Originally suitable for photography applications, this circuit from a Popular Electronics from the 90s can also be used to time plate baths. The components are common, and the sensor is remote.
The figure shows how to implement an alarm buzzer in an LM3914 to warn about crossing the scale...
This circuit can act as a shield being controlled by a microcontroller by pin 1 or 2. The...
This circuit was obtained from a 1980's QST magazine. The circuit can generate 1 to 15 MHz signals...