Help with relay/pulse circuit for wood pellet stove automation

[LaurB]

Hi all,

I am looking into building a circuit to turn a wood pellet stove on or off by making use of an interface on its rear that is intended by the manufacturer to be connected to a domotic (x10 type) home automation module. This interface consists of two pins which when momentarily shorted will turn the stove off (when it is currently on) or on (when it is currently off). Rather than invest in a home automation system I am considering using the existing central heating programmer (which at present is doing little for me other than turning the room thermostat on of off as it is unable to turn the stove on or off itself - this is currently done by a built in timer within the stove which is too inflexible for my needs).

My initial thoughts are to use a relay on the 240v ac output from the heating programmer in order to be able to provide a low voltage trigger to the circuit (from my limited electronics knowledge I guess this might involve a 555 timer?). The heating programmer provides a permanent live whenever its clock says the heating should be on, so I'd need to be able to generate a pulse whenever the heating programmer's output changes from 0v to 240v or from 240v to 0v.

So, the circuit would operate as follows:

1. Central heating programmer output changes from 0v to 240v (and remains at 240v)
2. Relay closes
3. 555 timer momentarily shorts stove pins and stove turns on
4. Later, heating programmer output changes from 240v to 0v (and remains at 0v)
5. Relay opens
6. 555 timer momentarily shorts stove pins and stove turns off

Am I on the right track here, and could anyone please give me a hand to put a schematic together? One concern I have is that if the stove received a spurious pulse it would become out of sync with the programmer and turn itself on when the programmer thinks it should be off, and vice versa! Any help or advice would be gratefully received

 

[JonBoy470]

Honestly, the easiest way to make this happen would be something like an Arduino. You'd use one pin as an input via the relay, and one pin as an output to close a relay to short the control terminals on the stove. The output might drive the relay via an emitter follower depending on the current requirements of the control relay's coil. Some minimal amount of code would be needed of course to drive the control logic...

 

[LaurB]

Thanks JonBoy, that sounds like an excellent idea. I'd not heard of the Arduino devices but I had wondered about using a Raspberry PI. From what I've just been reading, the Arduino looks perfect. Thanks very much for your help

 

[JonBoy470]

The Raspberry Pi would also work in this application, but is arguably overkill. It is also an immature platform for "physical computing" compared to Arduino.

The Arduino is easy and inexpensive to get started with. The integrated development environment (also called Arduino) is a free download for Windows, Mac or Linux, and allows programming the board in a C-like language based on the Wiring language (you can use straight C if you desire). You write a "sketch" on your PC, then download it to the Arduino board via USB. The Arduino board can also communicate with the PC via USB while running.