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Oh, where did I go wrong? - Circuit Quiz (Mystery Solved!)

I built this circuit:
a TTL input on s0 is supposed to turn on Motor 1, and a TTL input on s1 is supposed to turn on Motor 2.
Why the heck does both motors go on when either S0 or S1 go to 5V?

It never ends to amuse me how many ways I can goof stuff up...



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Yeah good point, I guess it is floating - I thought that just messed up FETs - I thought that transistors worked on flow of current through the base - so floating "should" stop Collector -to-> Emitter...

 But maybe its not floating but "bobbing" like a cork --- hmmm

 you may be on to something there - JIPs suggestion of pull downs might solve the problem ... also it might be the TTL logic which I was using before the optos had their own pull-downs - I'll check

 back to the lab ... bwah hahaha

yep, you got it -

double-transistor-dingy = big switchum (american) = darlington switch


Own design, which might have something to do with the proglem :) - the specs on the opto-isolator are here

Its a surface mount which makes it a bit difficult to breadboard. 

Where does the circuit diagram come from? Your own design or from a page with more info on it?


/ vzz-clck-"Maneuver"

Perhaps there's a little hole in your opto isolators and some of the light is escaping from the first one into the second one?

On a serious note, though, I can't see anything wrong with your circuit. Buzz it right through. 

First thing, pull the motors out of the circuit and measure the output voltage between +12 and the darlingtons. That'll answer the above question: are you sure you're not demanding too much oooph. If you get the wrong outputs, one of your your TIP122s might be stuffed.

Second, if you're still convinced the prototype hardware is good, pull the MCU out and apply +5V directly to the inputs of the opto isolators and see if it works. If it does, either your MCU is stuffed or your code is stuffed.

A quote from the Aerospace Indistry: "Always make sure the software is at fault. It's cheapest to fix."

Thanks Boa, 

I like the light idea, and I'll do the first test.  There is no MCU currently hooked up - so no code.  It might be the nut between the power supply and the circuit (me)

Yes, that's a good idea. Put a nut between the two opto-isolators. That will block the light. If you can find a nut with no hole in that will work even better.
Could you post a web link to the specs for the optos being used? Perhaps a resistor is needed between the 12 volt source and the collector (pin 5) of the phototransistor, 10kohm or so. 

Here is the link. I remember you are supposed to bias transistors... hmm maybe you are on to something.

I have to admit that I hooked the darlingtons up to a ttl ouput without biasing them and it "seemed" to behave just fine. 

If I remember correctly, an effectively biased transistor wastes less power - I guess i'm shorting them directly... I was kindof guessing that since its going through 3 gates that it might be enough resistance ... (bad thought) 

Lame question? - a very good question. I'll go back and verify again - as far as the voltage dip, I don't see why that would cause the other circuit to trigger.  The source is 400 watt 12 v power supply the motor is fractional amp, so I don't think that is it.  I'll poke at it a bit more...