Optical squelch

Many of the less expensive shortwave receivers have no squelch circuit and continue to emanate noise when no usable signal is coming in. Fortunately, they normally have an LED 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 ts via the contact of relay R1.