Here's my 2 cents on driving the transucer:

Have a look at Nation Semi's LM1812 Sonar Chip and app note, this is a chip
used for 'Humingbird' fishing sonar's. They use 1 transducer for transmit &
recieve.

I believe the spec's on the cheap 40KHz transducer is 300mw.
Measure the impedance of your transducer.
If it's over 75 ohms, at fr. you will need a driver because your PIC only
does 5volts.

So wrap a ferrite toriod coil to resonate with the transducer.
wrap a driver winding; each PIC port is good for +/- 20ma @ 5v, (100mw) so
you may want to parrallel 3 port lines.

>        1) Bang on the transducer for N pulses, where N < 10, with a pulse
>           frequency selected to match the 40 KHz center frequency.
>
>        2) Wait an integral number of cycles (however long you want the
>           chirp to be) plus 1/2 cycle.
>
>        3) Bang on the transducer again for M pulses (M<N), but now you
>           are 180 degrees out of phase with the transducer oscillation,
>           so this has the effect of _damping_ the oscillation, thus
>           giving a cleaner and quicker pulse.


You only need to do 1), because after you chirp the step-up transformer's
primary, you can 'short it' and allow the PIC's port pins to dissipate the
stored energy, damping the reverberations on the transducer.

After a SHORT damping interval, put the PIC port pins in HI-Z state, so they
don't dampen the transducer when you want to use it as a microphone.


For a reciever, you realy want TDC- time dependant gain control. I like the
idea of ripping off an IR remote module. Maybe a FET in the feedback path of
an op-amp, used as a voltage-variable resistor, and opened after the
transmit pulse through a time constant. Try to use a hi-z input amp directly
off the transducer. Maybe not too hard at only 40KHz, if Z-in is just 1-10
Kohms.

Have fun.

>