Driving those pesky indicators.

Well here we are again, and I am having to drive multiple of voltages for my indicator lamps…. This can be a pain in the rear… Let me see I have  5 volts, 12 volts, 28 volts. Take your pick..

You have several choices:

1. Do nothing, just buy lots of power supplies.
2. Change out all the lights to one voltage. (big bucks)
3. Something else maybe?

Me I’m out there anyway so I’m going with you guessed it: ‘C’.

The Analysis: First a little background…
Rule #1:  Computers Logic (TTL) does not like voltages over 5 volts dc.
So how do you drive a 28 volt lamp from a computer?  There are several ways..

1. Relays..(ADVANTECH PCLD-785B with 24 relays) This card retails for $240.00. Or you can find them used for about $50.00 on ebay. This tends to be some what costly as you have to buy the boards and then a driver for them. It’s more than little kludgy with tons of wire screwed down, on barrier strips. But it does work and its bullet prof. But remember all those power supplies.
2. HEX INVERTER BUFFERS/DRIVERS WITH OPEN-COLLECTOR HIGH-VOLTAGE OUTPUTS SN5406 and SN7406 have minimum breakdown voltages of 30 Volts DC. But when they fail it could put raw 28 volts on your computer! Ouch!
3. Optoisolator I like a little 6 pin dip a TIL111 will handle 0 to 30volts DC with TTL inputs.  If  you want you can buy them at Digi-Key for about 18 cents each, and if you look around you can buy them cheaper else where.   Add a few parts for decoding and latch, you can drive all of your lamps.

So lets see 18 cents x 64 optoisolators is less then $12.00. This means that with just 16 cards you could drive 1024 lamps…  But wait a minute I said DC right? (Who said we had to drive the lamps with AC). Now that we have DC you can easily create a divider or regulator for you lamps from one 28 volt power supply.