AIM tach amp protection
A while back I mentioned that I put some protection in the tach amp circuit. As I understand it, if the battery voltage drops below 9V while cranking, the op amp in the tach amp blows. They are only 88 cents, but it takes someone with the right tools to solder the tiny bug in.
I came up with the enclosed about eight years ago after two failures. None since. Petty simple, all parts from Digikey.
The switch allows you to take the amp out of the circuit when you know you are going o be doing a lot of cranking. Under normal operation with the switch closed, when you turn on the ignition switch the cap charges to the battery voltage minus 0.75-ish. When you hit the starter, the diode prevents the cap from supplying voltage back into the battery side of the circuit and holds the tach amp up until you release the starter.
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