Space Heater Controlled by Digital Thermostat




Introduction: Space Heater Controlled by Digital Thermostat

This instructable shows you how to use an off the shelf digital programmable thermostat to control an inexpensive space heater.

Most cheap space heaters have only an analog knob to coarsely set the temperature; even the fanciest models only allow you to set them to turn off automatically after a preset number of hours.

This project allows you to set the room temperature according to the time of day and day of the week, giving you much needed flexibility to save energy and avoid waking up to a freezing house! You can save energy by programming the thermostat to lower the room temperature at night, but still get out of bed to a toasty room in the mornings.

Step 1: Materials and Safety Warning

You will need the following materials for this instructable:

- A digital programmable thermostat. I found a used one on ebay for about $15. It is a Bryant and was originally used in a commercial building. Commercial thermostats typically don't have battery backup, something to keep in mind if you plan on moving the heater around and don't want to reset the clock. Commercial thermostats are also typically cheaper than name brand consumer models. Make sure you get one that is programmable, many digital models are not, shocking considering the minimal effort in adding the feature and the energy savings in a typical home!!

- A relay with a 24VDC coil voltage and around 700 ohms coil resistance. The contacts should be rated to at least 15 or 20A at 110VAC minimum. $3-$5 at your local electronics surplus store.

- A 110V to 24VAC transformer. My transformer was rated at 36VAC, 65mA on the secondary, and maintains about 20VAC under load between one end of the secondary and the center tap. 20VAC seems to be within the input supply range of the thermostat, the exact voltage is not critical. Another electronics store find - $3.

- An enclosure, power cord and AC receptacle. I gutted a fax machine power conditioner and got all three for about $2.

- Some parts you might already have in your junkbox - a 1k resistor, 1n4001 diode, 100uF capacitor. A terminal strip or some perfboard.

- And I almost forgot - a space heater. Mine is a Bionaire MicaThermic Convection Console Heater - about $40 (in store price) at Costco.

Read this!!
Safety warning:

Space heaters typically consume on the order of 1500W, or roughly 15A at 110VAC. All wiring needs to be sized appropriately to handle these currents. Undersizing the wire gauge used or poor connections could lead to a fire! Also, operating a space heater while you are not home is probably a bad idea. I recommend unplugging the space heater before leaving it unattended for an extended period of time. Be safe!

Step 2: Schematic

Here is a rough schematic of the circuit (also my first experience with Eagle!).

Only the W1 output of the thermostat is used.
C is the common terminal, some thermostats may not have this. Mine uses it to power the backlight and digital functions since it has no battery.
R is return and completes a circuit with the W1 terminal when the thermostat activates the heater.

C1 should be rated ~50VDC. The exact value is not critical.

The thermostat needs to be on the unswitched side of the relay so that the thermostat always has power. The hot wire should be switched for safety. The ground wire is not shown and just passes through the box from the cord to the receptable. If the enclosure is metal (not recommended) it should be grounded.

Step 3: Assembly

Assemble the 1k resistor, diode, and 100uF capacitor on a terminal strip or perfboard. The purpose of this circuit is to convert the output of the thermostat, which is AC, into a DC signal to drive the relay. There is a fair amount of flexibility in this circuit - these are just parts I had in my junkbox.

The thermostat uses a triac to switch the heater on and off. Triacs only work with AC signals, they can't be used directly to switch DC because they will "latch" on and not turn off until power is removed. The 1k resistor in the circuit ensures that a small amount of AC current can pass through the triac and avoids the latching problem.

Step 4: Assembly Continued

Assemble the remaining parts and complete the wiring. The thermostat is attached to the top of the box by its original wall-mount with three screws, and can be popped off to make the necessary connections. Be sure to use heatshrink or put tape on all 110VAC connections to reduce the chance of electric shock or shorts!

The pinout for the relay can be determined by the diagram on the top of most relays, or with an ohm meter.

Step 5: Final Assembly and Testing

Before you close up the enclosure, do some preliminary testing. With nothing connected to the AC receptable, plug in the power cord. Verify that the thermostat powers up. A test lamp or small wattage light bulb connected to the outlet should be off.

Set the thermostat for heat mode and increase the set temperature above the room temperature as shown. Make sure the relay closes and 110VAC appears at the outlet, or the lamp turns on. If it checks out, test it with a real space heater, and allow it to run at least half an hour on your bench. Turn it off and inspect for any overheated wires or hot components.

If everything checks out, congratulations! You now have a digitally programmable space heater!



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80 Discussions

I have a propane space heater mounted on the wall, is their any way to install a regular wall thermostat to this just like you did for the electric heater? The current thermostat is mounted on the heater but it does not evenly heat the whole cabin, I want something that will keep it running to heat the whole cabin. It does have a very small fan built into it. But not powerful enough to feel it blow.

Thanks for creating this! I'm using this to control the heat in an outdoor, insulated cathouse--or perhaps I should call it a cat-cabin!

Very useful article. A nice and safe alternative may be to use a Wattstopper power unit which is UL approved, has zero voltage switching and as a bonus has DC output at about 150 mA to power your controller. One such unit (B-120 EP) is available from Amazon for about $15. - Enjoy.

