Heat Control Unit
This circuit will tell the heater to "turn on" when the temperature of the water drops below the lower limit that you specify, and it will tell the heater to "turn off" when the temperatu…
Know the mission before touching a wire.
Understand what you are making, prepare the right tools, and make the workbench safe.
Project details
Heat Control Unit is a electronics project. This circuit will tell the heater to "turn on" when the temperature of the water drops below the lower limit that you specify, and it will tell the heater to "turn off" when the temperatu…
- Source pages
- 154-155
- Named parts
- 5
- Build goal
- Working, tested prototype
Tools you need
- Digital multimeter
- Wire stripper and side cutters
- Soldering iron with a fine tip
- Current-limited bench supply
Use eye protection, good lighting, and a clean insulated surface throughout the build.
Safety precautions
- Disconnect every power source before changing a connection.
- Check component polarity, pinout, and supply voltage twice.
- Use a current limit for the first power-up.
- Keep liquids, loose metal, and uninsulated wires away from the bench.
Gather, identify, and understand every part.
Use the standardized inventory, then open What's this? to learn each part's role, advantages, limitations, handling, and specifications.
What's this?Image, role, pros, cons, handling & specifications
Power supply
A power component supplies, converts, stores, or regulates energy for the project.
What it does hereIt must provide the documented voltage, polarity, isolation, and sufficient current safely.
Buy / compare this part ↗Advantages
- Stable power improves reliability
- Current limiting protects first tests
- Regulation reduces resets and noise
Limitations
- Wrong polarity can cause immediate damage
- Underrated parts overheat
- Mains circuits require qualified supervision
Handling
- Measure output before connection
- Use a fuse or current limit
- Insulate exposed conductors
Specifications to verify
- Use the exact model, value, package, and rating listed for Power supply; similar-looking parts are not always interchangeable.
- Confirm input/output voltage, current, polarity, connector, isolation, and thermal rating.
What's this?Image, role, pros, cons, handling & specifications
Operational amplifier
A named project component whose exact role is defined by the source circuit and build guide.
What it does hereIt performs a documented electrical, control, interface, or construction function in this project.
Buy / compare this part ↗Advantages
- Selected for this project
- Can be checked independently
- Supports modular troubleshooting
Limitations
- Substitutes may differ
- Generic names can hide variants
- Pinouts and ratings vary
Handling
- Compare the received part with the source
- Keep it labelled
- Do not force connectors or adjusters
Specifications to verify
- Use the exact model, value, package, and rating listed for Operational amplifier; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
What's this?Image, role, pros, cons, handling & specifications
NEGATIVE
A named project component whose exact role is defined by the source circuit and build guide.
What it does hereIt performs a documented electrical, control, interface, or construction function in this project.
Buy / compare this part ↗Advantages
- Selected for this project
- Can be checked independently
- Supports modular troubleshooting
Limitations
- Substitutes may differ
- Generic names can hide variants
- Pinouts and ratings vary
Handling
- Compare the received part with the source
- Keep it labelled
- Do not force connectors or adjusters
Specifications to verify
- Use the exact model, value, package, and rating listed for NEGATIVE; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
What's this?Image, role, pros, cons, handling & specifications
T1 - transistor stage identified in the circuit
A semiconductor stage performs switching, amplification, regulation, rectification, or logic.
What it does hereIt controls current or signal flow at a defined point in the circuit.
Buy / compare this part ↗Advantages
- Fast and efficient
- Compact
- Can control larger loads from smaller signals
Limitations
- Pin order varies
- Sensitive to overvoltage and reverse polarity
- May need cooling or bias components
Handling
- Verify the datasheet pinout
- Avoid static and soldering heat
- Check notch, stripe, or flat-face orientation
Specifications to verify
- Use the exact model, value, package, and rating listed for T1 - transistor stage identified in the circuit; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
What's this?Image, role, pros, cons, handling & specifications
PART LEARNING VIEWVR1 - preset potentiometer identified in the circuit
A named project component whose exact role is defined by the source circuit and build guide.
What it does hereIt performs a documented electrical, control, interface, or construction function in this project.
Buy / compare this part ↗Advantages
- Selected for this project
- Can be checked independently
- Supports modular troubleshooting
Limitations
- Substitutes may differ
- Generic names can hide variants
- Pinouts and ratings vary
Handling
- Compare the received part with the source
- Keep it labelled
- Do not force connectors or adjusters
Specifications to verify
- Use the exact model, value, package, and rating listed for VR1 - preset potentiometer identified in the circuit; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
Connect one verified path at a time.
Explore the named components and standardized signal flow, then use Source Check to verify exact physical pins, values, and topology before applying power.
