Let's Make Robots!

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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All right my friends,

The mystry is over - several lessons learned :

1. an ego only gets you in trouble (had i posted a picture, one of you would have immediately found it - and had a good laugh) - i did not post it for 2 reasons (a. not easy getting pictures downloaded to my computer (incorrect usb cable)  b. i was embarassed about such a crappy soldering job i did)

2. show the big picture first, its good to have schematics. but usually you should start with the "big" picture

3. never underestimate what a dufus i am

Thanks for going over it in such considered detail... 

Now the solution........ drum roll please....



3+ Days I spent trying to figure out why more than one motor would turn on when a single signal was high..
Notice THE BOLT THROUGH ALL OF THE DARLINGTONS !!!!!!   I originally thought when I was putting it together - hey, a nifty heat sink - and it will help stabalize everything as i free-form solder this crud together..
AHAHHAHAH - I forgot the heat sink was the collector and I effectively wired all the collectors together!!
What a maroon what a noob, thank you thank you ....

next time i'll start with the picture of my cruddy soldering job.

Thanks for all your attempts of help :)

Hehehe, yup, I figured there must be something not shown in the circuit. I've just recently been messing around with some of those TO220-packaged darlingtons too (TIP120/122's) , and it surprised me that the tab is connected to the collector, and not the emitter. I know transistors are often used in all different kinds of configurations, but it seemed like the most common arrangement would be with the emitter tied to ground, so if the tab were connected to the emitter, thena common heat sink like you have would work just fine. That's how they did it on 7805 voltage regulators -- the tab is ground, and that seemed to make sense. With the tab being the collector, I have to provide more space between my transistors to keep them from touching. Kind of a pain. Anyway, glad you got it working :)


LOL! That sounds like something I could have done too! I really like the whole assembly - reminds me of my first line sensor where 10 LDRs where mounted on a piece of cardboard from a cpu box and the connections were a mess of unisolated wires :-)... why the hell did they invent boards to solder onto? Who needs them?! :-)

Thanks for clearing up the problem for us.

Ah ha !

The current manual for my interface controller (PCI DIO 96) did not have any references - but the previous model (PC DIO 96 PNP) manual states this

"The PC-DIO-96/PnP facilitates user-configurable pull-up or pull-down. Each DIO channel is connected to a 100 kW resistor and can be pulled high or low using jumper W1."

So when I hooked it up directly to the DIO card there was a pull down involved...

I'll hook one up and see how it behaves... 

Thanks for the suggestion JIP! 

Have you tried adding pull-down resistors for the two base-pins of the darlingtons?

Hi Jip,

strange, I have not tried it but my electronic gut feeling, is you are right...  but i don't understand why..
The only experiment I have tried so far is cutting motor 2's E -> to ground.  Which did stop motor 2 completely (not really proving much).. 

So any guesses as to why? - Why would the opto Emitter be leaking on the base of the Darlington? 

Some more info would probably help:
I hooked this whole thing up to a TTL output without the opto-isolators - It appeared to behave just fine.
With the optos I soldered all of it together freeform - I now am a believer in not using freeform, since it does not lend to easy diagnostics or modifications


It's 15 years since I finished my electronics degree, so forgive me for getting a little rusty. I need a walk-through of this circuit..

When 5V is applied to the IRdiode inside the 630, the phototransistor opens up and lets through the 12V from Collector to Emitter. The 12V applied to the Base of the double-transistor-dingy (scandinavian hitech jargon) connects the motor to GND.
Is this a correct guess?

/ vzz-clck-"Maneuver"

15 years is a long time. We stopped using valves since then. Transistors are the same,  just smaller and they don't light up so much.

Joking aside, your reading of the circuit is the same as mine, HOWEVER, I just spotted the LEDs in series with the opto isolators. Try taking them out. There's a voltage drop across all those LEDs and if your battery's a bit rundown the optoisolator LEDs won't light.

But BOA,

the issue is too much light - or at least too much juice is being spread around - not too little.   1 signal -activates-> 2 motors 

Who know s what might happen if one of thoe opto isolator inputs is left floating....?