides a safe path for this stored energy when the fan is switched off. During normal operation, the diode does nothing, but when the fan is turned OFF, the diode conducts briefly. This allows the energy to dissipate harmlessly instead of creating a voltage spike.

Even though it’s recommended to use a 1N5819 diode in the parts list above, I had to use a 1N5817 as my flyback diode, because that’s all I had at the time of making my circuit. It works just as good for this project.

The reason why we place the flyback diode across the terminals of the fan is to:

• Protect the MOSFET from voltage spikes
• Reduce electrical noise
• Improve overall circuit reliability

The diode is oriented so it is reverse-biased during normal operation, meaning it does not affect the fan while it’s running.

To properly place the flyback diode on our current breadboard circuit setup:

• I took my 1N5817 Schottky diode and placed its anode lead at the point row 33, column H (inline with the green jumper wires we just placed in that row for the fan’s positive terminal to the drain pin of the MOSFET). Then, I placed the cathode of the diode in a point on the left-side positive power rail of the breadboard — this placed the diode’s terminals directly across the terminals of the fan (in parallel) as we want.

With the flyback diode in place, the circuit is now protected and ready for safe operation and testing.

Video: Operation of the 555 Temperature-Controlled Fan Controller

Video: Making a 555 Temperature-Controlled Fan Controller

How to Test It Using a Multimeter and Oscilloscope

Quick Multimeter Checks (Before Heat is Added)

To check and make sure that our circuit will function as it should, we can use the help of a multimeter to measure voltage at certain points on the circuit.

Take your multimeter and set it to a DC voltage higher than the supply voltage (VCC) — I set my multimeter to 20V, because that was the smallest value on mine that was greater than 12V. Once you have your multimeter ready for testing, you can perform the following measurements:

1. Measure V_CC at pin 8: This should be your supply voltage (12V).
   • Place your red probe at pin 8 of the 555
   • Place your black probe at a point on the negative supply — I chose the brown jumper wire connected to the negative return line of the power supply
   • You should have a reading of around 12 volts on your multimeter — the supply voltage (V_CC)
2. Measure SENSE voltage at room temp: This should be somewhere between 0V and V_CC.
   • Place your red probe somewhere on the established sensor node we created on the breadboard (row 22, columns F thru. J) — I placed my probe on the leg of resistor R3, where it’s connected to the sensor node at point row 22, column I
   • Place your black probe at a point on the negative supply — I chose the brown jumper wire connected to the negative return line of the power supply
   • My initial voltage reading on the multimeter was around 7.8 volts. Yours will most likely be different, but be somewhere between 0V and 12V.
3. Measure pin 3: Before we add heat to the thermistor, we want to take a voltage reading at pin 3.
   • Place your red probe at pin 3 of the 555
   • Place your black probe at a point on the negative supply — I chose the brown jumper wire connected to the negative return line of the power supply
   • Initially, there should be no voltage (0V) at pin 3.

Multimeter Checks (When Heat is Added)

Next, we want to take measurements throughout the circuit to see what it’s doing when we initially add heat to the thermistor using a heat source, like a heat gun.

1. Measure SENSE node: When we initially add heat to the circuit at the thermistor, we should observe a gradual drop in voltage at the sensor node.
   • Place your red probe somewhere on the established sensor node we created on the breadboard (row 22, columns F thru. J) — I placed my probe on the leg of resistor R3, where it’s connected to the sensor node at point row 22, column I
   • Place your black probe at a point on the negative supply — I chose the brown jumper wire connected to the negative return line of the power supply
   • As heat is added to the thermistor, watch your multimeter. You should observe your voltage reading go initially from the room temperature voltage you measured earlier (7.8V for mine) and gradually go down in voltage.
2. Measure pin 3: When we initially add heat to the circuit at the thermistor, we shouldn’t see an instant voltage change at pin 3.
   • Place your red probe at pin 3 of the 555
   • Place your black probe at a point on the negative supply — I chose the brown jumper wire connected to the negative return line of the power supply
   • Your voltage reading at pin 3, once heat is initially added to the circuit, should still be around 0 volts. This is because there has not been a sufficient amount of heat built up for the sensor node to trigger the fan to turn ON.

Multimeter Checks (When Heat is Sufficient)

Next, we want to take measurements throughout the circuit to see what it’s doing when a sufficient amount of heat has been added at the thermistor.

