Tweet-A-Watt Power Monitor modification

Here is an existing project that uses Xbee + P3 Kill A Watt meter:

My idea is to utilize a Photon instead of the Xbee.

I am a software developer, not an electrical engineer… could someone help with me with a wiring diagram (
resistors, capacitor, diodes, etc ) to the LM2902 ? It looks like only pins 1, 4, 11, and 14 are used.

( )
Pin 1: Output1
Pin 4: +
Pin 11: -/Ground
Pin 14: Output4

@mores, can you be a little specific and perhaps include a link to the schematic where the LM2902 is used?

They have a link to the original project.

It looks like the 4K7 and 10K divider drops the 5V analog to 3V3, so that’s fine for a Particle, but the supply voltage wasn’t enough for 50mA. Particle probably peaks well over 300mA, so you’ll need a separate supply, but a common ground between the two.

@CuriousTech, the OP’s question was about how to wire “the LM2902” which is quad op-amp which I don’t see anywhere in the article. This is why I asked for a link. I suspect the op-amp is used a simple follower after each voltage resistor divider but without any reference, it is hard to:

Step #9 says:

Now, the fun part — we’ll fillet, stuff, and reassemble the Kill A Watt!Open the Kill A Watt by removing the 3 screws. Be careful not to damage the ribbon cable holding the sides together!NOTE
: If your Kill A Watt looks a little different from this photo, that’s
OK. The innards are the same. You may just need longer wires to place
the supercap.Now it’s time to jack into the sensor output! Melt
a bit of solder to “tin” the ends of your power, ground, and sensor
wires from the XBee board, then tin pins 1, 4, 11, and 14 of the LM2902
op-amp chip inside the Kill A Watt. Connect power to pin 4, ground to
pin 11, analog in AD0 to pin 14, and AD4 to pin 1.

@mores, OMG I must be going blind (advanced age = more caffeine)! Then @CuriousTech is dead on with his suggestions.

The recommended supply for the Photon is 600ma though I have used a 500ma supply without problems. However, I recommend a high quality supply, not just a cheap phone charger (if you power via USB). Often these will not supply their rated current and can cause the Photon regulator to oscillate and possibly self-destruct. So if you power via USB or Vin at 5V or via the 3V3 pin at 3.3v, make sure the power supply you use is solid. :wink:

It wasn’t making sense to me either. That’s why I went to the other link. The first one was “put this wire here and that wire there.” :confused:

1 Like