In the past the sounding (sea bottom measurement) was done with the `bullet`, that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the `bullet` reached the bottom, the depth appeared directly from the calibration of the rope. This arrangement still exists in some yachts. The big disadvantage of this method is that it can be used only at a stop position or very low speed, and that it is also not easy for deep depth measurements. The electronic sonar that we are going to construct does not suffer from these drawbacks, and its indication can be done in the cockpit along with the other navigation instruments. It is essentially a sonar system that measures the amount of time between the emission of an ultrasonic pulse and the reception of its reflection from the bottom. An acoustic transducer called an underwater sound projector is used for the ultrasonic emission, while the reflected signal is received by a hydrophone.
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The usual arrangement of the electronic sonar consists of an ultrasonic transmitter that emits a short 150-200kHz ultrasonic pulse. This pulse is reflected in the bottom and its echo is detected by the hydrophone. The hydrophone converts the echo into an electrical signal, used to illuminate a small neon light moving at a fixed frequency by a motor, on a concentric calibrated disk. Thus, the neon light illuminates in the subdivision corresponding to the measured depth.
Since the ultrasound emission happens at the moment that the lamp passes from zero, the scale calibration of the disc indicates the depth directly. Experienced seafarers can still understand the type of seabed, from the way the light comes on. For example, the sandy bottom causes a brief glimpse, the rocks a longer fuzzy flashing, while the soft bottom causes a more protracted blink on a fuzzy basis.
Our construction has a digital sign that unfortunately does not give any detailed information about the type of the bottom. But it is smaller and gives a more accurate indication of the depth.
As the functional diagram shows, it is easy to build. An interesting simplification is that the transmitter and the hydrophone are embedded in the same housing. The transceiver connects to IC9, (LM 1812 of National Semiconductor).
The Sonar circuit
The ultrasonic pulse travels twice the distance of the water depth. As the average sound speed in the water is 1500 m/s (at 20°C and salt content 2%), the time it takes for two-way depth coverage, e.g. 7.5m is 10ms. If, therefore, the timer frequency of the IC1 is 750Hz and pulse arrivals at 10ms are recorded, the sounding is 7.5m. However, since the display only represents integer numbers, the display will be 7m. For more precise readings, the clock frequency can be 7500Hz, so the sounding will be accurate to the tenth of a meter.
Display, memory, and imaging drive are contained in IC1. When the echoes are received, the display receives a stop pulse from the IC9. The display information then goes into memory and is finally displayed in the seven-segment LEDs.
The new measurement cycle then begins with a zero-cycle pulse producing the IC5, every 200ms. At most 1500 beats can be counted. This means that the circuit can be used for depths up to 1500 tenths of a meter, ie up to 150 meters. The reset signal performs two other functions, starts the pulse transmission and activates the alarm via MMV4 and FF2. The output of FF2 announces the existence of "shallow" if the output level of MMV4 is logic 1 at the time of echo detection. The alarm threshold is adjusted with P1 from 1m to 10m.
Monostable MMV3 turns off the display when there is no echo detection for a certain time, set by P2. When no echo is received, LED D2 remains off. The display works until MMV2 is changed. When an echo is received, the D2 immediately starts blinking.
It is worth examining IC9 more closely. which is the heart of the device. Next diagram shows the individual steps of IC9 along with the necessary peripheral element.
When the IC5 provides a pulse duration of 0.5s every 200ms, the IC9 pin 8 activates the built-in modulator and generates the pulse to transmit the ultrasound which in our case is 200kHz. The modulator and the second high frequency amplifier (h.f.) share the coordinated circuit L1/C14. In the broadcast, this circuit is connected to the modulator, while the receiver is connected to the amplifier. This ensures the same tuning frequency in the broadcast and reception. The absolute value of this frequency is not particularly critical.
The output stage amplifies the 200 kHz pulse signal and drives the ultrasonic transmitter through the transistor driver T8 and the coil L2. L2, distributed transmit capacitance and C22 form a tuned circuit at 200kHz.
In the interval between transmission pulses, an echo is detected and evaluated. It is applied to the first high frequency amplifier (h.f.) and then via P4 to the second amplifier h.f. which is now connected to the coordinated L1/C14. The potentiometer adjusts the sonar sensitivity. The output of the selector amplifier drives a level detector that reacts for signals above a certain level. The noise pulses, present in the receive signal are discarded with a combination of pulse repeater and integrator detector. If the pulse train is interrupted, the pulse repeater detector judges the echo received as sporadic and causes the C15 integration capacitor to discharge.
If the received pulses are very short (eg, noise pulses), C15 is not fully charged and the pulses are rejected as random. However, if the pulse repeater detector reaches pulse of true echo, driving the imaging is activated. A protective circuit stops the display if it works for too long. This is done by charging C19 from the driving signal: When the C19 is charging, it runs a transistor built into the IC.
C9 ensures that amplification of the second h.f. is small immediately after the transmission of a pulse so that any oscillation of the transmitting element is not echoed. So the minimum measurable depth is about 2m. If this limit is not acceptable, the C9 value may be reduced. Note that in this case the sensitivity of the device must be reduced.
