View Full Version : 3 or 5 volt Opto Isolation for Printer Port

08-30-2009, 12:23 PM
Its a good idea to Opto Isolate your PC or Laptop from the Stepper motor drivers and the vooltages on them. It is actually very simple to do using almost anyone of several Opto Isolator chips available on the market.

It is also easy to make the opto isolator function with either 5 volt (PC) or 3 or 3.3 volt (some Laptop's) Printer Ports, just by changing one resistor (R1) in each signal line.

The disconnectiion in the 0 volt line is important too, do not connect the 0 volts from one side to the other....then you have FULL isolation.

I would also suggest that the chips are socketed, so in the event of an accident, the chip can be replaced quickly and easily.

I have not shown any LEDs for indication, fit them yourself, but use small high power ones, that allow you to run with as large series resistance as possible, thereby reducing the loading on the signals....

Check the simple schematic attached. Any questions, just ask.

08-30-2009, 01:07 PM
My personal preference would not be to drive an opto direct off the parallel port.
The Opto shown rates the normal operation current at 60ma, that would exceed the normal current capability of most parallel ports.
Allowing for the diode volt drop it would appear that the opto would be operating around 8ma, this also would exceed the rated current for most parallel ports in source mode, but be on the margin for the opto?
If the parallel port is damaged it can prove costly, I would prefer to buffer it at least with a 2n7000 or ULN2003 or ULN2803.

08-30-2009, 02:48 PM
Although the circuit as I specified is in use in several places with no problems seen, I have to fully agree with your comments as I am sure that somewhere is a PP that cannot supply the currents needed.

The chip you mentioned would buffer nicely the signals concerned, a good choice.

I thank you most kindly for your knowledgable input!

Have a great day.

08-30-2009, 03:39 PM
In the days of printer ports that had identifiable TTL drivers, the recommendation was that if you had to drive a high current device like a LED, then it should be driven sunk rather than sourced.
Now will MB. LSI chips, it is almost impossible to identify what is actually driving the port and its drive characteristics.
Gone are the days when if you blow the port, just solder in a replacement port driver.

08-31-2009, 03:41 AM
Absolutely true.