View Full Version : Needed: tach design for pc serial port and optical isolator for parallel port

09-10-2005, 06:31 PM
I saw a simple design the other day for tach to be connected to the serial port on a pc, now I can not find it. I don't remember if it was a reflective or interupt type, either would be okay.

Also I have been searching several websites including Fairchild trying to find a 5v optical isolator to be used with the parallel port of a pc. I have not been able to find one that is designed for 5v operation on both input and output.

Thanks, John

09-10-2005, 08:34 PM
Cheap or quick?

Quick = a Omron photo-emitter with darlington array amplifier inside. Take that through a Smidtt trigger or direct to the port. I think the lil horseshoe deal is about $30 Digikey is the one source, they are more than $65 at A?? what is the big electronic book? gone braindead.. Easy to hook up and it will last for years.

Cheap? a emitter.detector pair about a dollar. Use a transistor like a 3904 to amplify.

Your mileage may vary. What are you tring to control? I may have one.. I have a speed-control with digital setpoint w/10volt output. I bought it for my mill. BUt never really needed to know or control exact rpm.
I have a dozen opto22 boards left if you want to isolate it, I am getting short on the high speed modules. They have a ttl side, and a non-optical side you can put a 120vac module in, a dc module, or?? it sends or recieves 5vdc ttl on the left isolated.

Some autos, had optical ignition systems.. Not sure.. I had a ford magnetic unit on a harley w/555 timer to multi-spark.. It was a wad of wires you would not believe. I looked at that mess and went and bought a factory ignition.


[This message has been edited by ibewgypsie (edited 09-10-2005).]

09-10-2005, 08:39 PM
I'm not sure I follow on the opto isolator question. The input side is a LED and the output side is a photo transistor. For the input side you supply current with a current limit resistor to V+ of a size to keep the current within spec, say 5 ma, which would be a 1k pullup resistor with a 5v V+. This is within the capability of a TTL output to sink when driving the input.

The output side is connected with emitter to gnd and collector to input pin. If the input doesn't have a pullup then a 10k pullup to V+ from the collector will do.

If you are concerned about the spec on a opto coupler "maximum emitter voltage or forward voltage [meaning the LED]" don't worry about it. It is simply the voltage drop across the LED and will not be exceeded as long as the maximum current isn't.

Just about any opto coupler will do including gap style for encoders.


A Fairchild 4N35 will do.


[This message has been edited by Evan (edited 09-10-2005).]

09-10-2005, 08:54 PM

Here's a whole page of them from Jameco:


Should be able to configure many of these to work with a 5 VDC supply. Look through the specs on some of them - some even advertise TTL compatability. Pick pairs that will switch fast enough for what you are doing.

Hope this helps.