1. Measure SENSE node: Once enough heat has been added to the thermistor, we are watching what happens at the senor node once the fan kicks on.
   • Place your red probe somewhere on the established sensor node we created on the breadboard (row 22, columns F thru. J) — I placed my probe on the leg of resistor R3, where it’s connected to the sensor node at point row 22, column I
   • Place your black probe at a point on the negative supply — I chose the brown jumper wire connected to the negative return line of the power supply
   • As heat is being added to the thermistor, you should notice the voltage dropping at the sensor node.
   • For me, when the voltage at the sensor node reached around 4.3 volts and below, the fan kicked ON.
2. Measure pin 3: Once enough heat has been added to the thermistor, we are watching what happens at the pin3 once the fan kicks on.
   • Place your red probe at pin 3 of the 555
   • Place your black probe at a point on the negative supply — I chose the brown jumper wire connected to the negative return line of the power supply
   • After some time and a sufficient amount of heat has been added to the thermistor, the fan should kick on. At that point, you should immediately get a voltage reading on the multimeter at pin 3 — mine went from 0V, then almost instantaneously to around 11 volts.

Multimeter Checks (When Heat is Removed)

Finally, we want to take measurements throughout the circuit to see what it’s doing when we remove the heat from the sensor node and when the fan cools our circuit.

1. Measure SENSE node: Once we remove the heat source (heat gun) away from the thermistor, we are watching what happens at the sensor node as the fan cools the circuit.
   • Remove the heat gun away from the thermistor and continue measuring voltage at the sensor node with your multimeter.
   • As the fan is blowing air on the circuit, the thermistor cools and you should see that the voltage at the sensor node rises.
   • After some period of time of sufficient cooling to the thermistor, the voltage at the sensor node should reach near the threshold and the fan should cut off. When the voltage got to be around 7.8V, my fan turned OFF.
2. Measure pin 3: Once we remove the heat source (heat gun) away from the thermistor, we are watching what happens at pin3 as the fan cools the circuit.
   • Remove the heat gun away from the thermistor and continue measuring voltage at pin 3 with your multimeter
   • With your fan blowing on the thermistor — after some period of time of sufficient cooling to the thermistor, the fan should cut off. At that point, the voltage at pin 3 should drop almost instantaneously back down to 0V.

Oscilloscope Checks (Optional)

If you have an oscilloscope you can make a couple of checks on your circuit to see how things are working. The model oscilloscope I am using is a Hantek DSO5102P. All you need to do is take your probes and place them in the following manner:

1. CH1 Probe – SENSE node: To test the SENSE node, we’ll use the help of a jumper wire to gain a good connection to clip our probe to.
   • I took a yellow jumper wire (I used yellow to match my oscilloscope’s yellow CH1 section) and placed one end of it into the point row 22, column H — this is the row we established as our sensor node earlier.
   • Next, I took a black jumper wire and placed one end of it into a point on the left-side negative power rail (GND). Then, I clipped the negative supply alligator clip of the CH1 probe to the other end of the black jumper wire.
2. CH2 Probe – Pin 3 output: To test the output of pin 3 of the 555 timer, we’ll again used the help of a jumper wire to gain a good connection to clip our probe to.
   • I took a blue jumper wire (I used blue to match my oscilloscope’s blue CH2 section) and placed one end of it into the point row 13, column H (inline with pin 3).
   • You don’t need to connect the negative supply alligator clip of the CH2 probe because both probe tips are measuring nodes in the same circuit. All oscilloscope probe ground clips are internally connected together and are also connected to earth ground through the scope’s power cord. Because of this, you only need to connect ONE ground clip. Our circuit has a single common ground, and we already connected one ground clip properly. This is very common and totally acceptable in low-voltage electronics.

