The indicated circuit uses an unusual buffer from National Semiconductor. The frequency is given by resistors R1 and R2 as well as capacitors C1 and C2.
Found in a foreign publication, this sensitive circuit triggers a relay. The voltage depends on...
This circuit also functions as a phase detector. The components used are not common. We found this...
The presented circuit is suggested by Texas Instruments (www.ti.com) in its Application Report...