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Simple ultrasonic proximity detector Do you hove trouble parking close to a wall or getting out of tight parking spots? This ultrasonic distance indicator will save you from those embarrassing dings and scrapes. By JOHN CLARKE & GREG SWAIN Sometimes it can be really difficult to judge parking distances, particularly if you're trying to back up close to a wall or loading dock. It only takes one mistake and ... kerunch! To solve this problem, we set about designing a circuit that would warn the driver before the crunch 26 SILICON CHIP point. The result is the ultrasonic distance indicator featured here. It can be easily fitted to just about any sort of vehicle and has a range of about 1 metre. The idea behind the device is simple. A couple of ultrasonic transducers are mounted on the rear bumper bar and monitor the distance between the vehicle and any large obstruction which could be a wall or another vehicle. As the vehicle reverses and gets closer, the beep rate from a small piezo transducer gradually increases until, at a preset distance, the transducer gives a continuous tone which says "this far and no further". To make it easy to operate, the ultrasonic ranger is automatically switched on whenever reverse gear is selected. That's done simply by deriving the power supply from the switched line to the reversing lights. You can set the minimum distance trip point to whatever figure you like by means of a trimpot. As an option, you can also set 100 +BV ~+.;::.::;:......----------+--------------....---+BY +12V VIA REVERSING SWITCH 10 100 16VW! 16VW! .,. 100 + 1k 470k 470k + 16VW+ 2.2k 40kHz ADJUST VR1 22k 14 470k nm 22!l IC3 555 10k ULTRASONIC TRANSMITTER SCS•401T 1M 01 1N414B . 40kHz OSCILLATOR BURST OSCILLATOR - - - - - - - - - - - - - - - - - - - - - -- -~-----t1------t-----t1----+BV 1.2k ULTRASONIC RECEIVER SCM-401 4 470k .,. R D 14 15k - 2 IC5 Qlt,:--~IM,--+----, >----1-----13 CK 4013 s 6 1.2k 4.7k 7 RETURN DELAY DETECTOR AVERAGE SIGNAL 4.7k SCHMITT 1.8k 1M .---------+-~--------------..--+Bv 10k 2 'HIT' OUTPUT 10k EXTRA 0ts1:~c E_____. VR3 47k .,. 10k 10 + 16VW! .,. PIEZ0 TRANSDUCER AUDIO TONE RATE AUDIO OSCILLATOR CAR REVERSING ALARM Fig.1: IC2b is the burst oscillator and controls IC3 which drives the transmitter. When the reflected signal is received, it clocks Q-bar of IC5 low. IC5's output, in turn, controls the beep indicator circuit (IC2c, IC4 and IC2d). and select a second minimum distance trip point. This feature would be useful on station wagons and vans which require sufficient clearance for the rear door to be opened. A switch mounted on the dashboard will enable you to select between the two distances. How it worlcs Two ultrasonic transducers and five ICs are used in the circuit. In operation, the ultrasonic transmitter sends out a burst of signal and the receiver then detects the reflected signal from the object. The time taken for the signal to return is used to determine the distance. Fig.1 shows all the circuit details. Let's look at the transmitter circuit first, since this is by far the easiest to understand. IC3 is a 555 timer and is wired as an astable oscillator which runs at 40kHz. The output signal appears at pin 3 and drives the ultrasonic transducer via a 220 resistor. Trimpot VR 1 allows the frequency to be precisely set to 40kHz. Pin 4 of IC3 is the reset input and this is controlled by the output of burst oscillator stage IC2b. When pin 4 is high, the 555 timer oscillates; when pin 4 is low, the 555 stops. JANUARY 1989 27 r300us--------30ms _JlJlJLJUl___ I I ~~y _J I 3 ~3 T_Rs~_:~_fl_T_ _ _ _ _ _ _ _ _ PIN 1 IC2a _ I flJUl JUl ---------1 PIIC!52 _J _ PIN 1 IC2a I SHORT RANGE RETURN SIGNAL _ fUlJ I I..._______________, (LOW AVERAGE VOLTAGE) I LONG RANGE-fUUl RETURN SIGNAL (HIGH AVERAGE VOLTAGE) Q ___.flfill_JUl_ I 1-----.L .. L nnI.._______ _Ju I PIN 2 IC5 Fig.2: the top waveform shows the bursts of 40lcHz signal at the output of IC3. D-type flipflop IC5 is reset at the start of each burst which means that its Q-bar output is set high. When the reflected signal arrives at the ultrasonic receiver, IC5 is clocked and its Q-bar output goes low. It remains low until reset at the start of the next burst. ULTRASONIC RECEIVER I +12V VIA REVERSING SWITCH 100uf ~ GNU r ULTRASONIC TRANSMITTER +PIEZO- TRANSDUCER Fig.3: here's how to mount the parts on the PCB. Make sure that you don't swap the 555 and TLC555 timers because that would upset circuit operation. Leave out VR3 and switch S1 if you only require one . set distance (see text). IC2b is wired as a Schmitt trigger oscillator. A voltage divider consisting of two 470k0 resistors sets the bias on the non-inverting input while a third 470k0 resistor sets the hysteresis levels to 1/3 and 2/3 of the supply voltage. In operation, the .039µ,F timing capacitor charges mainly via D1 and its series lOkO resistor and discharges via the lMO resistor. This is because D1 is reverse biased during each discharge cycle. This gives a train of pulses