Trace this circuit.
Heat Control Unit: interactive parts, standardized terminals, responsive anchored wires, student explanations, and the original circuit reference in one shared system.
Trace before wiring
Follow power, ground, inputs, processing, and outputs in that order. Never guess a pin from package shape alone.
Connect with power off
Make short, labelled connections and share a common ground only where the schematic requires it.
Inspect every joint
Check continuity, polarity, adjacent shorts, and loose connections before the first power-up.
Open all source diagrams and build views 1 visuals
Confirm the hardware-only control path.
This project does not include firmware in the source. The circuit itself provides the required behaviour.
How to connect
- Match every controller label to the circuit view and source pin map.
- Join grounds before signal wires when separate low-voltage supplies are used.
- Keep motors, relays, pumps, and other loads on a suitable driver and external supply.
Common mistakes
Reversed VCC/GND, board-label versus GPIO-number confusion, missing common ground, and charge-only USB cables.
Troubleshoot
Disconnect loads, continuity-test one path at a time, then test with a current limit.
Software preparation
No IDE, board package, library, or firmware upload is required for this project.
If you add a programmable controller as an extension, document its pin map separately.
How to upload code
The original design is implemented entirely in hardware, so proceed after verifying the circuit and supply.
Assemble, deploy, test, and troubleshoot.
Use the complete source notes in build order, then pass the final checks before calling the project finished.
Build in functional stages
- Power and regulation
- Controller or processing stage
- Inputs and sensors
- Outputs and loads
- Enclosure and strain relief
Power up safely
- Inspect unpowered continuity first
- Apply the lowest safe current limit
- Measure supply rails before signals
- Add one load at a time
- Record expected and actual results
Work from simple to complex
- Confirm power, ground, polarity, and orientation
- Compare each pin with the source
- Test inputs separately from outputs
- Replace only one variable at a time
- Power off before every correction
Follow the documented instructions.
These notes come from this project's source and remain in their original order.
Project overview
Project build noteThis circuit will tell the heater to "turn on" when the temperature of the water drops below the lower limit that you specify, and it will tell the heater to "turn off" when the temperature rises over the higher limit that you specified. The circuit is comprised of both bridge and op-amp parts, and it is powered by a controlled 12 V supply of power. Resistors R1 and R2, a preset VR1, and a resistance temperature detector are the primary components of the bridge section (RTD). The operational amplifier component is constructed around IC1. Pin 3 of IC1 has a voltage that is greater than pin 2's voltage when water is at its typical temperature, which is approximately 30 degrees Celsius. As a result, the output of the operational amplifier is high, and the relay driver transistor T1 conducts, which causes the relay RL1 to become energized. The N/O contacts of RL1 are what make
the connection between the heater and the power supply. As a direct consequence of this, the heater will now begin to heat the water. The voltage at pin 2 of IC1 rises to a higher level than the voltage at pin 3 as soon as the temperature of the water reaches a certain threshold, say 60 degrees Celsius. As a result, the output of the operational amplifier IC1 becomes low. The conductivity of the relay driver transistor T1 is interrupted, which results in the de-energization of the relay RL1. Because of the pole contact with the N/C of RL1, the power supply to the heater is cut off, which results in an end to any further heating of the water. Now, once the water temperature drops below the level that was previously established, the relay will become energized, which will reconnect the heater to the power source so that the water can be heated. The loop keeps going around. Choose the resistance values for resistors R1 and R2 in accordance with the temperature requirements you have, but make sure that R1 and R2 have values that are proportionate to one another. The bridge can, in theory, be balanced by determining the resistance of the PT-100 (RTD) at the temperature that is required and then adjusting the resistance of the preset. A positive temperature coefficient is exhibited by PT-100. In other words, its resistance improves as the temperature rises. In order to calibrate, let's assume that the temperature of the water needs to be maintained at 60 degrees Celsius after it has been heated to that level. First, heat the water until it reaches a temperature of 60 degrees Celsius (measure the temperature using a thermometer). Keep sensor PT-100 (RTD) inside water. Adjust the setting in such a way that the potential difference between the inverting and non-inverting inputs of IC1 becomes negative while maintaining RTD at a temperature of 60 degrees Celsius (as determined by the thermometer). That is, the output pin 6 of IC1 should be low when the temperature of the water is 60 degrees Celsius. Construct the circuit using a PCB designed for general use, and then enclose it in an appropriate cabinet. After connecting the PT-100 wires to the PCB, determine the temperature of the water by dipping the section of the sensor that is metallic.
You built Heat Control Unit.
You followed the full workflow from understanding the mission to testing the finished project. That is a real engineering achievement - well done.
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