Low cost serial port to relay controller?


Thread Starter

George Marshall

I have an application where I need to control a single low-voltage relay from a PC serial port. This is quite cost-sensitive, so am trying to find a single channel (or closest approximation) controller, as 8 or 16 channels is overkill. Any
suggestions on vendor/model?

Serial port is ideal, USB or parallel port might work. Bonus points for a unit that can be powered from the PC.


Use Linux!

Oh, no, wait, that wasn't the point of the question. Got carried away, sorry.

I'd recommend Weeder Technologies ("www.weedtech.com":http://www.weedtech.com ) modules. They do daisy-chain RS-232 I/O modules and have been around a long time. $60 to $70 USD for their 5-point module.

I think for a supremely cheap module with just one point you'd have to build your own, probably with a PICMicro controller..... then the actual cost goes up for low volumes, so you'll have to weigh the buy/build decision.

Roland Lilja

Hi George,
The simplest way of doing it is to use one of the "Handshaking" signals in the serial port. For instance RTS (Request To Send). You need to make a "booster" circuit(1 transistor 1 diod and maybee som resistors) depending on witch voltage the relay works on. An alternative is to use a optocoupler, since the RTS e.t.c. has the power to drive it already.

Hope this vill help you,
Roland Lilja

Michael R. Batchelor

Just how cheap do you want? If your relay is small enough and has a 12VDC coil you can just drive the thing by setting the RTS line high or low on the port. Can't remember off the top of my head but I think you can do this just by setting the port's open property to true or false. If the coil draws too much current for the serial port to supply directly or needs 24VDC then use a cheap op amp to drive it.

Have you considered just installing a relay pc card that plugs into a pci or isa slot as an alternative. Advantech makes a board like this, and many others also offer this type of board. The boards are easily controlled via VB, or other languages and the plus side is they are powered by the pc as you desire
> Use Linux!

Speaking of linux, where can I get a few pointers on making the Weedtech relay board go under linux?

(I'd think it isn't that complicated, but still a kick in the right direction would be much appreciated.)

--Craig Meyer
Seattle WA

Scott Benninghoff

> Well, I have a diagram and the software if you would like.

I would like to see it. Thanks in advance.

Scott (scottben(AT)tampabay.rr.com)
Ben, I'm interested in seeing your schematics and software. Trying to operate cash drawer via delltouch programable keyboard button.