The second thing to note about this
circuit is that it is transmit only. To
receive the ultrasonic echoes, a second
transducer is used strictly for receiving.
This transmit-with-one-transducer/receive-with-a-second-transducer is the configuration used in
most hobbyist setups.
Since ultrasonic transducers
theoretically transmit and receive, some
transducers are sold as “transceivers;”
that is, devices capable of either mode.
Since the transmit and receive periods
are usually set up to occur at mutually
exclusive times, having a single
transducer to handle both functions is
entirely feasible. Figure 9 is such a
The trick here is to be able to deliver high voltages
during the transmit period without wiping out low-voltage
circuits in the receive amplifier section. This is made possible
by the diodes D3 and D4 along with R3, with the back-to-back diodes connected to VREF, a voltage midway between
the 3.3V (or whatever the supply voltage is for the receiver)
Also, when the circuit is to be in the receive mode,
there needs to be a means of putting the transmit drive
circuit into a high impedance state. Fortunately, this is
already built into the transmit drive. When TX+ is high and
TX- is low, both sides of the transmit drive circuit are off.
Most microcontrollers are quite capable of accurately
producing the 25 kHz, 40 kHz, or whatever other frequency
is needed for transmission. One thing to keep in mind when
doing this is that as with any half-bridge (totem-pole) circuit
like the ones in Figures 8 and 9, it’s good practice to first
turn off the active output transistor before turning on the
other output transistor, since turning one on simultaneously
with turning the other off may result in a momentary short
across the high voltage.
These first two articles have been all theory: first about
ultrasonic transducers; and then about the electronics
needed to drive those transducers. The next article will
show the actual circuits, plus a surprise announcement of a
new and unique approach to precision ultrasonics. SV
SERVO 03.2018 39
Figure 9. Ultrasonic transceiver circuit.
· 10mm-300mm stroke
· 6v-12v power supply
· 25kg+ available force
· 15g-100g net weight
MICRO LINEAR SERVOS
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