Tuesday, January 31, 2012

Why doesn't my circuit work?

Okay, so I finally managed to build something that didn't work! This is actually neat, since it represents a problem-solving challenge to be overcome, and something that goes beyond colouring in the lines. Well, apparently I couldn't colour in the lines, which is how I got here.


It looks like a fairly simple circuit. Basically, the motor is connected to ground at one end, and via a transistor to a 5V source at the other. Pin 9 there goes to the transistor, so pulsing that slowly ought to turn the motor on and off. There is also a diode, and I don't know what the hell it's for. To be more accurate, I don't know why it's necessary here, or what it's doing in this circuit. In general, the concept seems straitforward enough. I'm also not 100% confident I have the diode, the transistor or the various resistors the right way around.

The resistors, I'm pretty sure, could get plugged in backwards and still operate normally. The diode I realise will simply not allow current passed if plugged in backwards. Given that if I skip the transistor the motor spins, the diode 'must' be the right way in (assuming it's working correctly). So the fault seems to be with the transistor.

However, no matter how I connect the  transistor, it doesn't work correctly. One way, the motor is dead. The other way, the motor spins constantly. Total ignoring of the collector (Pin 9 there). Question: is it possible to break a transistor by sending current the wrong way through? How about with diodes?

Now, I did check. The transistor in NPN, which as far as I can tell translates as "on by default", so there is one explanation: Pin 9 is broken. I tried Pin 6, no luck. So I tried coding up a flashing LED to match the Pin 9 signal. The LED flashes as required, so unless both Pin 9 and Pin 6 are physically damaged on the Arduino, it shouldn't be that.

So, now I'm stuck. Without a multimeter (or maybe just some LEDs and alligator clips) I'm pretty much unable to determine which parts of my circuit are receiving the expected current. Did I short something out? Is my diode in backwards? What current is coming out of Pin 9?

Answers to all these exciting newbie questions, and more, in weeks to come.

In the meantime, to all you electrical engineers out there, I hope you enjoyed this brief sojourn into electrical bafflement :)

PS: Some people who spotted these photos on G+ have pointed out that the diode is likely present to protect against a back-spike of power coming FROM the motor as it spins down.