Optoisolator question

[farmallfan]

Hi all,

Newbie question for you... I've designed a controller board with 4N27 optoisolators between the parallel port leads and machine-side circuitry. I'm a software guy, and this hardware stuff is pretty new to me, so this question may do nothing other than show off my ignorance: I'm running motor-level voltage to the machine-side of the isolators. On the other side, for signals that are input to the parallel port (e.g. for limit switches), I know I can get a separate ground from pins 18-25, but where should I be getting positive power? Is it good enough to have separate grounds and share the positive voltage? (I am cutting the motor power down via a 7805 for logic.)

I appreciate any enlightenment you can send my way.

Many thanks,

- David

[abasir]

There should not be ANY electrical link between the input & output!

For the PC side, you can get the +5V from the power connecter in the PC (the red wire). For the motor side, you can use the output from the 7805.

Please note pin 18-25 are all connected to the same ground at the PC power supply. Do not connect any of them to the ground circuit on the motor side!

[NC Cams]

If you want to limit your 5v to being provided by the parallel port, simply tie all the "output" pins to a capacitor (say 100uf) THRU a bunch of 1N914 diodes. Use a diode array to save space/facilitate assembly.

Each time any output pin goes high, it will shoot 5v to the cap and the diode prevents voltage drain when pin goes low - been there, done that, works fine.

Probably should limit it with a zener. Otherwise, you'll need to hack into the PC to find a 5V source (wouldn't do it it it were my deal).

Other option is to provide remote 5V source from external mini DC transformer or 9v battery and 7805 regulator. As long as you tie negative of external P/S to same ground as pins 18-25, this will work as well.

[farmallfan]

Excellent! Thanks very much gents.

- David

[Al_The_Man]

As abasir said, if you need a 5v source referenced to PC side, any of the 4 pin power supply connectors should supply it. As this is what the parallel port is referenced to.
Al.

[Mariss Freimanis]

You can find 5VDC on:

1) Pin 1 of your game port (DB17 connector)
2) The red wire in a decapitated PS2 mouse cable
3) Any USB port

Connect a 220 Ohm resistor +5VDC, connect the opto-LED anode to the other end of the 220 Ohm resistor. Take the opto-LED cathode to the parallel port output pin of your choice.

Caveat: Many / most new PC parallel port outputs are 3.3V logic now. Toggle the port output pin, set a multimeter to DC Volts and measure the pin voltage. If you do not read at least 4V for a logic '1' ouput, you have a cursed 3.3V port.

If so, then do the following:

Connect a 470 Ohm resistor to the 220 Ohm / LED anode junction. Take the other end of the 470 Ohm resistor to parallel port ground. Then it will work with 3.3V logic as well as 5V logic.

Mariss

[farmallfan]

Ah...great suggestions -- I'm using a laptop, so I'm not particularly excited about hacking in to the internal power supply leads, but I hadn't thought about the USB port. That's a great idea.

Thanks,

- David

[lete002]

Hi Mariss
I know this is an old topic, but I want to ask you a question about your setup for 3.3-5V, compared to the one PMinMO uses in his opto board. I think yours is better, only not sure why.
The other setup is:
5V (USB port), anode of opto-LED, 470 Ohm resistor, pin of the parallel port. There is also another 2k2 resistor between the parallel port pin and 5V, I guess to avoid conduction of the opto-LED with the 1,7 V difference, in case is a 3.3V port.
Which one would you use?
Thanks

Rafael