I'm trying to design a circuit that can both transmit a 40kHz square wave and receive a 40kHz sine wave using the same ultrasonic piezo transducer. Switching between them doesn't have to be very fast (I'm not trying to catch an echo from the same sensor)

When testing the receive side separately I have found using a charge amplifier works significantly better then using a normal voltage amplifier. I understand the reasons for why as described in here and here. I'm putting one lead of the capacitor at a virtual ground to avoid having a split supply. Charge Amplifier

On the transmit side I'm using a MAX3232 RS-323 level shifter. This works very well as I have drive the piezo with +-5V (or even +-10V if I use both channels) without having to generate high voltages myself.

What I'm struggling with is how to I electronically switch between those two circuits? Despite the supposed high impedance of the MAX3232 when off (and the charge amplifier making stray capacitance less impactful) it makes the signal on the charge amplifier disappear.

I found some circuits to do the switching for example this one by QingStation. Excerpt reproduced here: TX RX switch

It appears to use one leg of the piezo to transmit and the other to receive. I don't fully understand this circuit but it seems to work for him. When I've built it I get a lot of noise - definitely not as good at the charge amplifier.

I've also seen transformer based circuits like in this question but haven't built and tested them yet.

What are some methods of switching between the charge amplifier receive side with the MAX3232 transmit side?



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