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Head Ache with many variations of similar detectors, some one who is free can combine all the nice features of the circuits here, and build a new detector.
Features like, power supply isolation at Tx coils , opamp stages and speaker
And also look into isolating the stage 1 for eg Lm318 inputs from coil with a capacitor, instead of feeding the powers too into the opamp like in stuart 1,2. I doubt this capacitor reduces the Rx signal !!! Or use 2 coils one for Rx Tx isolating the powers.
Use a mosfet for damping resistor, and clock it with 555 and a variable resistor in series with the gate when the Tx off's if not using a microcontroller.
Features like, power supply isolation at Tx coils , opamp stages and speaker
And also look into isolating the stage 1 for eg Lm318 inputs from coil with a capacitor, instead of feeding the powers too into the opamp like in stuart 1,2. I doubt this capacitor reduces the Rx signal !!! Or use 2 coils one for Rx Tx isolating the powers.
Use a mosfet for damping resistor, and clock it with 555 and a variable resistor in series with the gate when the Tx off's if not using a microcontroller.