at pin 14 which are each high for about 28 SILICON CHIP 300µ,s and low for about 30ms. What this means is that IC3 provides a 300µ,s burst of 40kHz oscillation every 30ms. This 30ms gap is to allow sufficient time for each burst to be reflected back to the receiver before the next burst is transmitted (see Fig.2). As well as controlling IC3, the output of IC2b is also wired to the reset input (pin 4) of D-type flipflop IC5. When pin 14 of IC2b switches high, it resets IC5 so that its Q-bar output goes high. As we shall see, it is IC5 that allows the circuit to determine the delay time between the transmitted and reflected signals. Receiver circuit The reflected 300µ,s burst signal .is picked up by the ultrasonic receiver and fed to pin 3 of ICla. ICla is an AC-coupled non-inverting amplifier with a gain of 11 as set by the 47k0 and 4.7k0 feedback resistors. The .01µ,F capacitor in series with the 4.7k0 resistor rolls off the frequency response below 3.4kHz. Following ICla is an identical op amp stage, ICl b, which is also wired with a gain of 11 (ie, the two stages together provide a gain of 121). The amplified 40kHz signal appears at pin 7 of IClb and is applied to pin 6 of Schmitt trigger stage IC2a. DC bias for the noninverting input is derived by filtering the output of IClb via a 1.2k0 resistor and 10µ,F capacitor. IC2a's job is to monitor the output of ICl b for amplified 40kHz burst signal. As soon as the signal input on pin 6 exceeds about 400mV, the output at pin 1 swings high and low (ie, it squares up the signal). This output signal then clocks flipflop IC5. What happens now is that Q-bar (pin 2) of IC5 immediately switches low on the first positive going clock input from IC2a. If you're not too familiar with D flipflops, just remember that the data (D) input at pin 5 is transferred to the Q output by the clock input. Because the D input is connected to the positive supply, this means that Q goes high and thus Q-bar must go low. Fig.2 shows the waveform details. Note that additional clock pulses from IC2a have no further effect on the Q-bar output after it switches low. It remains low until reset high again by the output of IC2b at the start of the next transmitted burst signal. What this all means is that Q-bar is set high at the start of the transmitted signal and remains high until that signal is reflected back to the receiver. The further away the object is, the longer it takes for the signal to come back and the longer the Q-bar output stays high. The output signal from IC5 is PARTS LIST 1 PCB, code SC05102891, 105 x 60mm 1 plastic case, 130 x 67 x 43mm (or larger) 1 plastic case, 83 x 54 x 30mm 1 40kHz ultrasonic transmitter 1 40kHz ultrasonic receiver 1 piezo transducer 1 SPOT toggle switch Semiconductors The PCB clips neatly into the specified case but you can use a larger case if you wish. Use shielded cable to make the connections to the ultrasonic transducers and note how the 3-terminal regulator is oriented. D 7808 3-terminal regulator 1 6V 1W zener diode 1 N4148, 1 N914 diodes LM339 quad comparator LF353, TL072 dual FET input op amp 1 555 timer 1 TLC555CP CMOS timer 1 4013 dual D-type flipflop Capacitors 2 1 OOµF 1 6VW PC electrolytic 4 1 OµF 16VW PC electrolytic 1 .04 7µF metallised polyester (greencap) 2 .039µ,F metallised polyester 2 .01 µF metallised polyester 1 .0068µ,F metallised polyester 2 .001 µF metallised polyester t~ ~ ~ 1 1 2 1 1 ~~~!!:::::~~~~~-~ Fig.4: the printed circuit board is coded SC05102891 and measures 105 x 60mm. filtered using a 15k0 resistor and 10µ,F capacitor and fed to pin 5 of comparator stage IC2c. This stage compares the filtered 'average signal' with the voltage on pin 4, as set by either VR2 or VR3. Switch Sl selects between these two trimpots, which provide the distance set adjustments (ie, the distances at which the transducer emits a continuous tone). Actually, the way in which the circuit works from here is really quite devious. What goes on? Well, comparator IC2c controls oscillator stage IC4, which in turn controls Schmitt trigger oscillator IC2d. Let's look at this in greater detail. The 'average signal' from IC5 is also connected to the frequency control input of IC4. This IC is a We mounted the ultrasonic transducers CMOS version of the .more familiar in a small plastic case. Mount the case 555 timer and oscillates at a frequency of about lHz when its pin 5 so that it can be easily removed when the car is to be washed. is at + 8V. Resistors (0.25W, 5%) 2 x 1 MO, 7 x 4 70k0, 1 x 1 OOkO, 2 X 47k0, 1 X 27k0, 1 X 22k0, 1 x 15k0, 7 x 1 OkO, 3 x 4. 