I know this instructable is old as dirt, but I was wondering if this could be modified to work with a car's a/c and heater. i.e. set it to 70 degrees and never have to touch it again, save to turn on the defrost. or just leave it on defrost and cool/heat your car that way.

 $32 at amazone... programable!... I'm not trying to be a jeark but I just don't get it... 

  this is a great idea. However, I wonder... why not simply use a line voltage thermostat?

2 replies

Because that thermostat (and most like it) don't have a timer.  The whole idea behind the project was to set the heater to turn off at night while I'm sleeping, and then turn on before I get up so the room is nice and toasty in the morning.

Thanks for your comment!
- Jeff

Here's an idea that I don't think anyone has thought of yet. A relay has two states, normally open, and normally closed. You can use both of those at once if you want to switch between two different things.

You can take advantage of this idea without any modification of the circuit. You only have to make sure that you connected your hot wire to the relay so that the "Normally Closed" terminal has power when the relay is not activated. Then you would run that hot wire out to appliance #2.

Maybe you want a heater running if it's below the set point, and you want a fan running if it's above the same set point. Maybe in your greenhouse you want some bright (and hot) lights on if the temp is below the set point, or some not so bright (but cooler) lights running if the temp is higher than the setpoint.

I don't understand your scematic. Can you explain it in more detail? I don't see how the relay turns on the heater. What part of the scematic is the relay? What does the capacitor do? Thanks

7 replies

The relay is labeled K1.  The heater is connected to the switched 110V terminals labeled in the schematic.  The terminals of K1 complete the circuit to the heater when the relay is in the "closed" position (the contact marked C).

The capacitor, resistor, and diode are part of a circuit that converts the AC control voltage from the thermostat into a DC voltage used to switch the relay.  Most relays need a DC voltage on the coil and won't work with AC. 

Oh, now I see. The relay is drawn twice. Now makes more sense.

I know you can convert ac to dc using a bridge rectifier using 4 diodes. I guess just one diode will block 1/2 the voltage so that what comes through is 1/2 strength dc.  What do the resistor and the cap in that circuit do?

Thanks for making that more clear for me.

The relay is drawn once, but it has two parts, the coil and the contacts.

A bridge rectifier would be more efficient but requires more parts.  The single diode will output the same voltage but as a series of pulses instead of a constant level.  The capacitor smoothes out the pulses into a DC level. 

I used a 1.1k resistor, instead of the 1k. Also used a 100uF 160v capacitor. How will this effect the circuit?

I just measured voltage from line 1 of the transformer, and then to both sides of the resistor. The unmodified reading was 21.4vac, and the reading across the resistor was 21.1vac. Does that sound right to you? If so, why do we need a resistor at all for such a small voltage change? Just wondering cause I thought the voltage change was gonna be a lot more.


Very good question.  The reason the 1k resistor is there is because on my thermostat, without the resistor the relay would get stuck on never shut off.  Not all thermostats are the same, so you can try removing it on yours and see what happens...

21VAC should be fine, as long as everything works, don't worry about it.

Sorry, I didn't mean to bother you. Just wanted to learn a little bit about how/why this circuit works.

I've heard of voltage drops with resistors. I looked it up on wikipedia. It gave me a lot of info, but nothing to tell me how a resistor works. Like "What does 1.1 K mean?" "What does 1/2 w mean?" If I use different values how does it change the overal circuit?

You are right. If what I have works, then I shouldn't be asking any questions then, right?

Again, don't mean to be a bother.

1.1K means 1.1 kilo ohms, or 1100 ohms.  An ohm is a measure of resistance, that is, a way to describe how the resistor affects (slows down) the flow of current through itself.

1/2W is 0.5 watts.  A watt is a unit of power.  The resistor turns some of the electrical energy flowing through it into heat, and the wattage rating describes how much power the resistor can handle before getting too hot and burning up.

The reason the resistor is there is to allow current to flow in both directions through the triac switch inside the thermostat.  If the resistor wasn't there (or was too large), then the diode in the schematic would only allow current to flow in one direction through the triac, keeping it from turning off.  If the resistor was too small, the output of the thermostat would get shorted and the relay would never turn on.  I chose 1000 ohms because it's a good middle of the road value (a guess), and it seems to work ok so I never changed it.

If you want to learn more (for example, what a triac is), here's a link to my favorite beginners guide to electronics:

I highly recommend that you pick up a copy and start reading.  There are many resources for learning electronics online and in print, but I think Forrest Mims' book is a good place to start.

This is a great instructable. It's easy to build, the parts are not expensive, and it helps a person save power, and therefore save money. It's great that you are so helpful when a person trying this runs into a snag. Only weak point is that the schematic is not very clear where it comes to the relay, but then you explained in the comments.

I gave you a 5 Best Instructable Ever.

Just wondering though, if you (mightyohm) is the author, why does it list P914 as the author?

Thank you for the great instructable, and your help with it.

1 reply

Long story short, I wrote this instructable a very long time ago under a different username.  I don't use p914 anymore and use mightyohm instead.