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Power Supplies
Supplying this project with the right flavors of power is something of a challenge. There are three systems to consider: the fish motors, the sound amplifier, and the raspberry pi.
- 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 watt - more if it is also supplying usb or ethernet.
- The amp burns about 2 watts. All told, that's about four or five watts.
My eventual goal is to have this project self-contained, supplying power with a battery. Rather than tackle the battery problem at the same time as tackling the programming and hardware problems, I'm opting to use wall warts as an temporary solution.
My original idea was to use the power supply recommended for the billy bass. It's highly unlikely any bmbb 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 - but don't bother going this route - explanation in a second.
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 can 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!
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 power supply was only supplying about 800 ma. Something had to give, fortunately it was the LM7805, which will reset when it cools down.
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 - or a full-blown computer power supply.
The computer power supply was immediately unworkable. They supply 5vdc - perfect for the RPi - but then only supply 12 v or more. Getting 7.5v would require some circuits - none of them as clean as the LM7805 - so there isn't any real advantage.
Surrendering to the two-wart solution just seems wrong. I'm not ready for that just yet.
If I was willing to provide the fish motors with five volts, then I could power the entire fish with USB. That would also mean I can trim down to one small power supply (a USB hub) and 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. For now, I have a USB hub plugged into the wall with two cables running into the fish. One connector goes directly to the RPi, another goes to the breadboard.
The Raspberry Pi is delighted 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.