I created a sziklai pair with a 2n2222 and a tip42c to drive a motor running on 2 AA batteries. It works when I connect the 2n2222 base pin directly to 3v on the Arduino, or to a digital pin. However, I can't get it to fire on a pin from a 74hc595. I know the 2n2222 is being fired from the shift register, it drives an led. I also know the shift register can drive a relay, I tested that. At 35ma out, I'd think the 74hc595 would be able to drive a sziklai pair. Any ideas? Here is an elementary drawing of the circuit.
Have a look at the third diagram here: Sziklai Transistor Pair Circuit Tutorial which matches your configuration of an NPN transistor switching a PNP transistor. You will see that the sziklai pair behave together like an NPN transistor. That means your motor should be connected directly to the positive power rail and grounded via the sziklai pair. You need also a resistor, say 1k, in series with the base of the NPN transistor otherwise you may damage your shift register.
So if I'm reading that correctly, battery positive would go direct to the motor positive, and battery ground would connect to the 2n222 collector, and also the tip42c emitter, and common ground to arduino would connect to the tip42c collector. Is that correct?
It is this. I've annotated the transistor junctions which you seem to have interchanged in your description.
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You are awesome, thank you! I've been struggling for days to wrap my head around this. I'll give it a shot tomorrow. What did you use to draw the schematic? Also I've been told it needs a diode somewhere in the mix. Would that go on ground between the tip42c and the Arduino?
Indeed it should have one across the motor terminals (google for flyback or freewheeling diode). The cathode of the diode is put on the positive terminal of the motor.
I used LTspice to draw the schematic. It is also a simulator for analog circuits but there is a bit of learning curve for the simulator part. Drawing a schematic is quite easy though.
thanks again for the schematic, it's working perfectly.
No. You must reverse it.
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