While I was hanging around waiting for the price to drop on the Pirate Audio HAT I stumbled across this
https://thepihut.com/products/adafruit- ... spberry-pi. Not only is it cheaper than the Pirate Audio version, but it doesn't have an LED Display (which we don't need) and the GPIO connector slides over the pins and allows them to protrude, so I won't need the breakout board to get to the GPIO pins to drive the solenoids. I will continue to pick up the fan power from a 5 V Distribution board, rather then GPIO Pins 4 and 6, because it mean that the fan current doesn't have to go through the Pi. The other good thing about this 'Bonnet' is that they specify the performance somewhat better than Pimoroni
do for their HAT and the 3 W per channel is only available if 4 Ohm Speakers are used. At 8 Ohms, the power output falls to 1 W. I suspect that the Pimorini device is the same, but at least we know that we can increase the volume by up to three times by paralleling up additional speakers or finding some 4 Ohm types, should it be necessary.
I have made some changes to the mechanical design as well as the circuit diagram to accommodate things that have come to light as I constructed it. I'll post the updated diagrams in due course.
(BTW. I'm very impressed with the Dual Fan Heat-sink; it straddles the SoC and the other hot chip on the top of a Pi 3 and is whisper quiet. I ran a stress test for around 15 minutes and the CPU temperature only got up to ~50 degC.)