relay to control 18v DC drill

I am trying to use an arduino to control a relay which will connect 18V DC to a cordless drill. I converted a charger into a battery dock and gutted an old battery pack to allow power into the drill via a couple of connectors. The idea is to physically pin the trigger of the drill down the turn it on and off via a relay in between the drill and power source.

I have a cheap eBay solid state relay designed for AC/20 amp and DC 3-30VDC input. If I put it inline and apply power to the relay then pull the trigger on the drill it works great. If the trigger is already pulled on the drill when the relay is powered however it does not work.

Is this due to the relay being designed for AC?

Can anyone suggest a way to resolve this or a relay that would work?

Thanks :-)

Is the relay switching power to the drill? (check with a multimeter).

If so, then it could be the drill has a safety feature that prevents the drill turning on when the trigger is already pulled when power is applied.

woodc: Is this due to the relay being designed for AC?

Probably. The SSR is probably a zero-crossing type, which only turns on when the voltage across it is close to zero. The other problem you may find is that the relay won't turn off.

If you need isolation between the Arduino and the drill power supply, get an SSR designed to switch DC. If you don't need isolation, you can use a logic-level mosfet instead. In either case, connect a flyback diode across the drill motor.

I believe the drill is not the culprit since you can insert a battery while the trigger is depressed and it fires right up :slight_smile:

Thanks for the info…I will try to find a good source for a DC SSR that can handle some decent amperage. Would the diode be a good idea still if the SSR is opto-isolated?

woodc: Would the diode be a good idea still if the SSR is opto-isolated?

Yes, although if the SSR contains a snubber network then the diode may be optional.

You could use a brushed motor ESC and effectively replace the whole control circuit for the drill. The power connector connects to your battery, the signal (white) ONLY to arduino and the output to the drills power drive. Gets rid of the whole power activation circuit.

It works with standard remote control output signals, easily replicated with arduino. Just a thought. :)