Thursday, September 18, 2008

Receiver Saver -- Updated Circuit

I've made two modifications to the circuit that is described in a posting on 6/10/08. Click on the revised schematic below for an enlarged view.

By changing the values of R1, R2, and C1 as noted, the transition from transmit mode (receiver grounded) to receive mode (receiver connected through to its antenna) takes about 1 second. A smaller value of C1 can be used for a shorter time delay. The transition from receive mode to transmit mode is still very quick, IF the transceiver control lead provides a low resistance to ground on transmit, thereby discharging C1.

This circuit change enables an operator using full break-in CW to use the circuit on a spotting receiver without having the circuit's relay chattering in time with each CW character transmitted. Since the status indicator LED has its own switching transistor Q2, it now transitions from red to green a fraction of a second after the relay switches from transmit to receive state. The values of R3 and R4 could possibly be adjusted to change this behavior.

Diode D8 was added to prevent the powered-off circuit from presenting a resistance-to-ground of about 1 megohm or less (R1 in series with base-emitter junction of Q1) to the transceiver control circuit. In one user's application this circuit was connected to the control lead from a transceiver to a linear amplifier. The amplifier apparently interpreted the resistance-to-ground of the receiver saver circuit as a transmit control command from the transceiver. The addition of D8 presents a much higher resistance-to-ground on the control lead that should prevent false keying of the linear amplifier.

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