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There are a lot of different products that use heating elements. Unfortunately, many of them are not adjustable. For instance, most soldering irons are either on or off.

It would be much more useful if you were able to change the temperature of the iron. This would allow you to use the soldering iron for lower temperature applications.

So I designed a simple control circuit that will let you adjust the output of a heating element.

Project Steps

The Control Circuit

This is the control circuit that I designed for this project. It is based around a 555 timer IC. This configuration is known as “Astable Mode.” In this mode, the output of pin 3 is a series of HIGH and LOW pulses. This turns the relay off and on. The frequency and duration of these pulses is determined by the values of the resistors and the capacitor.

The time that the relay will be on is based on the formula:

t = 0.7 x R1 x C

The time that the relay will be off is based on the formula:

t = 0.7 x (R1 + R2 + R3) x C

In these formulas “t” is measured in seconds, “R” is measured in ohms and “C” is measured in Farads.

For my application, I used C = 0.000330F (or 330 micro farad), R1 = 22000 ohm (or 22 kohm), R2 = 000 ohm (or 1 kohm), and R3 = a 100000 ohm (or 100 kohm potentiometer). With these values the “ON” time was 5 seconds per cycle. Based on the setting of the potentiometer, the “OFF” time ranges between 5.3 seconds and 28.4 seconds.

You can change the timing of the on and off cycles by changing the values of the resistors and the capacitor. You can make the values whatever you want. But keep in mind that you always need to have at least 1 kohm of resistance between pin 7 and pin 8 on the IC chip. If you don’t, the IC can be damaged when the output turns on and off.

Prototype the Circuit on a Breadboard

With any electronics project, it is a good idea to prototype the circuit on a breadboard before soldering it together. This give you a chance to find any problems and make any necessary adjustments. For testing, I connected an LED to the output of the relay. This made it easy to see when the relay was on.

R2 from the circuit diagram is not shown in these pictures; I added that later when I was soldering the circuit onto the circuit board.

Prepare the Power Cord

In order to connect the heating element to the control circuit, we need to modify the power cord. First, I cut the power cord in half using a pair of wire cutters. Then I pulled apart the individual wires on each piece. Next, I stripped the insulation off of the cut ends.

One wire from each side was connected together using an insulated twist-on connector cap. This effectively reconnected one of the cut wires. In hindsight, it would have been easier to simply leave this wire connected and never cut it in the first place.

Now you have one pair of cut wires that you can attach to the control circuit.

Solder the Circuit Together on a Perf Board

Once the circuit was working properly, I soldered all the components onto a piece of perf board. In the final configuration, I replaced the small potentiometer from the previous step with a larger potentiometer that could be mounted to the side of the housing. I also attached the wires from the power cord to the switch contacts on relay.

Mount the Circuit in an Insulated Project Enclosure

Because we are connecting to an AC power cord, it is absolutely essential that all the parts be mounted inside an insulated project enclosure.

I started with a generic project enclosure that I purchased from RadioShack. I drilled a hole in the top so that I could mount the potentiometer. Then I cut slots in the sides so that the power cords cold go in and out of the box. I fit all the parts into the enclosure and closed it up. Then I added a knob on the shaft of the potentiometer to make it easier to turn.

Finished Temperature Controller

Now plug in the power cord of the appliance and plug in the 5V power supply that powers the control circuit. You can now use your control circuit to adjust the output of a heating element and control its temperature.

You can use this circuit for anything that uses a simple resistive heating element. You can use it on a soldering iron, a hot glue gun, a candle warmer/coffee warmer, a heat lamp, or anything else.