I am working with a baseboard thermostat to replace the low voltage control side with some custom controls. The thermostat is divided into two sides, a high voltage board and a low voltage board. There is a 4 pin connector(with only 3 used) between the two that is labeled to have ground, power and control(triggers the load when high). The chip on the low voltage board is an ATmega169P.
Right now, I can control the load(in my test setup, not an actual baseboard) from an external controller if I power it from an external power source(ie USB) and connect the 3 wires to the controller as well. Without the external power, the controller I'm using(just a Adafruit Trinket for now, not my final plan) does not boot up - it's power light blinks dimly and that's it.
I don't have any great scope or similar for checking things, only a multimeter and probes. When I check the voltage across power-ground I get 6.8V, probing at the original ATmega169 power lines get's me 3.3V(as it should) after a on circuit power regulator. If I run the 6.8V through a LV7805 I get a solid 5V there. All this makes sense, but once I put a load on the power coming from that circuit, it fails to keep the voltage up.
Connected to the Trinket, and running through the onboard regulator, I only see 3.0V on the other side of the regulator. If I build a quick and dirty voltage divider and measure it's output(setup to divide in half), I get 1.43V.
At this point I'm well past my knowledge level. I did not expect this part of my project to be what messed with me. A couple of theories that I don't know how to test: Is the output really low amperage? How can I see what the max output on this power source is? Is the voltage changing heavily? I don't see any fluctuation on the meter. Is there something odd with the power source coming from the high voltage side due to it only being a two wire thermostat? The design works with the existing board, but I don't know how it deals with the fact that it only connects inline on the hot wire, no connection at all to neutral or ground. I assume it's allowing some current through all the time to get power for the main processor.
As I try to figure this out, I'm looking for pointers to help move it forward, and I've either not found what I want online, or I don't know the proper stuff to search for. Suggestions on what to try are appreciated, I do want to understand what I'm working with better over time.