Solid State
BUILD YOUR OWN SONAR SYSTEM By Lou Garner
SONAR  the name alone has an used as a submerged object detector, essential electronic circuitry for a com-
exciting quality, whether you're old depth finder, or fish locator, or, with a plete sonar system within a chip area of
enough to remember World War II, few modifications, for underwater data only 80 by 93 mils. Affectionately
young enough just to have read about it transmission and remote control appli- dubbed the fishfinder by the firm's
in history books, or simply a viewer of cations. If you're not a yachtsman application engineers, the device, type
the late-late TV movies. You have (yachtsperson) or fisherman (fisherper- LM1812, was released just recently for
visions of determined, steely-jawed son), you can use the same IC to general distribution, although it has
destroyer skippers searching relent- assemble an air ranging version called been in production on a semi- custom
lessly for killer U-boats. You hear the sodar (for SOnic Detection and Rang- basis for over a year. Joining the manu-
 ping-ping-ping background sound as ing) suitable for remote sensing, facturer's growing family of special-
a tense and sweating American subma- collision avoidance, and intrusion or purpose devices, which includes the
rine commander attempts to elude an burglar alarm systems. LM3909 LED flasher, discussed in last
enemy patrol. Now, with a little skill, a Utilizing a number of novel circuit year's July and October columns, and
dash of patience, a single IC, and a few design techniques, engineers at the the NSL4944 universal LED, examined
accessory components, you can build National Semiconductor Corporation in our May issue, the LM1812's unusual
your own sonar  not a military ver- (2900 Semiconductor Drive, Santa circuit contains a 12-watt ultrasonic
sion, to be sure, but a practical down-to- Clara, CA 95051) have developed a transmitter and a selective receiver fea-
earth (water?) instrument which can be monolithic IC which contains all the turing a 10-watt display driver. Despite
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Popular Electronics, July 1976  Copyright
Gernsback Publishing, reproduce for personal use only
its high peak power capabilities, the IC, through its external transducer, which off, momentarily disabling the receiver
in an 18-pin Epoxy B molded DIP, can also serves as a pick-up device. section. The sine-wave signal is inter-
be operated without an external heat Between pulses, the receiver section is nally amplified and squared, then
sink in most applications. activated to detect any returning signals applied to a one-shot multivibrator,
Designed for use on standard 12-volt reflected by solid surfaces, such as a where each leading edge triggers the
dc sources, the LM1812 has a maxi- lake or river bottom, schools of fish, or generation of a 1-µs pulse. Applied to
mum supply voltage rating of 18 volts, submerged objects. These echo signals the power amplifier, each pulse drives
coupled with a maximum power dissi- are detected and amplified, then used to the stage into saturation, resulting in
pation of 600 mW. Its specified drive the output display. The time dif- high-efficiency class-C operation. The
operating temperature range is from 0°C ferential between the original amplified 200-kHz output signal is then
to +70°C. Under normal operating con- transmitted pulse and any returning sig- coupled to the piezoelectric transducer
ditions, its receiver section has a typical nals is directly proportional to the by means of an impedance matching
sensitivity of 200 µV p-p, with its dis- distance from the object(s) causing the step-up auto-transformer, L2. The final
play driver supplying a maximum echo, permitting the output display to be transmitted signal, then, is a narrow
current of 1A for 1ms. The unit's trans- calibrated in distance units (feet or burst of 200-kHz sonic energy. At the
mitter power output stage is capable of meters) rather than time intervals. end of each timing pulse, the transmit-
delivering a 1-A, 1-µs pulse to a suit- A single resonant circuit, L1-C3, ter stages are deactivated and the
ably matched load. Although generally time-shared by both the receiver and receiving section gated on. During this
used with a neon bulb or LED output transmitter sections, establishes the sys- period, and until the next timing pulse is
display device, the LM1812 can be used tem's exact frequency of operation, thus applied, returning (echo) signals picked
in conjunction with a clocked digital eliminating the need for special align- up by the transducer are applied to the
readout or a CRT display. ment procedures and insuring that the receiver through coupling capacitor C1.
A basic sonar system using the two sections track over a relatively wide An external gain control, P1, is pro-
LM1812 is illustrated in Fig. 1. As in temperature range. The system's trans- vided between the first and second r-f
most conventional sonar systems, the mit mode is activated with the amplifiers, coupled to the second stage
basic design employs the  echo-rang- application of an externally generated through dc blocking capacitor C2. From
ing principle  that is, the system positive-going timing pulse to the mod- here, the amplified signal is applied to a
transmits short, high-intensity ultra- ulator control, pin 8. At this point, the threshold detector which responds only
sonic pulses at fixed intervals and gated oscillator is switched on, develop- to signals above an established level.
detects any resulting echoes. In prac- ing a controlled sinewave signal across Impulse noise is rejected by the com-
tice, the LM1812 transmits pulses of resonant circuit L1-C3. Simulta- bined action of the pulse train detector
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about 200 kHz for approximately 80µs neously, the second r-f stage is gated and integrator stages. The two circuits
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Popular Electronics, July 1976  Copyright
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require a reasonable number of signal be used, including digital control and a typically 40 kHz rather than 200 kHz.
