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Re: Interfacing to AC; lamps, relays

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 06:01 PM 10/30/2005, you wrote: 
Original poster: Davetracer@xxxxxxx

I would like to ask for some help in interfacing a microprocessor to some unusual parts. Well, unusual for me, anyway.

I was very impressed with the "singing Tesla coil". I would like to try a few things along this line.

What I would like to do is interface an 8255 PPI chips (essentially, three 8 bit I/O ports, programmable) to several things:

A: 24 volt AC relays. I am especially concerned about the inductive "kick" that's going to happen when an energized relay opens. (If necessary I can set these up for DC, but I happen to have a lot of these relays at this voltage).

    B: 6 volt incandescent light bulbs (e.g., pretty much flashlight bulbs).

    I could (and probably should) change these out for LED's.

I need to go both ways; e.g., turn on the relay, and also sense whether or not it is on. Same for the light bulb; turn it on, and also sense if it is on.

    I'm thinking that perhaps optocouplers might be a good idea.

If there is good information online, I'd appreciate you pointing me to it!

    Thank you,

    Dave Small

p.s. Whatever we do, I'm going to have to repeat it 24 times per 8255 chip, and there will be several 8255's, so hopefully it'll be nice, simple, and very robust.

I would suggest that unless you particularly like 8255s, you take a look at the offerings from Allegro. They have a 32 bit relay driver with a 3 wire serial input (clock, data, latch).

You might also look at standard breakout panels using the little modular interfaces from Opto22 and so forth. Most of the data acquisition companies (Data I/O, Measurement Computing, National Instruments, Graybar, etc.) have them in one form or another, and by the time you buy and wire up your own relays (even surplus), it's pretty hard to beat the prebuilt panels.