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Author Topic: simple transistor switch help  (Read 7675 times)

trials_modder

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simple transistor switch help
« on: July 19, 2006, 01:25:51 PM »

I am controlling a low power NPN 2N3904 transistor by the +5V from my parallel port. Turning that transistor on switches another, medium power NPN TIP41C transistor which in turn powers a motor (I know I can trigger the medium power transistor directly from the port, but I don't want to). I would like the low power transistor to also power an LED in addition to the second transistor. When I place the LED at the emitter of the 2N3904 and disconnect the lead to the TIP41C, it lights. But, when I connect the TIP41C to the circuit (in parallel with the TIP) it won't light but the motor will turn.
I'd like the LED to be connected to the 2N3904 and not in parallel with the motor. What would I do to keep the LED on and the motor turning?
The schematic currently, without the LED:
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Re: simple transistor switch help
« Reply #1 on: July 19, 2006, 01:33:26 PM »

how did you wire up the LED (since you don't have it on the schem posted)?
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trials_modder

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Re: simple transistor switch help
« Reply #2 on: July 19, 2006, 02:43:33 PM »

I had the LED in parallel with the second transistor, both connected to the emitter of the 2N3904.
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wonko

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Re: simple transistor switch help
« Reply #3 on: July 19, 2006, 04:55:31 PM »

Are you sure your anode and cathode are wired up correctly?  Just curious.
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trials_modder

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Re: simple transistor switch help
« Reply #4 on: July 19, 2006, 05:04:07 PM »

Problem resolved. I neglected to consider that each transistor puts a 0.7V drop, giving a total drop of ~1.4V. The motor was okay with the 1.6V left over but the LED wasn't. One question remaining, which way does the cathode on the diode point? I've seen diagrams on various websites pointing towards GND and away from GND. I know the motor will make current in the reverse direction it was given, so I'd say with the way it is in the above diagram.
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viridius

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Re: simple transistor switch help
« Reply #5 on: July 19, 2006, 07:19:46 PM »

When forward biased, the diode points towards ground.
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trials_modder

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Re: simple transistor switch help
« Reply #6 on: July 20, 2006, 12:01:17 AM »

OK. New question: how would I get the motor to reverse direction, say by another pin? Would I hook up a circuit identical to this to pin 3 of the parallel port, and link it to the same motor but in reverse polarity?
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viridius

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Re: simple transistor switch help
« Reply #7 on: July 20, 2006, 01:00:26 AM »

You'll need to rig an H-Bridge.
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trials_modder

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Re: simple transistor switch help
« Reply #8 on: July 20, 2006, 02:08:37 PM »

Yep.  This is becoming quickly more complicated than I thought. The point of this was to get some experience with motor control before building, say, a robot. I only have 8 pins to spare on my parallel port. Looks like I will have to find out what this PIC and STAMP chips are all about. And perhaps control it by USB (much cooler).

EDIT: Appears to be far too expensive to mess around with programming chips. I'll have to look for a better alternative.
« Last Edit: July 20, 2006, 02:17:13 PM by trials_modder »
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Skylined

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Re: simple transistor switch help
« Reply #9 on: July 20, 2006, 05:01:43 PM »

You can control 12 pins, not just 8.
I LOVE ED (x24) might help you. http://x24.x24garcia.com/prog_LEDtest.html
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