Convert a temperature signal to a servo signal to control air to a wood stove?

[mnelson07860]

Could anyone suggest a cheap and easy way to control the air intake of a wood burning furnace using a servo? I would like to take a milliamp signal from a temperature sensor and convert it to a servo signal to throttle open and close an air control on the furnace using a servo to achieve a constant burn temperature. If temperature goes above a preset level then the servo would start to close the air control and vice versa.

[Soeren]

Hi,

Could anyone suggest a cheap and easy way to control the air intake of a wood burning furnace using a servo? I would like to take a milliamp signal from a temperature sensor and convert it to a servo signal to throttle open and close an air control on the furnace using a servo to achieve a constant burn temperature. If temperature goes above a preset level then the servo would start to close the air control and vice versa.

The fastest and cheapest way would be a small microcontroller with an A/D-C, but I don't know how easy you find that.

If 8 bits of A/D-C resolution, even a PIC10F220 (or PIC10F222) should suffice


If you need through hole components, you can get 10 bit A/D-C in eg. PIC12F675 or PIC12F683.

It could be made analog, but would require a larger circuit, much more work and a lot of experimenting and tuning and each change would be time consuming - compared to changing a line of code.

What you consider easy is not for me to say though, so you be the judge and decide whether you want to go with a small microcontroller based circuit or a larger analog circuit.

If you have the sensor installed already, what kind is it?
And what output does it give over the temperature range you need the stove to be within?

[mnelson07860]

I don't have the sensor yet. I was thinking of using the sensor in a electronic house thermostat and a servo from an old remote control car. Would that be possible? Thanks.

[Soeren]

Hi,

I don't have the sensor yet. I was thinking of using the sensor in a electronic house thermostat and a servo from an old remote control car. Would that be possible? Thanks.

A house thermostat is not a temperature sensor, but a temperature switch, so that won't do. placing a house thermostat in a place near the flames (I assume in the output going to the chimney or whatever) sounds a bit destructive as well.

You'll need a thermocouple, eg. a type-K thermocouple that stand up to 1300°C (a B-type TC handles up to 1800°C, but is probably harder to come by).
Unfortunately, this means that you need a thermocouple amplifier as well.

If a placement can be found that stays under ~850°C, a Platinum RTD can be used, but the placement need to be proportional to the actual temperature that  you want to keep.

If the servo is a real servo, rather than what's in many cheap R/C-cars, it should work, but you might also wanna consider a geared down DC motor.

[blackbeard]

I don't have the sensor yet. I was thinking of using the sensor in a electronic house thermostat and a servo from an old remote control car. Would that be possible? Thanks.

that's what i would do. for the servo there's 2 things you might want to consider.

A) using a CD drive. it's essentially a linear actuator for all intents and purposes and it's allot more disposable imo then an RC car. plus you wouldn't need to program anything and would simply need to replace the switch that triggers the opening and closing action with a transistor. if you wanted to scrap the circuitry in the drive all together then i could also share a simple logic latch circuit that fully opens and closes a CD drive door. it requires about $3.50 in chips if you can't steal them from something else (1x 7408, 1x 7404,  1x l293D (or 4 nchannel mosfets), a bunch 3904 transistors, MAYBE an op amp for what you're doing). essentially most CD drives use a mechanical switch to detect when the drive is fully open or fully closed and using a D latch you can detect the state it's in. by using transistors you can tell the motor to move if it detects that the drive is in a certain state and another variable is in another state.

B) buy a servo from DX. it's $4 free shipping! again much better then gutting an RC car imo. then go the root with that tiny ass pic