This circuit generates the busy tone of ordinary telephones. The transistors can be the BC548 and the components of the T filter and the multivibrator can be changed to obtain the exact tone.
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...