The audio signal is also passed via R13 and C1 to the base of Q1. C7 across base and emitter of Q1 help to prevent any RF pickup that could falsely trigger Q1. Q1 is a general purpose PNP transistor such as BC212. When the telephone line is called a higher AC voltage (approximately 90V AC) is sent down the line. This passes through the transformer T1, and R13 biases Q1 into conduction. The signal passes through R9 and D1 and is rectified to DC. This slowly charges C2 (which has been previously discharged by R8). Once the charge is sufficient it will turn on Q2 which will apply a negative going input pulse to the 555 timer. The time constant of C2 and associated circuitry is designed for a slow response, and thus prevents false triggering from voltage spikes, clicks and other line influences.
The 555 timer is wired as a mono-stable, its pulse duration being set by R6 and C6. As C6 charge current is in micro-amps, it is important to use a low leakage capacitor and tantalum type is recommended. The output from the 555 timer is passed via R3 to Q3 which acts as an inverter and level shifter. The voltage on Q3 collector is passed via voltage divider R1 and R2 and turns on Q4 which operates the relay. The pulse duration controls the "on" time that the relay operates, this is approximately 30 sec with values shown. After the timing period has ended, then if Q2 is still active, then the mono-stable will be re-triggered and another timing period starts.
The circuit was originally designed for a 24V relay which was available. However, these may be hard to find and a common 12V relay may be used instead. If a 12V relay is used then the regulator IC2 is not required and Q4, R1, R2 and C8 may also be omitted. The relay would then be powered from Q3 collector to ground and LED1, R14, D1 moved accordingly. A suitable transistor for Q3 could be BC490 or similar. If working from 12V supply, C4 should also be increased to 100uF and the supply should be stabilised.
In the prototype, T1 was a transistor radio output transformer, but many small mains transformers could also be used. Ideally a step-down transformer of 6:1 should be used and some adjustment to the value of R13 may be required. R13 should not be lower than 4.7k but could have a 47k potentiometer in series with it. All electrolytic capacitors except C8 should be 16V working or higher. C8 should be rated 35V or higher. C6, tantalum type should also be rated at 16 Volts. The input capacitor should be a poly-carbonate type rated 300V or higher.
If working from 12 Volt supply then R14 may be reduced to 1k. VR1 is the Bias potentiometer and should be adjusted for best sensitivity without false triggering. D1 can be 1N4148 or 1N4001 or any general purpose silicon diode rated 25V 0.5A or higher.
The transistor types can be substituted for BC478 or BC108. If using a 2 Volt relay then use a BC490 for Q4.
End of UK Telephone Service and Using the Circuit on VOIP
If you don't already know the UK PSTN service for all landline telephones is due to close in 2025. The reason for closing is that 99% of all calls are made on mobile telephones and the landline service will end. It will be replaced by VOIP (Voice-Over-IP) requiring an internet connection and a VOIP adapter. You will also require a subscription to a VOIP provider who will provide you with a telephone service and issue you a number. This is more complicated than the present landline and may also work out more expensive than a mobile contract. You are advised to search on google for VOIP service providers and adaptors. The VOIP adapter will have an outlet compatible with a type 631 socket and so this circuit will still work from a VOIP adapter. As a landline telephone uses a higher AC voltage to ring the telephone bell, the VOIP adapter will generate the ringing voltage and circuit can be adjusted to trigger on the incoming call, as with an ordinary landline.
Line Connection UK Only
In some countries including the UK a direct connection to a telephone line is illegal, so this circuit must be connected via an appropriate line cord. The standard connection is a plug 631 and a suitable line cord is available from CPC Farnel:
t Telephone Line Cord
The outer connections are all that's required and as this lead, terminates with a type RJ11 plug, then a suitable RJ11 socket is also available:
RJ11 PCB mount socket
Although the socket contains all 8 contacts only contacts 2 and 5 are required for the input to this circuit.
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