Sound for both will be generated of the same pin on the UNO, and it's just simple pulsed tones at two different frequencies, so sound quality isn't too big a concern.
Having had a search of the forum, I've found this diagram -
This project will be getting powered from a 12V car battery, but to keep power consistent for the speakers, I'll probably add a 9V regulator just for them.
Is this circuit the simplest way to get reliable sound, or should I be looking at something different?
Do a search for MOSFET Amplifier, I have one posted that runs from 12V and is quite loud thru a 4 ohm speaker from www.mpja.com
A logic level N-channel MOSFET, 220uF cap, couple of 68 ohm resistors are all you need. I have the resistors wired with a switch so I can have them in series or in parallel to act as a simple hi/lo volume switch. The 220uF is definitely needed to let the AC out to the speaker.
If you put in the 9V regular you will give up power output: Pmax = V^2/R, so 9V vs 12V makes big difference.
But you will keep car alternator noise out - guess its a tradeoff.
The reason I was thinking of using a 9V regulator, was because the battery won't be getting any charge while in use, the voltage will gradually drop, along with the speaker volume.
However, thinking about it, other than a bit noise (there's also 4 electromagnets being switched by the UNO at the same time), it's probably more hassle than it's worth to add a seperate regulator.
I'm needing to get this project in a useable state for the weekend, so I can always add some noise suppresion at a later date if it's going to be an issue.
IRF3707 zee
International Rectifier
Turns out that particular part went obsolete after I bought a tube of 10 (and not long after - like, by the first time I recommended them here!)
This one looks like a good replacement
Not marketed as a Logic level Mosfet, but Rds with Vgs of 4.5V is still really low, 0.0055 ohm