A guitar has an on-board battery powered pre-amp between the pickups and the output jack. This is great until the battery fails, when it goes dead until the battery is replaced. I would like to have a safer failure mode for live performance, whereby when the battery fails it falls back to passive mode, with pickup connected directly to the output jack.

Knowing very little about this, my idea for a solution is simple and probably naive:

There is a connection from the pick-up to the output which passes through some kind of semiconductor device or devices. This device is normally “open” so no signal passes to the output, but it receives an input from the battery which keeps it open. When this signal drops because the battery has failed, the semiconductor switches the circuit from pickup to output Jack to “closed”. Clearly the output would be lower than with the preamp, and without any EQ, but turning up the amp mid song is much easier than changing the battery!

Can anyone suggest a circuit to do this?