Got a nifty little program to create simple schematics. Before I was using Paint. Never really liked the higher end programs like Eagle.
Anyway, This is a small circuit using Opto Couplers for isolation from the parallel port.
Didn’t have a Opto Coupler template so I just used a LED, NPN transistor and a rectangle
Lets talk about the circuit real quick. Since the Parallel Port tosses out a digital voltage (IE a low voltage/current Square Wave) it can power a LED just fine and with out a current limiting resistor. You could add resistors to the circuit if you wish but with my testing with different rated LEDs it works just fine with out them. If you plan to use resistors then a 220 ohm should work. The Opto Coupler is just a LED and Transistor with a photocell on the base. Basically it does a priezo effect to conduct enough current to turn on the base of the transistor. From there the Emitter is grounded and the Collector acts like a virtual ground. From the Virtual ground you connect the negative side of your device/circuit. In all it acts like a solid state switch.
Connect all of the negative side of the LEDs of the Opto Coupler to the Parallel port grounds, pins 18 to 25 are ground. With the Transistor Emitters shown in red go to the external power supply’s ground.
Here is another circuit that uses a R-2R ladder.
Yay! I figured out how to make the runs smaller.
I would recommend on watching the video from the last post on the R-2R ladder. In a nut shell it is a Digital to Analog converter. This circuit has a Op Amp that is acting as a voltage follower and giving isolation to the circuit. Be sure on the ground to the second resistor on D3 to use the ground to the Parallel Port pins 18 to 25. The Op Amp doesn’t have to be a LM358, any general purpose rail to rail Op Amp will work. You can use a external power supply for this but I would stick with a 9v battery to power the circuit.