Ultrasonic Circuits
These ultrasonic circuits are all quite old: my notes date them at mid-70s
so they don't use ICs. Nevertheless there are several places where an op-amp
would possibly simplify things. Despite their age I hope they are of interest:
certainly basic principles don't change.
Ultrasonics: remote control and Intruder detector
This transmitter is designed to work with the next circuit as a remote
control transmitter/receiver. It is only a single channel and you could
once get multi-channel chips for the whole job. However ultrasonics have
fallen out of favour commercially so I think most of these chips are now
obsolete.
Utrasonic transmitter
This is a fairly commonly published self-oscillating driver for ceramic
(piezo) resonators. However with some resonators it can be a bit temperamental:
the 1n0 capacitor across the top transistor of the totem-pole helps it
to start up. Some resonators won't need this.
Also, the impedance curve of a piezo device is very complex and I found
that some of them resonated at the wrong frequency (too low). A 1n0 cap
in series with the resonator cured this - this cap is not shown here.
The two transistors to the left are a conventional multivibrator oscillating
at 480hZ to modulate the transmitter. If I re-designed the circuit today
I would use a 555 here as the old multivibs used to have problems starting
sometimes. Use of the assymmetrical resistors and capacitors values helps
this.
Utrasonic receiver
This is the matching receiver. You should note that the circuit easily
divides into 3 stages (each two transistors) with a final transistor driving
the relay.
Stage 1 is the preamplifier: it is a fairly conventional circuit wilth
gain fixed by feedback.
Stage 2 is a second amplifier: you may notice a Wein network formed
by the input and feedback into the base of Tr3. This tunes the response
to around 40kHz.
The output from the second stage is rectified and fed to the third stage
which is tuned to the modulating frequency by a twin-T feedback network.
This picks out the modulating frequency. The ouptut from the third stage
is fed to the relay driver which is arranged as a rectifier so it stays
on when the modulation frequency is present. Note the presence of a 25µ
capacitor and the absence of a flywheel diode on the relay: this is another
way of removing the switching spikes. It works pretty well - as long as
you don't require fast relay switching since each time the relay operates
the transistor has to discharge the capacitor which (if you don't get the
values right) can cause a large current surge in the trasistor.
Utrasonic Intruder detector
The next circuit is an ultrasonic intruder detector. It uses Doppler shift
from a moving object. Ultrasonics aren't used much for this commercially
as they can be a bit temperamental to set up and tend to detect cats amd
curtains moving in a draft. Modern detectors tend to use a narrow beam
microwave system which only scans a small area such as a doorway. Ultrasonics
tend to scan the whole room.
The transmitter circuit is similar to that already shown. Not identical
(I think I did this one first - but it was long ago). ThePreamp (Tr5 &
6) the same as the one in the receiver, but with different values (both
work - you can always recalculate values starting with a different initial
point and they'll both work).
Tr7 takes the received signal and combines it with the transmitter's
signal to get a beat frequency. This is fed via a sensitivity control and
Tr8 to a rectifier stage and a threshold detector. This turns on the relay
once the beats exceed a preset level. Theis detector is latched so the
relay remains on as is required for an intruder detector.
Utrasonic Preamp with AGC
The next circuit shows how an ordinary diode can be used as a variable
resistance element to incorporate automatic gain control.
Utrasonic Preamp with Twin T tuning
The last circuit is an ultrasonic preamp with a twin T feedback to make
it highly tuned.
Afterword
At the time I did these circuits I was writing a series of projects up
for the British edition of Popular Science. The magazine folded after only
a short run. One of the circuits (the intruder detector, I believe) was
published in the last edition but none of them were really much more than
experimental so never got sustained tesing. Nevertheless they should prove
interesting and I hope you enjoy them.