MIDI Project: First Success! Throwing an Electrical Switch

Last night I spent my evening hacking at Pumping Station: One, Chicago's premiere hacker/makerspace. It was my first time really getting down to work there after joining a few weeks ago. What I love about working there vs. at home is the community. A couple members, Steve and Patrick, who much more experienced in electronics than me were incredibly helpful, teaching along the way. I wouldn't have had this first success without their help.

So what am I making at PS:One? Well, in short it is a MIDI-controlled channel selector for my guitar amp. Anyone who's ever seen my guitar rig will usually comment on my pedal board first. For all the complexities you see at first glance, it's actually rather simple and elegant. A few years back, I bought a box that plugs into the amp that selects between its clean and distortion channels. Then I started it on fire. I've since replaced the fried diode inside it and it works again (also thanks to the guys at PS:One), but in the process realized just how simple it would be to build one of these boxes myself. My guitar amp has three quarter-inch mono phone plugs, one for each channel. When the tip of a cable is connected to its ground (i.e., shorting the cable with a SPST switch), the amp switches to the corresponding channel.

Thus, the goal for my project is simple: Build a box that will receive MIDI program change (PC) messages and select the desired channel on my guitar amp (clean, rhythm, or lead). For example, MIDI PC 11 would be the clean channel, 12 would be rhythm and 13 would be lead.

The components used thus far in the project are as follows:

  • 1 x Aruidno Uno board
  • 1 x JFET TIP120 transistor
  • 1 x 1k Ohm resistors
  • 1 x standard green LED
  • 1 x solderless breadboard & jumper wire set

Throwing the switch: At this point in the project, "success" comes from just getting the transistor to act as an electrical SPST switch, which is what we achieved last night. When power is applied to the transistor, the LED will light up. We're using an LED here to simulate the channel switching on the amp; essentially, when the LED lights up, the channel on the amp will switch. The 1k Ohm resistor goes between the power flowing to the transistor (aka "V+") and the transistor itself. This is necessary because without it the JFET transistor will charge itself and stay "latched" even when power is not applied to it.

Simply connect and disconnect the 5v power from the 1k Ohm resistor to turn the LED on and off. The next step for me is actually testing this with my guitar amp later tonight. To translate from simulating with an LED to the real world application, I'll plug a quarter-inch mono phone cable (aka a standard guitar cable) into one of the amp's channel selectors, with the tip of that cable wired into the transitor's collector pin and the ground into the emitter. When the transistor is powered, the channel should change on the guitar amp.

This circuit diagram below shows how to hook it all up.

Next steps: Test if it works on my guitar amp, then start working on the MIDI input. More to come!