It looks like connecting GND to GND of the spark and the - of the SSR board will work.
The ‘active’ state of the SSR minus terminal is that it is pulled down. This means in your circuit, the relay is not powered. The ‘not-active’ state that the pin is pulled to 5V through a resistor and LED. the LED can cause the pull-up to only reach 3V on the pin, but that is still enough to open the darling transistor on the relay board.
The downside here is that active on the SSR board means not-active on the relay board. On a digital actuator, you can set it to inverted. However, the fallback non-configured state, when no digital actuator is driving it could still mean that the relay coil is powered. That’s a risk and downside of using the SSR boards this way.
ok thanks for blowing up my brain here - take a min to digest.
Simple breakdown - are you telling me that the exp. board and power can potentially always be on? Cause the Relay boards can be wired to be (NO) normally open or (NC) normally closed if that needs to change. I just want to make sure cause the relays will power all my pumps, AC Unit, heater element etc. and I dont want them to be on all the time.
Also should the expectation be that the LED of the relay board should always be ON or only ON when they are engaged?
I doubt ive said this enough - but thank you for all your guys help!
I should probably ask for a pen & paper sketch of what you mean (im visual) for a connection from the exp. board and from the Spark. I just want to make sure Im on the right page. Ill ask my electrician friends but that will take more time for sure.
The transistor on the relay board has a pulldown. You are connecting the output pin of the SSR board that has a pull-up. If the SSR board has the pull-down not activated, then the pull-up is stronger than the pull-down on the relay board and the transistor opens.
The SSR-board was designed to be minimal in components and just work with SSRs. When used as a digital output that should be driven high for an active state, it just doesn’t fit very well. I can’t make the situation any prettier.