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Reply to "Using an optocoupler to connect insulated rail to input pin of shift register"

Very true, John.  In my case, I use resistors on my LEDs that limit their draw to a few mA each and they are still plenty bright (though I had to sample a number of different LEDs to find high brightness at low current.)  For the 16-relay boards, I power the coils with a wall wart, so the shift register only needs to source or sink (I forget which) a few mA to close each relay.  I've confirmed this with a Fluke multimeter.

Without an external power source, the relay coils on those 16-relay boards draw about 30mA each.

If you were driving LEDs at the more conventional 20mA, and driving the relay boards without an external power supply, there could be serious problems.  It's a very important consideration!

When using the shift registers to monitor inputs going to ground, such as pushbuttons and relay contact closures (typically with a pull-up resistor), I don't think current sink/source limitations are a factor - at least not as I have been using them.

Randy

OGR Publishing, Inc., 1310 Eastside Centre Ct, Suite 6, Mountain Home, AR 72653
800-980-OGRR (6477)
www.ogaugerr.com

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