Found in a Popular Electronics from the 90s, it applies to the old answering machines that used tapes. It emits a warning beep on the drive.
This conventional astable multivibrator circuit with operational amplifier can be found elsewhere on...
The LDO series (Low Dropout) TPS80 from Texas Instruments (www.ti.com) can provide output voltages...
This circuit was found in a Hands On Electronics from the 80's. The circuit emits an audible alert...