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[on:]

I believe I have burnt out my Freetronics LeoStick with my hardware wiring. I have spent days reviewing my circuit and can't see what I've done wrong. I've attached a diagram, but in short I have a 12v fuel injector i'm trying to run by sending the arduino signal to a tip42c transistor. I'm powering the Freetronics LeoStick through a voltage regulator 7805. Any tips would be appreciated. Thanks.

[Reply #1 on:]

You really should have some caps on your 7805, at least something on the output so it doesn't oscillate.  You should always use a resistor between the Arduino pin and the transistor base.  Do you need high-side switching?  If not, consider something like an NPN darlington TIP-120 or (better yet) a logic level FET on the bottom side.  The gain of the TIP42 is not very high, that's why I recommend anything else.  You want to install clamping diodes across the injector leads to safely vent the high voltage spikes to the supply rail.  Look around for examples of how it is done with relays.

EDIT:  Does your Leo work at all?  Start by disconnecting Pin 3 from the transistor.  Can you reprogram it with the Blink Example?  If so, then you may have damaged pin 3, otherwise, sadly, if the board acts completely dead and won't do anything, or parts on the board are getting roasting hot, then it's probably curtains.  Still might be only the mega328 which can be replaced.

[Reply #2 on:]

Leostick is (as the name suggests) based on the Leonardo, but in a usb key form factor, with the ATMega (not a 328 but don't recall which one) being SMD.

High side switching will be required as the fuel injector may connect its low side to its stainless steel casing, and in a metal water heating aparatus which is grounded to the rest of the electronic system, it won't be possible to disable the output.