alternately turn on using the microcontroller signals labeled
TX+ and TX-.
The bottom switch is straightforward: An NPN
transistor (Figure 6) will switch on when its base resistor is
presented with the three to five volt signal from the
microcontroller. Its collector can withstand the high voltages
while isolating the microcontroller from such potentially
The high-side switch is a little more involved. A PNP
transistor makes a good complementary switch, but the
base voltage of such a transistor will
always be within one diode drop from the
high voltage, and therefore cannot be
driven directly by the microcontroller.
Instead, an additional common-base
NPN stage is added to interface between
the microcontroller and PNP output
(Figure 7). Note that Q3 is turned on
when TX+ goes low.
Putting these together gives the
transmitter drive circuit of Figure 8.
Note the following things about this
circuit. First, diodes D1 and D2 have been
added across the transmitter. Neither of
these will turn on unless the transducer voltage goes one
diode drop higher than the +30V supply voltage or one
diode drop lower than ground! Why would we need
It turns out that when we drive the transducer at its
resonant frequency, then suddenly stop changing the
voltage, the transducer diaphragm continues vibrating
(remember this is a resonant contraption).
As the diaphragm flaps around, it actually produces
voltage (it’s a piezoelectric crystal), so it can produce
voltage well above +30V or well below ground as it rings
Such a situation probably won’t wreck your circuit,
but the diodes are there to make sure you don’t have to
test that theory. Figure 7. High-side transducer driver.
Figure 8. Complete ultrasonic transmitter circuit
(receiver transducer and circuit separate).
Figure 5. Ultrasonic transmitter block
Figure 6. Bottom switch for ultrasonic driver.
38 SERVO 03.2018