Telephone hold button

The on-hook (no load) voltage across the red-green wires will be 48 V or slightly less when all telephones are on-hook (disconnected). When any telephone goes off-hook the load current flowing in the telephone causes the voltage to fall below 5 volts dc. Although the telephone hold is connected across the red-green wires, silicon control rectifier SCRl is open; so there is no current path across the telephone line. To hold the call, depress normally-open switch SI and hang up the telephone (still depressing SI). When the phone goes on-hook the red-green voltage jumps to 48 volts dc. Since switch SI is closed, a positive voltage is applied to SCRl's gate, which causes SCRl to conduct, thereby completing the circuit across the telephone line through Dl, LED1, Rl, and SCRl. The current that flows through those components also causes the LED to light up—indicating that the telephone line is being held. The effective load across the red-green wires is the 1500 ohm value of Rl, which is sufficient to seize the line while limiting the current through the LED to a safe value. When the telephone, or an extension, is once again placed off-hook the red-green voltage falls to 5 volts or less. But diode Dl has a normal voltage drop—called the breakover voltage—of 0.7 volts, and the LED has a forward drop of 2.0 volts. Excluding the voltage drop across Rl there is a maximum of 2.3 volts available for SCRl, which is too low to maintain conduction; so SCRl automatically opens the hold circuit when any telephone goes off-hook.
Telephone hold button  - schematic

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