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Parallel port LEDs; current limiting?

 
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HyperHacker
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PostPosted: Sun Apr 19, 2009 5:38 am    Post subject: Parallel port LEDs; current limiting? Reply with quote

Hey, so I'm a software guy, very limited knowledge of the physical side of electronics, but I've got a simple setup with some strings of LED lights being controlled by my computer's parallel port.

The design is amazingly simple. I just wired some LEDs directly to a parallel port connector, and attached them to some "solar control" sensors, that are basically just an outlet with a CDS cell built in. The light strings are plugged into those so when the LEDs are on, the sensor sees "sunlight" and the lights are turned off.

Someone suggested that just driving 8 LEDs directly from the parallel port could burn out the controller on some motherboards though, and that I should have some current-limiting resistors in the circuit to avoid this. My board's survived so far but I don't like the thought that if I ever upgrade, it might fry the new one.

The problem is these sensors don't have any sort of threshold to turn on or off. They're just ordinary CDS cells, that give more resistance as the light intensity increases. So for them to turn the lights "off" (really just very dim), the LEDs have to be as bright as possible. Even now, when the lights are "off" they still glow dimly, because the LEDs aren't bright enough to get them completely off. So the question is how to add current limiting to the circuit without significantly reducing the LEDs' brightness?

On a related note I've noticed some strange behaviour with longer light strings. Some are almost unaffected by the sensors (you can see them dim slightly), but if plugged into the end of another string, they do go on and off as expected. Even stranger, in some configurations half the string will go off and the other half stays on. O.o What's up with that?
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Turd
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PostPosted: Sun Apr 19, 2009 7:15 am    Post subject: Reply with quote

I would use an output buffer maybe something like this Very Happy
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Alan
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PostPosted: Sun Apr 19, 2009 7:48 am    Post subject: Reply with quote

After the buffer you could then use a resistor calculator to determine what the correct resistance for the LED should be.
http://alan-parekh.com/led_resistor_calculator.html
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HyperHacker
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PostPosted: Sun Apr 19, 2009 8:53 pm    Post subject: Reply with quote

I see. So how does an output buffer work?
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Turd
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PostPosted: Mon Apr 20, 2009 5:20 am    Post subject: Reply with quote

There's 8 gates in that chip each has an input and an output. The weak signal from the parallel port goes to the input and it outputs a stronger signal on the output. You have to power the chip with 5 Volts. Use a 5V regulated power adapter or use a 9V or 12V power adapter with a 5V voltage regulator circuit Very Happy
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HyperHacker
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PostPosted: Mon Apr 20, 2009 3:54 pm    Post subject: Reply with quote

Oh, I didn't know such a thing existed. Is it like a relay, then? I was going to use relays originally but I wanted to be able to do PWM dimming.

Although if there's more current coming out, I guess I'll need a big fat resistor before sending it back to the port's ground?
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