This simple circuit can use general purpose transistors and SCRs like the TIC106. The warning device depends on the supply voltage. It consists of a small buzzer. Note that the SCR voltage drop of 2 V. must be considered.

This simple circuit can use general purpose transistors and SCRs like the TIC106. The warning device depends on the supply voltage. It consists of a small buzzer. Note that the SCR voltage drop of 2 V. must be considered.