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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.

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    \$\begingroup\$ This question is all a bit random, we can't see what you have. Can you post any schematics? Start with the power source. Simplify the question \$\endgroup\$
    – Icy
    Nov 18, 2015 at 14:31
  • \$\begingroup\$ Attach some pot as a load to draw ~100µA and then slowly wind it down until the voltage becomes unstable and calculate the current, then add a bit of headroom and you have what the PSU can supply. \$\endgroup\$
    – PlasmaHH
    Nov 18, 2015 at 15:08
  • \$\begingroup\$ I'm not really sure what the best way to simplify it would be. I guess the base question is: how do I find out what the power supply is capable of? This is some very new stuff for me, so I don't know what the best questions to ask are. \$\endgroup\$
    – Geek42
    Nov 18, 2015 at 18:15

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The ATmega169P runs on 330µA (that's micro amperes.) So, 0.33mA.

The Trinket uses a ATtiny85 that runs on 300µA. It also, however, has an LED that turns on when it is running. The Trinket schematic shows the LED connected through a 470Ohm resistor. Given a 2Volt forward voltage for the green LED, and powered from 3.3Volts, you get about 3mA for just the LED. That's a whole order of magnitude more than the original processor (ATmega169P.)

It is entirely possible that the high voltage side supplies very little current to the low voltage side since the original processor didn't need much current. Your Trinket pulls 10 times as much current, and so may be just too much for the power supply.

You might try removing the series resistor from the LED on the Trinket (open the circuit to the LED so it doesn't light up.) That should greatly reduce the current, and may get things running.


I've answered what I think is the meat of your question. Let me know if I've guessed wrong, and please do consider simplifying the question.

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  • \$\begingroup\$ I will give that a try and see what comes of it. There is an active LED on the original board along with the ATmega, it is used as a sidelight on the small LCD display panel. I will see if I can figure out the draw it uses, but I suspect it's still very low draw. I think it's safe to bypass the LED resistor with power this low. I did run an LED with resistor by itself on the power supply to see if anything odd happened(cannot remember the resistance, I think it was a 330 Ohm) and it ran fine by itself. Thanks \$\endgroup\$
    – Geek42
    Nov 18, 2015 at 18:19
  • \$\begingroup\$ DO NOT bypass the resistor. Remove it to open the circuit so no current flows through the LED. \$\endgroup\$
    – JRE
    Nov 19, 2015 at 8:08
  • \$\begingroup\$ Yeah, I just grabbed one of my ATTiny85 chips straight from my parts bin and used that without using the Trinket. I was using the trinket to get the voltage regulator that it has onboard into the circuit. With the ATTiny programmed to just set a pin high or low every second, it would run and then reboot. After testing a bunch of situations it still did not work properly, and I'm sure the draw is too much, it would turn on the load and then reset the chip. Looks like I won't be able to do what I want, that low power will not run WiFi as my plan was to do this. \$\endgroup\$
    – Geek42
    Nov 19, 2015 at 13:51

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