Problem is I need a Normally-Closed DC Relay switched with 3vdc, rated for say 5 amps and 18volts. Enough to drive a fluid circulator pump by a small solar panel. Ideally it would also be a clean packaged unit suitable for a mechanical room.
You’d hook the VDD to 5v and the imp IO pin directly to the control input. This control input is connected to the gate of a 2N7000 FET (if his schematic is to be believed) and this device has a maximum Vgs(th) of 3v - ie the imp is guaranteed to be able to control this no problem.
If you look at the schematic for Sparkfun’s COM-11041 (there are several other similar examples), you can see the circuit is pretty simple. A couple of resistors, a transistor, a diode and the relay. You could fairly easily get a small breadboard and make the same circuit only using the NC contacts in lieu of the NO.