[solved] PWM & Transistors

I have two seperate circuits. One circuit is with an arduino doing some sensor readings. The second circuit is extremely high current, to create heat for moving nitinol.

This is what I am doing to test the circuit:

plus of the battery to the collector of the transistor digital out (pin 9) of the arduino to the base emmiter of thransister to led, to resistor, to minus of the battery (this is only to test, later on nitinol will replace the led)

I am sending pwm signals, fading back and forth from on to off on pin 9. However, nothing happens.

I will need to use PWM for my final circuit as well - is PWM what is messing this up?

Am I doing something obviusly wrong?

Any pointers would be apreceated



The basic idea is there, though some important details are missing:

  • What transistor are you using?
  • What is the resistor value?
  • What is the battery voltage?

The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

sounds like you have it wired all wrong, but only a drawing can really communicate what you have done.

What you need is a NPN transistor and wired just like this solenoid example. Replace the solenoid coil with your nitinol wire and you won't require that D1 diode. Don't forget the base resistor R1 and of course you have to wire it to a Arduino digital output pin that supports the PWM commands. Also note the requirement that your external battery negative terminals has to connect to a arduino ground pin.


Good luck;


Its not wired wrong, it's a classic emitter-follower circuit and it should be working as described - check the pin-out of the transistor and all the connections. In this configuration it is OK to drive the base without a current-limit resistor (in fact it helps to reduce voltage droop on the output).

However an emitter follower will not be best for high-current as the transistor doesn't reach saturation and will dissipate more power than a common-emitter switch.

When dealing with high currents you have to do the math - we need to know the transistor and the current and voltage of the load to comment on the feasibility.

Thanks for the feedback so far. I believe the problem was that, as retrolefty pointed out, I need to connect minus of the battery to the arduino ground.

I havent had time to test if that actually solves the problem. I'll check it later in the day I hope (bah. two exams coming up next week ... we'll see how the day goes) and the post proper schematics etc.