Re: [CAD_CAM_EDM_DRO] Opto Isolation ??

Hugh Currin wrote:

> I've been thinking it would be good to
> opto-isolate
> the computer from the drive circuits, either to be conservative or
> because I'm
> paranoid. I think these circuits are very simple but I'm
> electronically
> challenged.
> I searched around the net and found some information but not quite the
> circuits
> I need.

These circuits might work. But, they might not. The darlington
transistor has two
Vbe voltage drops in series when conducting, so it can not bring the
output down
much below 1.4 V! This may cause the circuit you are driving to not
recognize
that as a logic low level. You can check it with a voltmeter.

The 1 K resistor can't deliver more than 5 mA to the LED, and because of
weak
pullup on the parallel port, probably a LOT less. You will likely get
no more than
1 mA through the LED, which is probably nowhere near enough to saturate
the darlington.

The input circuit might work better, but again, you will need more
current
through the LED. I recommend a resistor of about 360 Ohms, to get
about 10 mA through the LED. This is for driving it with a 5 V supply
and a switch. For driving the LED with a logic signal, you need even
lower resistance. For the parallel port output, it may also work better

to tie the LED to +5 V, and then have the port pull the cathode low
through the resistor. This requires the software to know to pull the
port bit low to turn on the LED. But, the port may be able to sink
much more current that it sources.

Jon