Construction and assembly
The most interesting point of construction is the installation of the transmit/receive element. Next diagram proposes some solutions.
It is essential to be positioned perpendicular to a conceivable line along the vessel and also perpendicular to a conceivable line corresponding to the width of the vessel. It may be necessary to finally place the transmit/receive element in an adapter box, as shown in above diargam. If the hull is fiberglass, the whole device can be placed inside the hull. The connection cable of the cell with the rest of the circuit must not be tied to other cables, in order not to be affected by noise pulses, which would degrade the operation of the circuit. Caution here: DO NOT shorten the cable to the transmit/receive element! If you already have such an item. you do not have to buy a new one, since what you have is almost certain that it will work well with the Sonar circuit.
The VDO Echo Soynder Modis 120 (operating at 200 kHz), Sacece, Euroromarine, Seafarer (all running at 150 kHz) have transmit/receive elements that are difficult to distinguish. You will find these items in most marine electrical/electronic equipment stores.
The construction of the circuit board on the board is a toy for children, compared to the difficulty of positioning the transmit/receive element. The L2 coil has to be wound in hand but the L1 can be purchased ready. The three-digit display is made on a second board.
The voltage stabilizer and its refrigerator are placed on the copper face with suitable insulators or, after being suitably insulated, on one of the walls of the box. Between the two boards, a metal sheet must be inserted for shielding. The contacts of the two boards bearing the same symbols must be connected to each other.
Caution: The ground connection is not on the same side as the board with the CL.
DS contact on the same board must be bridged on the ground and the DP is connected to +5V.
The box may be plastic or metallic but impermeable. The axes of potentiometers, switches, LEDs and sockets must be waterproof during installation. The red display window should be stuck in the box with waterproof glue. Do not forget the connections at 12 ± 2V. The settings must be made before the boards are placed in the box.
First set P4 for maximum sensitivity of the receiver. Then place the transmit/receive element vertically and within 0.5m of a reflective surface. If the item has already been placed in its permanent position, put a reflection surface at a distance of 0.5m in front of it (the boat is not in the water !!). Then set the core of L1 until the display shows 2.3m. This is because the sound in the water spreads at only 0.217 of its velocity at sea. Since the target distance in the air is 0.5m, the equivalent in the sea would be 0.5m / 0.217 = 2.3m.
Then change the distance between the transmitting/receiving element and the reflecting surface. In the air the change is from 0.5m-1.5m, corresponding to a depth change in the sea from 23-6.8m. The depiction should show changes in distance. If not, the L1 core needs to be adjusted to achieve true maximum sensitivity.
If you have an oscilloscope, the settings are somewhat simplified. WARNING, because if you touch the oscillator terminal at the same time on two pins of IC9. you will need a new IC9.
Connect the oscilloscope probe to IC9 pin 1 and synchronize the oscilloscope with the IC9 pin 3 signal. Then set the core of L1 for maximum echo amplitude. which is visible a few ms after the transmission pulse.
The current consumption of the sonar with the display in operation is about 200mA or an average of 40mA at 12V.
Some final details
Coil L2 is handmade. It is wrapped in a suitable core of approximately 18mm in diameter and 11mm high. The inductance of the secondary winding L2b must be such that the resonant frequency of the circuit forming the L2b, the distributed transmit/receive transmittance and C22 are identical with the same transmit/receive element frequency.
This frequency is given by the relation f = 1 / 2π x L x C, where f is the resonance frequency in Hz, L is the inductance at H and C the total capacity in F.
Reversing the terms, L = 1 / 4π2 x f2 x C, and for f = 200kHz, C = 3n2 we have L2b = 198 μH.
The corresponding number of turns N is calculated from the relation N = L2b / Ls. where Ls is the specific inductance of the ferrite core. If, for example, Ls = 250nH, the number of turns becomes 28.
If the coil ratio is 1:9, L2a must have 3 turns.
If a ferrite core with a different special inductance value is used, the above calculations should of course be re-generated. The turns ratio can be held at 1:9. Correspondingly, if a different transmit/receive element is used, L2 inductance must be recalculated.
Also, if the frequency is not 200 kHz, C14 must be recalculated by: C14 = 1 / 4π2 x f2 x L1, where f is the new frequency and L1 = 630μH.
The depth at which the alarm is triggered for "shallow" is determined by the following relation: depth (m) = 9 x 106 x (P1 + R16 + R17) where R16, R17 and P1 are in Ω.
If the transmit/receive element is not in the deepest point of the vessel, measure the distance Dk (depth difference) between the position of the element and the deeper point of the keel.
Replace 4098 (IC6) with 4538. Make C9 12n and connect in series with R13 a resistance Rk whose value is calculated from the relation Dk = 9 x 106 x (Rk + 104), where Dk is in m and n Rk in Ω.
Therefore, Rk = (106 Dk / 9) - 104.
For example, Dk = 1.5m Rk = 157k. After that, the display will show the distance of the sea bottom from the deepest point of the vessel and not from the position of the transmitting/receiving element.
Caution: When setting P1, Dk should be taken into account.
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