Power Supplies
Supplying this project with the right flavors of power is something of a challenge. There are several parts with different needs:
- The fish motors are happiest at about 6 to 7.5 volts and seem to want about 1 watt.
- The Raspberry Pi needs 5 vdc (no more, no less) and about 1, maybe 2 watts - more if it is also supplying usb or ethernet.
- The audio amp burns about 5 watts per channel and does not like sharing it's power or ground with the Raspberry Pi. (Signal noise)
- I'm using various integrated circuits. Some of them want 3.3 volts, some want 5 volts.
- I have a USB hub that needs five volts. Depending on what is plugged it to it, it can require several watts.
My original idea was to use the power supply recommended for the billy bass. It's highly unlikely any bass you find at garage sales will also have a power supply, so if you go this route, you'll need to find one - the specs are printed on the case.
The power supply I found supplied 7.5 vdc (at this point, I was unaware I needed to worry about amps) - which was fine for the fish motors - but not so fine for the Raspberry Pi, which requires 5vdc. Not to worry - I figured I could use the LM7805 5vdc solid-state voltage regulator. Hooking it up is simplicity itself; voltage in, ground, 5vdc out. I bread-boarded this up, fed 5 volts to the Raspberry pi via GPIO pin 4 and it booted! Whoo hoo - I'm a genius!
Well...until the LM7805 overheated and tripped it's internal circuit breaker. The LM7805 handles 1.5 amps, which should be fine for the Rpi - but the wall wart was only supplying about 800 ma. Something had to give, fortunately it was the LM7805, which will reset when it cools down.
By the way, I found a lot of discussions about using two LM7805 chips in parallel to increase the amount of available current. This is a really bad idea since it is likely that one LM7805 will output a different voltage or current than another - resulting in back current and a fried circuit.
If I can't be clever with an LM7805, I might just as well surrender (temporarily) to the idea of two wall warts - one per power requirement. This sort of worked - but still didn't supply the necessary current if I plugged things into the RPi USB - especially a WiFi USB adapter.
If I was willing to provide the fish motors with five volts, then I could power the entire project with a USB hub. That would also mean I can trim down to one small power supply. I could find them with sufficient wattage.
I had a USB cable I was willing to sacrifice and another with a RPi-compatible micro-USB connector. I powered the USB from a wall wart, then ran a USB cable to the breadboard and another to the Raspberry Pi. The breadboard supplied power to the fish and amplifier.
The Raspberry Pi is happy with this arrangement. The fish isn't as energetic as it might be with 7.5 volts, but it manages. The amplifier could be louder - but is sufficient for a fish. The static from the amplifier is annoying.
The fish just isn't impressive at 7.5 volts. The mouth opens up - but the tail and head barely move. I need something different.
I wandered down to Freegeek, the source of all Portland Surplus Electronics goodness. Seriously - if you're in Portland, stop by and salivate. At Freegeek, they have $10 ATX power supplies. ATX supplies 3.3 volts, 5 volts, and 12 volts with 45 watts. Way more than what I could ever use. The power is clean, so the amplifier isn't noisy and the Raspberry Pi doesn't lack for wattage.
In the end, this is what I went for. It solves a lot of problems with power. Unfortunately, it's large and can't be installed in the standard wall-mount base.
At some point, I'll learn how to do this with batteries. For now, it's a sufficient trade-off.