Thermo Switch Kit: Fan Control & Fire-Alarm Models
Build temperature-responsive science projects with the Pragmatic Thermo Switch Kit. Follow the assembly photographs, sensor reference, fan-control ideas and educational fire-alarm demonstrations.
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Thermo Switch Kit: Fan Control & Fire-Alarm Models
Build temperature-responsive science projects with the updated sensor reference, step-by-step assembly photographs, and practical ideas for fan control and fire-alarm demonstrations.
A fan that responds to heat makes a memorable school science model. The Thermo Switch Kit introduces temperature-responsive switching through a hands-on electronics build: identify the parts, place them on the PCB, solder the connections, and observe the circuit’s response.
This guide follows the Pragmatic kit shown in the photographs. Keep the reference sheet supplied with your own kit beside you, as component values and connections can differ between board revisions.
01 / Start with the complete kit. Sort the labelled components before assembling the board.
Watch the project in action
Use these quick links to view the Thermo Switch project on YouTube, Facebook and Instagram. Keep this assembly guide open for reference as you watch.
How does a Thermo Switch work?
A temperature-responsive element changes its electrical behaviour as it warms or cools. A switching circuit uses that change to control an output. This kit’s placement guide identifies a BC548 transistor, an LED and a relay, which are parts of its switching and indication circuitry.
The assembled-project photograph shows a fan beside the board during a heat-response demonstration. The exact activation temperature, switching direction and reset behaviour depend on the supplied sensor and circuit. Test those behaviours on your build; this guide does not assume an adjustable or calibrated temperature setting.
1. Check the parts and prepare your tools
Use the following markings to help locate parts on the supplied reference drawing. Confirm each value against its component bag before fitting it.
| Reference | Marking visible in the guide | Assembly check |
|---|---|---|
| R1 / R2 / R3 | 1 kΩ / 330 Ω / 680 Ω | Confirm values using labels or a multimeter. |
| D1 | 1N4007 diode | Match its banded end to the correct orientation. |
| D2 | LED | Identify anode and cathode before soldering. |
| Q1 | BC548 transistor | Check body orientation and the actual part’s pinout. |
| C1 / C3 | 220 µF | Electrolytic capacitors: observe polarity and voltage rating. |
| C2 | 100 nF | Match the supplied part to the placement guide. |
| C5 | Value not clearly specified | Use the kit’s labelled part or ask the supplier before fitting. |
| K1 | Relay | Confirm the relay model, coil voltage and contact arrangement. |
| J1 / J2 | Power / switch | Follow the kit’s connector and polarity instructions. |
| J3 | Waterproof temperature sensor | Connect the kit-compatible temperature probe as specified by the supplier. |
| J4 | Motor / pump or device control | Device-control connection. Confirm load ratings, supply arrangements and terminal wiring before connecting a load. |
Tools: a soldering iron and stand, electronics solder, side cutters, a PCB holder, a multimeter and eye protection. Work on a clear, ventilated surface. Disconnect the power before soldering or changing connections.
2. Understand the PCB and placement guide
The PCB artwork shows track routing and footprints. The placement drawing below is easier to read when identifying component locations. Do not copy left-to-right positions directly between views without checking the board orientation.
Identify the sensor input and device connection
J3 — waterproof temperature sensor: use the supplied compatible probe for temperature-response projects such as fan control. The label applies to the sensor, not the PCB, battery or connectors. Keep the board dry, and confirm the probe’s temperature range and immersion limits before any liquid-temperature demonstration.
J4 — motor, pump or device control: use this connection for the compatible demonstration load according to the kit wiring instructions. J4 is not a fire-sensor input. The drawing alone does not establish whether these terminals provide switched power or require an external load supply, so confirm this before wiring.
For a temperature-triggered warning model, the temperature probe remains connected at J3. A suitable warning output can be used through the device-control arrangement once its compatibility and wiring are verified.
Before fitting a part: identify its reference, confirm its value, check its orientation, and only then solder. For Q1, verify the manufacturer’s BC548 pinout as well as the printed outline. For electrolytic capacitors, align the positive lead with the “+” marking.
3. Assemble the board step by step
Fit the low-profile components first
Start with R1, R2 and R3. Bend the leads to suit the hole spacing, insert each resistor at its matching reference, and hold it in position. Fit D1 with the correct polarity. Add C2 according to the reference sheet. Resistors are not polarised; the diode is.
Add the LED, transistor and capacitors
Identify the LED’s polarity before trimming its leads. Fit Q1 only after confirming its pin arrangement. Install C1, C3 and the supplied C5 part with the correct polarity where applicable. Do not select C5 by appearance or assume that it matches the other capacitors.
Fit the relay and connectors
Place the relay and connectors in their marked positions. Seat the parts without forcing them, then solder them so that they remain mechanically secure. Follow the supplied wiring instructions for the sensor, switch and output. The photographs are not a substitute for a verified relay pin diagram.
