millivolt thermostat interface

[Ttelmah]

Yes.
I must admit, it does sound 'weird'. The existing thermostat is doing what I'd expect for a millivolt interface device, using simple switching, yet the manufacturers seem to be talking about a voltage source device.

One thought. You have tested that the thermostat gives a contact closure output. Have you tested this "on it's own", disconected from the boiler, with a _voltmeter_ on the output?. If the boiler is actualy built to take a voltage signal, using this as a detection for temperature, but with a simple 'on off' switching occuring at a particular voltage, then this could be simulated, by a contact closure, with a resistor presenting a voltage output, which is turned on/off by the relay. This would increase the battery consumption a little, but might be what is happening...

Best Wishes

[jaymiller]

Dan(the OP). If a read, and reread your original post I think you're trying to send 'somewhere' the woodstove's temperature using the millivolt thermopile as the 'temperatur sensor'. The info about the millivolt stat is not necessary.
IE..
millivolt sensor---PIC---433xtmr......433rcvr....PIC...Display

If this is true, you'll need a load resistor for the milivolt sensor to 'dump into' and use the ADC of the PIC to measure it.The PIC then calculates the temperature and transmits it to the remote receiver.

The mfr of the millivolt sensor or Google' should be able to provide the data you need...or just start a fire and record heat vs output voltage of the millivolt pile.Can also put it in a pot of water and heat it up..

You can even use the millivolt pile for the power to the PIC circuit using a smps boost circuit.

Jay

[dan king]

I decided to probe around the stove control board a little and found some interesting things. When the stove is set to use an external thermostat the terminals to connect to measure 5V.dc across them. From this I think I know what is happening. I think one terminal is connected to a pull up resistor and then sensed by the on board Atmel Microcontroller and the other terminal is ground. This would then explain why a contact closure indicates a call for heat and may explain the millivolt spec. with the series pullup resistor, a thermostat that "steals" power from the connection would be limited by this resistance. Since I'm not stealing any power, my thermostat is self powered, then it should work fine.

I now have a separate issue, the 434Mhz receiver I'm using is being drowned out by 60Hz noise as soon as I connected my receive board to the stove control. My electronics had all been mounted and working perfectly until this final connection was made. I confirmed this on a digital storage scope monitoring the receive packets presented on the output of the receiver board.

Does anyone know if I can implement a high pass filter on the antenna side of a rf receiver? If I can't remove this "noise" I'll need to change to another receiver module.

Thanks for all the input.

Dan

[dan king]

Ok, I got the wireless link to stabilize and ignore the 60Hz noise. All I have left is to test the tstat function of turning the stove on and off. Thanks all for your input.

Rgds,
Dan

[adrian]

[fuji0010]

Millivoltage System
These systems are not very common in a home as they are used often in direct /top vent wall furnaces. Thermostats that run on a millivoltage system rely on very low voltage, usually 750 millivolts (mV) or 0.75 volts. They do not require a step down transformer reducing line voltage of 120 volts to 24 volts as do low voltage systems. Instead they are self powered by a special thermocouple device called a powerpile or thermopile generator that produces direct current (DC). The thermopile is a device with thermocouples connected in series to increase the millivoltage output to 750mV.

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programmable Thermostats

[fuji0030]

All the appellation implies in this context, is that the voltage the thermostat has to control, is a low voltage signal, with low accepted abounding as well. As such you can about-face this with things like reed relays, which would not handle the accustomed college ability for added systems. However the arrangement can appropriately able-bodied be switched by abounding (most) of the accepted top voltage relays as well, provided they are cocky powered. It is acclimated as a term, to analyze from 24v, or mains switched systems.
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reign santiago