Hooking up LEDs and fans to low power GPOs

NoRemorse · Post by **NoRemorse** » Tue Dec 23, 2003 11:48 am

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.

Paradigm · Post by **Paradigm** » Wed Jan 07, 2004 10:57 am

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

NoRemorse · Post by **NoRemorse** » Wed Jan 07, 2004 11:37 am

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?

Paradigm · Post by **Paradigm** » Wed Jan 07, 2004 2:13 pm

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.