Monday, February 21, 2011

MOSFET's or Relays?

Originally I had a plan to use relays to control the power to the various solenoids and coils, but as I found out, relays for +48VDC aren't exactly cheap. Something like 60$... and I need around 10 of them. Yikes!

After doing a little research I learned about the MOSFET transistors IRF640 which are ideal for this. They act like a relay but costs around 4$ each instead! A much more comfortable pricerange...

The best thing is that I can control the IRF640 directly from my Arduino board, since the requirements for "flipping the switch" are very low. There are also smaller versions available for even less which I'll probably be using in case I need higher powered lights than my Arduino board can power on its own.

A big thanks to Benjamin Heckendorn at  for helping a poor soul out regarding the MOSFET's. In fact - his Bill Paxton pinball machine was and still is an inspiration to my project - Great stuff!


  1. Hey, I just stumbled across your blog post. I too am building a pinball machine. :) Do you have a schematic diagram of the MOSFET circuit? More specifically, how to connect them to a TTL signal (like a microcontroller) in order to drive he 24v solenoid circuit?

  2. Hi there!

    Cool! Do you have a webpage for your build as well? :)

    I actually ditched using MOSFET's, well more specifically - a custom MOSFET board.
    I'm using a MOSFET kit by Sparkfun to keep things simple (more about that here -> ).
    I later found out that the IRF640 are quite slow to trigger (from a logic-level device, but there are lots of MOSFET's better suited for being run by logic-level.
    The light board I'm currently building is using N-channel MOSFET's from Sparkfun as well.

    I don't have a schematic, but the connection is simple enough. For a N-channel, for example: Connect trigger pin to Gate, Gate to ground with a resistor, Source to ground, Drain to the load. Connect the positive side of the load to a device and the other end of the device to Drain.

  3. By the way, I'm using 48V.
    Power to the flippers! :)