Telephone Music Hold-on Circuit

[Harry126]

Can tell me briefly how this telephone hold-on music circuit works ?

 

[betwixt]

It's a crude toggle control. Pressing the switch charges up C1, eventually making TR1 conduct. This in turn makes TR3 conduct and supplies voltage to the UM66 which is a 'jingle generator', pre programmed with short tunes. The output of the UM66 changes the current through TR4 and therefore the phone line so it can be heard in the earpeice. When the switch is closed a second time, the combined current through R3 and R7 make TR2 conduct which discharges C1, charges C4 and makes it ready for the next time it is used.

Brian.

 

[Harry126]

1) When TRI conducts, it makes TR3 conducts. Why?
2) What is the purpose of 10V zener diode (ZD1) and diode (D5) in the circuit ?
3) Can you tell me more about the combined current through R3 and R7 when the switch is closed the FIRST and SECOND time?
4) When TR2 conducts, it charges C4. How?

 

[betwixt]

1. When TR1 conducts, it's collector voltage drops close to 0V and R5 provides base current to TR3 making it conduct.
2. A phone, when off-hook clamps the phone line voltage to around 10V, if it goes higher because the user replaced the handset, the diodes conduct and discharge C1, thus turning the circuit off. If it didn't do that, there would be a risk that this circuit held the line open and no new calls would arrive.

Before going into more detail, you should be made aware that this circuit is NOT well designed and would be illegal to connect to the phone line in most countries. It may not work on some lines at all. A proper line seizing circuit uses a constant current generator and audio would be connected through an impedance matching network. I suggest you use a circuit simulator to see it's operation and look at how it behaves under different line conditions. Bear in mind it has to work with voltages across "tel in" between about 8V and 150V, including reversed polarity.

Brian.