Posted on Aug 7, 2012

A sensitive-gate SCR provides a line-holding current of 20 to 40 mA, depending on loop resistance. It also lights an LED to give the user a positive indication that the telephone line is on hold. The 20 to 40 mA should prove sufficient to hold the majority of lines, but it might require increasing-by decreasing the size of Rl-in individual instances. When any receiver in the same loop is lifted, the low impedance of the offhook telephone set shunts holding current away from the SCR, thereby releasing the line and extinguishing the LED.

Click here to download the full size of the above Circuit.

Zener diode D2 ensures that the line-holding current drops below the SCR"s minimum conduction current. If the calling party tires of waiting on hold and hangs up, the release of the central-office relays from the calling side also releases the line from the hold mode.

Leave Comment

characters left:

New Circuits



Popular Circuits

20W Car Booster Audio Amplifier
Phone Bill Calculator
Simple TV Remote Control Jammer
ypbpr to rgb video converter circuit
amplifier electronic circuits audio amp schematics 170w lm4651 lm4652
auto power off schematics
Long duration timer circuit
Application Circuit CW4960 CW4962 dual voltage + 5V 1.5A -12V 100mA output