• Outputs (heat added): We want to observe what’s happening at our test points when we add heat to the thermistor using our heat gun.
  • Before Fan Turns ON: When we add heat to our circuit, but before the fan kicks on, we should observe that the voltage at the sensor node gradually drop in voltage from our initial measurement (my initial measurement was 7.8V). The CH1 (yellow) line on my oscilloscope screen started at around 8V, then the line gradually declined in voltage as time went on and heat was added to the circuit.
  • Fan Turns ON: Once a sufficient amount of heat has been added to the sensor node and the lower threshold voltage has been reached (~4.3V), the fan should turn ON. That threshold voltage for my circuit was when my multimeter read a voltage of about 4.3V or below when the fan kicked on. So, my CH1 (yellow) line on my oscilloscope screen went down to around 4V, and my CH2 (blue) line on my oscilloscope screen went from a reading of 0V, initially – to almost instantaneously to around 11V, once the fan turned ON. The CH1 (yellow) line still gradually went down, but held a level above 2V, before I took the heat source (heat gun) away from the circuit.
• Outputs (heat removed): We want to observe what’s happening at our test points when we remove heat to the thermistor by taking away our heat gun.
  • Fan Turns OFF: When we remove the heat source away from the circuit and allow the fan to cool the circuit, we should observe the voltage at the sensor node gradually rise again. The CH1 (yellow) line on my oscilloscope screen started to rise from the lowest voltage reading it obtained and gradually inclined in voltage as the fan cooled the circuit. Once the upper threshold voltage was reached (~8V), then the fan turned OFF. That threshold voltage for my circuit was when my multimeter read a voltage of about 7.8V or above when the fan cut off. When the fan turned off I observed my CH2 (blue) line on my oscilloscope screen almost instantaneously drop back down to around 0V.

We can see that the behavior shown on the oscilloscope screen for the sensor node and for pin 3 were almost identical to what we observed on our voltage readings from the multimeter. Both methods allow us to have a visual as to what’s happening within our circuit when different stages occur due to temperature changes.

Video: How to Test the 555 Temperature-Controlled Fan Controller Using a Multimeter and Oscilloscope

Troubleshooting Your Circuit

⚠️ Issue 1: Fan Turns ON When Voltage Applied and Stays ON

I created the circuit on the breadboard and when I supply +12V to the circuit, the fan turns on immediately and nothing else happens. I try turning the potentiometer and that doesn’t change anything to the outcome of the circuit. I tried grasping the thermistor with my fingers and still, nothing changes to the outcome of the circuit as its running — the fan just spins at a steady speed. What’s going on?

What You Built (and Why It Matters)

This temperature-controlled fan circuit demonstrates several core principles that appear in countless real-world systems: analog sensing, decision-making with hysteresis, and safe power switching.

The circuit you made in this project makes its decisions using nothing more than voltages, thresholds, and clever feedback. You learned how a thermistor converts temperature into a voltage, how the 555 timer can be used as a Schmitt-trigger comparator, and why hysteresis, decoupling, and protection components matter in real designs.

Just as importantly, you saw how measurements, observation, and methodical troubleshooting turn confusion into understanding. That process is what electronics is really about!

Wrapping Up

In this project, you didn’t just wire up a fan — you built a circuit that senses the real world, makes a decision, and takes action.

From here, the path forward opens up quickly. The same ideas used in this circuit — sensing, thresholds, and control — appear everywhere in robotics, automation, and embedded systems.

The fundamentals you learned here will carry forward into every future project. Understanding why a circuit works is far more powerful than simply making it work.

What’s Next?

In the next part of the series, we’ll take these same concepts and explore how a microcontroller can replace the analog decision-making while still relying on the same sensor fundamentals. But before we move onto Part 5 of this series, we’re going to take another detour into our discussions of what we just learned here.

Next, we move onto another subseries for this 555 Temperature-Controlled Fan Controller project we just accomplished, where we delve deeper into the subjects of:

• Schmitt-trigger comparators – what they are
• Thermistors – what they are and what they do
• Hysteresis
• and MORE for this 555 Temperature-Controlled Fan Controller project, like:
  • What type of thermistor should be used for this project?
  • Looking at using different kinds of thermistors and which one is better
  • Comparing different MOSFETs that can be used for the 555 Temperature-Controlled Fan Controller circuit
  • Debugging issues with your 555 Temperature-Controlled Fan Controller circuit

These side discussions are meant to strengthen your understanding, not slow you down. By taking a closer look at the building blocks behind this project, you’ll gain confidence in why the circuit behaves the way it does and how to troubleshoot it when things don’t work as expected. Once we’ve covered these topics, we’ll return to Part 5 with a much stronger foundation — ready to move from analog decision-making to microcontroller-based control.

We hope that you’ve enjoyed this series, thus far. We’d appreciate your feedback by leaving a comment below and encourage you to join our newsletter to stay in touch as to what’s new and going on here at Motbots.

Remember to keep at it and stay motivated.