# Power and Heat

This post concerns my progress on my Physical Computing final project– an assistive device that will allow a musician with limited dexterity in their left hand (from, say, accident or illness) to again play the guitar. A series of buttons underneath the fretboard will, with the aid of the Arduino microcontroller, trigger a combination of solenoids to depress the strings to create a chord.

ULN2003 – \$1. ULN2380 – \$1.75. Finding the right transistor array, one that will be sure to be able to supply enough current to all the solenoids I’m going to be using — priceless.

I hadn’t thoroughly investigated the amperage requirements of the solenoids I am using until now– and boy, was I in for a surprise. Even though they’re 1/2″ in diameter, and barely an inch long, they like to suck on about 400 milliamps between 9V-12V– that’s a lot of juice, and we’re not talking about tropicana. Damm.

See, those transistor arrays I was looking at CAN put out about 500 milli amps — *IF* you’re using just one circuit. By the time you’re running juice to two or three solenoids, that amperage is going to drop by half. And my design calls for the possibility of ALL of the solenoids staying on for, possibly, an extended period of time.

Previously, I was going to order up a triad of ULN2003′s, but they don’t meet the power requirements. I looked at another, beefier transistor array that I found through this but I realized I needed to do some solid math.

400 milli amps x 18 solenoids = 7.2 amps. Sheee-it.

And I thought I was going to have some cute little battery series on board the guitar fretbox. Ain’t gonna happen. We’re talking something akin to a motorcycle battery. I could be off a little, but it’s the same order of magnitude. I’ll probably look for some kind of computer power supply, but either way, this is going to be tethered instrument.

The other issue, of course, is heat. With this consideration in mind, I think I’m just going to use 18 individual transistors. It’ll be easier for the heat to dissipate, I think. And I won’t have the power problems that plague transistor arrays when they have to fire multiple circuits concurrently. Hah– con’currently.’