Good day all! Long time lurker / reader but first time posting…
I am working on monitoring energy usage in my shop / makerspace. I have a heater that is triggered by a mechanical thermostat - essentially when the thermostat triggers the heater to turn on, it simply shorts the two wires which tells the heater that it should turn on. There is no voltage on the two wires.
I’m trying to think of a way to have an Arduino that those two wires were “shorted” by the thermostat to call for heat.
The challenge is that I can’t send voltage over those wires so I can’t use an output pin to send voltage to an input pin to read that the circuit is closed…
If that were true then no current would flow whether the contacts were open or closed so shorting them would do nothing
How about switching a relay with 2 sets of contacts in place of the heater. One set would turn on the heater and the second, separate set would trigger the Arduino to do something
What type of thermostat is it? An electronic one that runs on battery or its own power supply? If so, it may put out a voltage when heat is called for. What is that voltage?
If it is a simple mechanical thermostat, then there is no way for the heater to sense whether the thermostat is calling for heat, unless it puts some voltage on those wires.
To further clarify - if I use a piece of wire to short the terminals on the heater, the heater will turn on.
So pretty much I need to do a continuity test on the two wires…
I will open up the heater and see what type of relay is being triggered when heat is called for - perhaps I can pull voltage from there to sense when it is running instead of doing a continuity test.
It is indeed a simple mechanical thermostat. I would need to do a continuity test between the two wires.
I’m going to look at the relay that is triggered when the heater turns on, but may just build an Arduino thermostat to replace the old mechanical one (the mechanical thermostat is old enough to have a mercury bulb in it lol)
I will check again - you are right / has to be something there otherwise it couldn’t trigger a relay or do anything.
Thanks for all the quick replies guys - will look at it again and see if there is low voltage coming off one of the wires - hadn’t considered that something has to be there - no voltage = no action so will look again today.
i've been considering using a sense transformer to sense that a current is flowing thru the wires. they are used for model RR block detection where the track voltage reverses polarity every ~100 usec. a pulse from the transformer can drive a transistor to discharge a capacitor.
an alternative is to measure the voltage from the switch, which i believe is AC, so thru a diode charging a capacitor
Also measure the current that flows between the thermostat terminals when the heater is on. In other words use your multimeter in current measuring mode to short the terminals instead of that piece of wire. As always when using current mode with your multimeter, start on the highest range, then go down the ranges until you get non-zero measurement.
Hopefully that current will be no more than a few mA.
If the current is low and the voltage is low, you could use an opto-isolator to trigger the heater, instead of a relay.