cycles for operation. If there is not a clocked readout or an oscilloscope dis- The basic circuit modifications needed
continuous train of pulses in the ampli- play with a calibrated linear sweep, but for operation in an air medium are given
fied signal (if 2 or 3 are missing, for one popular method is illustrated in Fig. in Fig. 2. A different transducer is
example), representing a valid echo, the 1. Here, a small permanent magnet and required, of course, together with a
pulse train detector will  dump the a neon bulb are mounted near the rim of matching drive coil, L2. In addition,
integrator, discharging the integration a wheel, with slip rings provided for bypass and coupling capacitor values
capacitor to ground. On the other hand, applying a voltage to the bulb. The should be increased as indicated and the
if the signal is valid, the display driver wheel is rotated by a constant-speed dc tuning elements (L1 and C3) changed to
is switched on, activating the display motor. The magnet serves to generate achieve 40-kHz resonance, while an
device. An additional protective circuit modulation pulses inductively as it external  pulse stretcher must be added
momentarily disables the receiver if the passes a fixed pickup coil, L3. The neon to lengthen the drive pulse from 1 to
display driver is kept on for too long a bulb serves as the display device, driven 5µs. Driven by the LM1812, the pulse
time period; this is accomplished by by the receiver's output stage through stretcher consists of a simple RC inte-
feeding back a signal from the display transformer T1. Shunt diode D1 is gration network and pnp power driver.
predriver stage to integration capacitor included to suppress switching tran- Except for these few changes, the cir-
C8 which, in turn, furnishes a control sients, while a series filter, R2-C9, is cuit arrangement and component values
bias to the duty-cycle control transistor. provided to limit excessive current build are identical to those of the system
Although the circuit's basic operation up in the transformer's primary under shown in Fig. 1.
is the same whether it is used for sonar, rapid flashing conditions. The trans- When using the LM1812 in practical
data communications, or remote con- former must provide a substantial designs, special attention must be given
trol, the external drive and output voltage step-up (from 12 to 100 volts or to ground loops and common coupling
circuitry must be altered to meet indi- more) to insure flashing the bulb. In paths due to the close proximity of
vidual system requirements. Generally, operation, the wheel's position at which transmitter and receiver circuits in the
much less power is needed for commu- the initial pulse is transmitted is consid- same package. Three ground pins
nications and remote control ered  0, while the arc length traveled (5,10,15) are provided on the device to
applications than for echo ranging since by the bulb before it flashes an echo simplify layout problems, but the
the latter requires signal transmission represents the time required for the ground path(s) still must be adequate to
over twice the distance (to the target ultrasonic pulse to travel to the target handle peak currents of as much as 2
and back). In remote-control systems, and back. Since this time period is amperes when the transmitter and dis-
the display unit might be replaced by a directly proportional to target distance, play are energized simultaneously.
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relay or control device, such as an SCR a fixed calibrated scale can be posi- Local sources of high-energy impulse
or power transistor. On the other hand, tioned around the wheel to indicate noise, such as lightly loaded motors, if
if the LM1812 is used for communica- distances in feet or meters. Within sys- not shielded properly, can cause errone-
tions, a high-impedance detector and tem sensitivity limits, the sonar's ous display signals or  blips. Ideally,
audio amplifier should be connected to maximum scale range is determined by these noise pulses should be filtered at
pin 1 for reception, with another used the repetition rate of the transmitted the source, but their effects can be mini-
for modulation. Of course, a single pulses, for echoes can be received only mized by connecting a small capacitor
amplifier can be used, if preferred, during the intervening intervals. With a (about 30 pF) across the first r-f stage
switched back and forth between the system design similar to the one shown (between pins 3 and 4) to reduce ampli-
modulator and receiver sections for in Fig. 1, then, the scale range is deter- fier bandwidth. Finally, for optimum
transmission and reception. Variable mined by the display wheel's rotational overall performance and maximum effi-
rate pulse or other modulation tech- speed (hence motor rpm), for this deter- ciency, the transducer driver coil (L2 in
niques may be used for digital data or mines the pulse rate. Figs. 1 and 2) should be designed to res-
code communications. Considering the relative attenuation onate at the proper frequency (200 kHz
Naturally, some means must be pro- of high-frequency ultrasonic signals in for water and 40 kHz for air systems)
vided for measuring the time interval water and in air, a much lower operat- with the sum of all output circuit capac-
between the transmitted and echo pulses ing frequency is recommended when itances, including distributed wiring,
when the LM1812 is used in a sonar the LM1812 is to be used in air trans- that of the coax cable feeding the trans-
system. Any of several techniques may mission systems, such as sodar  ducer, and the transducer itself.
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Popular Electronics, July 1976  Copyright
Gernsback Publishing, reproduce for personal use only