Solder and inspect each connection
Heat the copper pad and component lead together, apply a small amount of solder, and let the joint cool without movement. Look for a smooth joint that wets both surfaces. Avoid excessive heating, loose joints and solder bridges between neighbouring tracks. Trim excess leads after soldering while controlling the cut-off pieces.
Inspect the entire copper side under good light. Use a multimeter to investigate suspected shorts and confirm continuity where the board reference requires it. Resolve uncertain connections before connecting power.
4. Power up and test the response
- Check the supply. The project photograph shows a 9 V battery. Confirm the supply required by your particular kit and relay, and verify J1 polarity. Do not treat the multiple supply markings on the drawing as permission to use any adapter.
- Record the starting state. With the sensor at room temperature, observe the LED, relay and connected demonstration load. Note whether each is on or off.
- Apply gentle warmth. Use the sensor supplier’s recommended test method. Avoid flames, direct soldering-iron contact, and temperatures that could damage the probe or insulation.
- Observe the change. Note whether the relay switches and whether the fan changes state. Remove the warmth and allow the sensor to cool; record whether it resets automatically or requires a switch operation.
- Repeat the demonstration. Check that the response is repeatable. To report a trigger temperature, measure it with a suitable thermometer rather than guessing.
For school demonstrations: use compatible low-voltage DC loads and supervised soldering. A small fan or pump may need its own suitable supply. Do not connect mains appliances to this exposed educational board.
Test a fire-alarm demonstration
Build this as a temperature-triggered alarm demonstration: with power disconnected, connect the compatible temperature probe at J3. Use a suitable low-voltage active buzzer or alarm module through the J4 device-control connection only after verifying the terminal wiring, load ratings and supply arrangement. A buzzer is an additional output option; it is not established as an included kit component by the photographs.
Power the model and record its normal state. Gently warm the temperature probe within its specified limits, then observe the indicator, relay and alarm output. Remove the warmth and check the reset behaviour. This demonstrates a temperature response; the supplied reference does not establish smoke or flame detection. Do not use open flames for a classroom demonstration.
Educational fire-alarm model: this build demonstrates sensing and warning. It is not a certified smoke detector or fire-alarm system and must not replace installed life-safety alarms.
Quick troubleshooting
| What you observe | What to check |
|---|---|
| No visible response | Battery condition, supply polarity, switch wiring and solder joints. An unlit LED alone does not prove the whole circuit is off. |
| LED changes but fan does not | Relay contact identification, load wiring, fan voltage and load supply. |
| Relay chatters or the supply drops | Weak battery, loose connections, unsuitable load or sensor operating near its switching point. |
| No response to warming | Sensor connector assignment, sensor continuity as specified by its supplier, Q1 orientation and resistor values. |
| Unexpected on/off behaviour | Relay contact choice and the kit’s intended switching/reset behaviour. |
| A part becomes unusually hot | Disconnect power immediately and inspect polarity, shorts, component values and supply compatibility. |
5. Turn the circuit into a working model
Temperature-responsive fan
Demonstrate how a sensor can command a small cooling fan when conditions change.
Greenhouse ventilation model
Place the sensor in a model greenhouse and use a compatible fan to illustrate ventilation control.
Miniature pump demonstration
Show temperature-triggered water movement with a compatible low-voltage pump and suitable supply. Soil-moisture-based irrigation requires separate sensing.
Smart-house science model
Build a cutaway room with a fan or indicator to explain the relationship between sensing and switching.
Educational fire-alarm model
Demonstrate a temperature-triggered warning using the J3 probe and a compatible buzzer or indicator through verified J4 device wiring. This illustrates heat response, not smoke or flame detection.
Temperature warning model
Use the compatible J3 probe with a warning indicator to demonstrate a temperature-triggered alert. Measure the response instead of assuming a precise setpoint.
Frequently asked questions
Does this project need coding?
The kit shown uses discrete components and a relay; assembly does not involve uploading an Arduino or ESP8266 program.
Can I set an exact temperature?
The supplied images do not establish an adjustable, calibrated setpoint. Check the kit specifications and measure your build’s response before describing a particular trigger temperature.
Can I build a fire-alarm model?
Yes—as an educational temperature-triggered warning model using the probe at J3 and a suitable alarm output through verified J4 device-control wiring. No dedicated smoke or flame sensor is identified in the corrected reference. This model does not replace a certified fire alarm.
Which connector is for the waterproof temperature probe?
The corrected drawing labels J3 for the waterproof temperature sensor and J4 for motor, pump or device control. The PCB itself is not waterproof.
What should I explain at a science exhibition?
Describe the input, switching stage and output. Show the room-temperature state, demonstrate a controlled temperature change, then explain what the output does as the sensor cools.
Build your next electronics project
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Which model will you build—a fan controller, fire-alarm demonstration, greenhouse or temperature warning model? Share your project idea in the comments.
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