Attaching a standard car relay to the Digital sidecar

I have a 40AMP car relay and wanted to know if i could hook it up to the digital side car through the ‘relay’ port. If i can how would i hook it up? It has a coil circuit (2 connections) and the switch itself(2 connectors).

Please post a datasheet (or link to) for the relay. My first guess is that the sidecar can’t directly provide sufficient current to drive the control coil since none of the outputs can drive more than about 10 milliamps.

You might, however, be able to use the Robot Signal Light output but I’m not sure whether the FRC-specific software allows you to directly control the RSL (since it’s primary function is for safety indication and debugging). Even if you could control the RSL output, I’d wager that there’s a fair amount of start-up code that would leave it blinking and twiddling while the cRIO is booting.

If you don’t mind sacrificing a sidecar since mods are illegal (per 2009 FRC rules) and you’d lose the RSL functionality, you could check out the schematics for the sidecar and rewire it to drive the RSL FET using a GPIO.

Another couple of options that wouldn’t break the sidecar would be to use a Spike to drive the control coil (a bit overkill since the Spike can almost drive your 40A load directly but it would work) or fabricate a little power switch that’s controlled with a GPIO from the sidecar to provide the 100ma(?) or so for the control coil.

An even better option, I think, would be to use the NI 9472 module and the Solenoid Breakout adapter since each of its 8 channels can source 3/4 amps of current. I’d be surprised if that “relay output” module couldn’t drive the relay’s coil.


It is almost identical to this one at radioshack.

also what is a Spike?

Spike = Spike relay module from IFI Robotics

Easy to use, bulletproof relay for upwards of 20A at 12VDC with an optically-isolated input structure that’s typically connected to something like the Relay Output lines from the sidecar.

If you don’t need a full 40A but 20A is sufficient, you might want to consider using a Spike.

If you really need a full 40A, I’d recommend using the NI 9472 “digital output” cRIO module to drive the coil in your relay. It can easily handle the 100ish milliamps of the relay drive coil.


The Digital Sidecar generally cannot drive a relay directly.

If this is for a non-FRC application, the easiest would be to use the Solenoid Driver module -these can source 750 mA per channel (2A max all channels), easily enough to drive a relay. For FRC, the 2009 rules stated these may only be used to drive pneumatic solenoids.