Many of the less expensive short. Wave receivers have no squelch circuit and continue to emanate noise when no usable signal is coming in. Fortunately, they normally have an LE t) which lights to indicate that the level of an incoming signal is at or above a predetermined limit. This LED can be used to control a light dependent resistor (LDR). The potential across the LDR can serve to actuate a circuit that switches the loudspeaker or headphones off in the absence of a suitable signal. This is possible without any work in the receiver itself by connecting an external loudspeaker to the headphone output. Inserting the plug into this socket automatically switches off the internal loudspeaker. The connection to the external loudspeaker is via the contact of relay Re 1. The circuit is based on comparator ICI. P1, R1, and R2 provide the reference voltage at the inverting input of the opamp. which in the quiescent state (when no light falls onto RI, which then has a high resistance) is higher than the potential at the non-inverting input. When light falls onto RI. its resistance decreases, whereupon the level at the non-inverting input of the comparator becomes higher than that at the -eye input. The output of the comparator then changes state and switches on T1, whereupon the relay is energized. The relay contacts then connect the loudspeaker to the headphone output. During construction, It should be borne in mind that the less ambient light falls on to the LDR, the better the squelch will perform.
Preset P1 should be adjusted so that. the title, the relay is not energized when the LEE) is out, but actuated as soon as the diode lights. If difficulties arise in tile setting 01 i), they are probably caused by the LDR having an incorrect value. In the diagram, the values of R1, R2, and P1 are suitable for use with a new that has a resistance when light falls onto it of about 6 Measure this resistance with a multimeter; the value of R1 should be made (about) equal to this, and that of P1 about twice as high. The value of R2 and R: should be roughly the sum of the values of P1 and R1 The power supply may be taken from the receiver: if that is not possible, a separate mains adaptor should be used. The circuit draws a current of about 5 mA plus the relay current.
