Terminals A and B of the circuit in figure 2 are connected to one of the sensing circuits in figures. . .If, of which the pros and cons will be dis-
cussed shortly.
Whichever of these circuits is used, A is shorted to B immediately the power is switched on. This cuts off transistor T1 instantly, which causes capacitor C1 to charge.
After a few seconds, the voltage across C1 causes zener D2 to breakdown. Transistor T2 and T3 then conduct; the relay is energized, and the loudspeakers are connected in circuit.
When the power is switched off, Tl conducts and this causes CI to discharge very rapidly. The voltage across CI quickly drops below the breakdown level
of D2; transistors T2 and T3 are cut off, and the relay returns to its quiescent state, which disconnects the loudspeakers.
Input circuit 1a relies on a light-dependent resistor (LDR) fitted close to the mains on indicator lamp.When the lamp lights, the resistance of the LDR
drops sharply, so that terminal A is virtually shorted to B.
The input in 1b relies on a reed relay connected to the secondary winding of the mains transformer. As soon as the mains is switched on, the relay contacts close.
The third possibility, shown in lc, is that the mains on/off switch has a third contact that connects A to B when the mains is switched on.
A further option is illustrated in Id, where a transistor is connected to the secondary of the mains transformer via a diode and resistor. The transistor
conducts when the mains is switched on.
The inputs in le and If also provide power for the protection circuit. That in le has a bridge rectifier connected across the secondary winding of the
mains transformer. When the mains is switched on, the BC 547 conducts and shorts A to B.
Finally, the circuit in If is connected direct to the mains. Here again, as soon as the mains is switched on, the BC 547 conducts and terminal A is shorted
to B.
Whichever of the input circuits is used depends on circumstances and/or individual preferences. If one of circuits la … Id is used, a separate power supply is required for the protection circuit. As suggested, the output voltage, Uv, of this should be 40 … 60 V d.c. For lower values of Uv, the rating of D2 must be reduced accordingly.
Resistance Rv depends on the relay used, and is
calculated from
Rv = [(Uv – Ur – 2.5)/Ir] ohm
where Uv and h are the operating voltage (in volts) and current (in amperes) of the relay used respectively.
The relay contacts must be able to carry a large current: 10 A is not unusual in many amplifiers. The rating of Rv is [UJr] W. If the ‘plop’ is still heard, increase the value of R3 as required — in reasonably small steps.
*this article reprinted by 303 electronic circuits.
