Once the COM and NO wires for the 5v pump are connected, the relay will just very briefly kick on for a few milliseconds, and reading the voltage of pin 0, it spikes up to ~~0.09 volts briefly.
Despite it being a relay module ive tried using NPN transistor, with ATtiny pin 0 controlling base and still nothing...
I had this exact same setup running with an Arduino nano (and wanted to get my Arduino back and was trying to swap it for an ATtiny) - micro USB in to Arduino, and wire connected to 5v Arduino pin
Power source is a 5v 1.5A output box connected directly to the wall.
If thats not enough info let me know and i can clean it up for a picture
I think you may have nailed it with this here... im just getting back to messing around with this and when I tested a separate power source for control side of the relay it worked fine, going to try running it over a capacitor now..
ATtiny13 20U
5v DC pump (working voltage 3.3-5V)
5VDC Relay Module
I tried the exact same setup with A) separate power sources, and B) a DC-DC stepdown (without actually stepping down the power, using it as a power source isolator) and both of those worked... Im assuming I need to isolate the power in some way just not sure exactly how?