This circuit was found in the September 1992 issue of Radio Electronics magazine. The circuit does not use a transistor to drive the relay and has no adjustments.
The LDO series (Low Dropout) TPS80 from Texas Instruments (www.ti.com) can provide output voltages...
The presented circuit uses an operational one of the time. The circuit is from an 80's manual and...
The operation of this circuit is old, but the configuration can be elaborated based on modern...