I need to monitor the coil side of an SSR (not actually a coil I know). The problem with my circuit is it allows enough current through the resistor divider to 0V to actually turn on the SSR. Just wondering if anyone has a better way. I initially designed it thinking the SSR circuit would have been PNP.
Wire the LED of a 6N137 in series with the SSR input (most SSRs draw no more than 10mA for 3-32V in, which you should verify).
Add a pullup resistor on the 6N137 output, plus power and ground.
Done.
You could also do this slightly cheaper by wiring a PNP transistor base in series with the high side (with a parallel resistor of perhaps 1K) and keep your divider. The optocoupler gives more isolation from EMI coupled back through the SSR.
Edit:
If you can't open the circuit, you can just increase the resistors by 100:1 (say 2M and 500K), add a 10nF cap to ground and use a 2N7000 transistor to buffer the voltage. Input to the ATTiny85 alone would actually work too (typical <50nA leakage), typically, but 1uA max leakage is too much to absolutely guarantee it. 2N7000 is 100nA max.
If you have access to both sides of the SSR you could just parallel the SSR input with a resistor but it sounds like that's another unstated limitation.
