Matrix Orbital Corp

I have a BLK-202A, which has 6 low power GPOs. The manual says they're ground controlled, 20mA max with 240 ohm current limiting resistors.

I'm planning to use this (http://www.lcdforums.com/forums/viewtopic.php?t=1036) to connect two fans to my GPOs. Would it matter if I used 5.6k resistors instead of 4.7k?

This circuit will invert the behavior of the GPO. When GPO is on, it will turn the fan off and so on. Can I use this circuit for LEDs too? Would I then just add a resistor for the LED in series with the "High power output" on the schematic? I definitely want this inverting behavior for fans. I want to invert the LEDs as well for consistency in LCDC (I can just rename on/off states there).

One more question. I have some LEDs laying around at my place. How can I figure out their voltage and current? Are there some ballpark figures for different types of LEDs? Can I just measure the voltage (someone said on this forum that voltage drop across an LED is constant and depends on the LED itself)?

Thanks in advance.

1) The 5.6K resistor shouldn't make too much of a difference.
2) Leds will work fine with the proper resistance in series
3) This is a little tricky. Experiment. I usually estimate LEDs at 1.2-1.5V and 15-20mA. Good starting point. If they pop, increase the resistance

Thanks for the reply. The circuit works great. I even hooked up relays to control fans (clamped with diodes).

Unfortunately, a piece of metal got stuck to the circuit board and shorted two of the GPOs together. One of them burned out, so now I only have 5 working GPOs. I'm wondering why it burned out. The current limiting resistor would prevent the GPO from burning out if it was shorted. So what could be worse than a dead short?

Glad to hear it worked for you.

As for your short, that is strange. You must have done something a little different than just a short. If you applied over 5V or a negative voltage to the GPO, the resistor won't protect you.