Electronics Question - Parallel Relay

**blazinlow** · 11-07-2004, 12:41 PM

Hey guys,

Im trying to build a very simple single relay card for just an experiment using this design:

I have built the circuit but I am having some problems with it- its not working!!

From what I can tell from this schematic, the + pin 2 on the parallel triggers the transistor, feeding ground from pin 18 to the relay, activiating it. I have a 12V source on the other side of the relay coil.

I am getting 3.3V coming off pin #2 on the parallel port. Ive used a multimeter on various parts of the ciruit to try to figure out why its not working.

Also, Im using a reed relay if that matters.

When as soon as I hook the 12V source to the relay coil, it trips - i get continuity between the in and out on the relay. When I put my + probe on the 12V source, and - probe on the ground, i show about 7V, which I dont understand becaue the parallel port is set to off. This is why the relay is engaged.

I cant figure this out, Im about to give up, so if anybody can throw me some ideas that would be great. Thanks!

**brooksware2000** · 11-07-2004, 01:39 PM

1. Do you have pins 18-25 on the parallel port grounded to the ground of your transistor?
2. Are you making sure that the port is addressed correctly i.e. 0x0378 for port 1?
3. What voltage are you using to energize the relay coil?

Take a look at my site in the signature and go to the projects section. There is an example there of a relay controlled H-bridge. Just use half of that design and it should work.