Interface Relays for low current TTL output

  • Thread starter Donald C. Johnson
  • Start date

Thread Starter

Donald C. Johnson


I am in need of an interface relay compatible with a low current (10mA) 5V output. My ultimate goal is to drive a dry contact that will allow me
to interface to a PLC input. (without building a
circuit board!)


It may not be pretty but you can accomplish this by soldering together a switching transistor and a couple of resistors from Radio Shack. I use this method when I need to pick up one or two digital outputs from a PC serial port. Snag one of those little transistor notebooks while you're at Radio Shack. It has several examples of switching circuits with very hi input impedance.

Steve Myres, PE

That should be adequate to operate a hockey puck style SSR from Crydom, Continental, Idec, etc. So you will not need any additional DC supply, and the AC contact output is optically isolated from your control voltage. Expect to pay $30-$60 depending on the amount of current you want the output to handle.

Curt Wuollet

At that current level, I would look at a packaged optoisolator. The relays available tend to be PC mount or at least not socketed. SSR's might be an option as well. Check with Opto22 and the DAQ board vendors. They would stock TTL compatible relays and stuff.


You can not avoid a circuit board. If your requirement is a single output, the suggested information by others will be the best way. If no isolation is required and you have multiple 5 v dc outputs, then a TTL to higher voltage interface IC like ULN 2000 series is the best way to go.

Best regards,

Lynn August Linse

Check out the Crydom DO061 at - sells them for about $8 each. They'll allow 15mA 5v control to safely drive 10-60vdc load (ie: a 24vdc PLC input). They also have an AC version with same control input. They are 4-pin SIL for PCB mount, but I suppose you could wire-wrap/solder leads onto it and seal it into a cable shell or something. I've had good luck with these on PCB.

Best Regards
- Lynn

Alan Hartwell

If you are determined to have a dry contact output, there are relays that can be driven directly by logic level 5 volts at low current. The TYCO / Potter & Brumfield R10S sensitive relay series come to mind. You can get a socket to have screw terminals; no circuit board needed. I would suggest a reverse-biased diode across the coil to damp the inductive kick.