7k0, 1 x 2.2k0, 2 x 1.8k0, 2 x 1.2k0, 1 x 1 kO, 1 x 220, 1 x 100, .2 x 47k0 miniature vertical trimpots, 1 x 22k0 miniature vertical trimpot Miscellaneous Hookup wire, shielded cable, solder etc. But IC4's operation is a bit trickier than that. As the voltage on its frequency control input (pin 5) decreases, its output frequency increases. And this is precisely what happens as the car gets nearer to the obstruction (ie, a wall or another car) behind it. The closer the car gets, the lower the 'average signal' voltage and the higher the output frequency from IC4. This, in turn, increases the audio beep rate. This situation continues until the 'average signal' voltage drops below the voltage on pin 4 of comparator IC2c. When that happens, pin 2 of IC2c switches low and JANUARY 1989 29 For the prototype, the audio transducer was mounted on one end of the case but this will not be pratical for all installations. Mount the unit towards the back of the vehicle to keep the leads to the transducers short. 7 CAR REVERSING ALARM _J L Fig.5: here is a full size reproduction of the front panel artwork. shuts down the IC4 beep oscillator. The pin 3 output of IC4 controls Schmitt trigger oscillator IC2d. When pin 3 is low, D2 is reverse biased and IC2d oscillates at about lkHz and drives the piezo transducer via a 4.7kO resistor. When pin 3 of IC4 subsequently goes high, the .0068µ.F capacitor at the inverting input of IC2d charges via D2 and the IC2d oscillator stops. In practice, this means that we get a series of lkHz beeps which increase in rate as the car gets closer to the object. This beep rate continues to increase until IC2c shuts down IC4. When that happens, pin 3 of IC4 goes low and IC2d drives 30 SILICON CHIP the piezo transducer continuously. Power for the circuit is derived from the + 12V supply rail to the reversing lights. This is fed to a 3-terminal regulator to give a + 8V supply, thus making the circuit independent of changes in battery voltage. Zener diode D3 ensures that high voltage spikes on the supply line are kept out of the regulator. Construction The circuit for the reversing alarm is built on a small PCB coded SC05102891 (105 x 60mm). Fig.3 shows the wiring details. Start construction by installing The completed unit can be easily tucked away in a corner of the boot. Power for the circuit comes from the + 12V rail to the reversing lights. all the low profile components first. These parts include the resistors, diodes, wire links and ICs. Note that the ICs, diodes and electrolytic capacitors are all polarised parts and must be correctly oriented as shown in Fig.3. The 3-terminal regulator must also be installed the right way around. It should be mounted with its metal tab facing towards the adjacent edge of the PCB (see photo). Push the regulator down onto the board as far as it will comfortably go before soldering the leads. Attention can now be turned to the wiring. Shielded audio cable is used for the connections to the 40kHz transducers while all other connections can be run using medium-duty hookup wire. Be sure to make the leads long enough to reach their required destinations when the unit is mounted in position. Switch Sl and its associated wiring can be deleted if you only require one minimum set distance. If you do decide to leave Sl out, delete VR3 also and connect a wire link between the wiper of VR2 and pin 4 of IC2. Just wire the link between two of the vacant pads that would normally be used to terminate the switch wiring. The reversing alarm can be bench tested by connecting it up to a 12V power supply. Switch on and check that there is + 8V at the regulator output and on the IC supply pins. Now lie the two transducers side by side away from any obstructions and check that the piezo transducer beeps at about a one second rate. If this checks out, move your hand close to the sensors. The frequency of the audio beeps should increase. If the beeps don't increase in frequency, try adjusting trimpot VRl. This trimpot sets the transmit frequency and should be adjusted for maximum sensitivity. VR2 and VR3 should be adjusted to give the desired minimum set distance (ie, a continuous audio tone] for each setting of S1. Installation We housed the prototype PCB in a plastic case measuring 130 x 67 x 43mm but you can use a larger case if you wish. The ultrasonic sensors were housed in a separate small plastic case measuring 83 x 54 x 30mm. The PCB is secured to the inside of the plastic case using the integral side pillar supports. You will have to drill holes in the case for the screened transducer leads, the power supply wiring and leads for the piezo transducer. Just how you mount the unit will depend on the type of vehicle you have. In any case, the unit should be mounted towards the rear of the vehicle (eg, in the boot] to keep the leads to the ultrasonic transducers short. This will also make it easy to tap into the reversing light circuit for the power supply rail. The two ultrasonic transducers can be mounted below the rear bumper bar and secured using a suitable bracket. We suggest that you come up with some sort of plug and socket arrangement so that the transducers can be easily removed when the car is to be washed. If the transducers do get wet, they will operate normally again after they have dried out. But be warned - if the transducers are saturated with water (eg, when it rains] you will not be able to rely on the circuit when parking. !§;l .,..__,SPECit~1tR ~ <:;;:i vKa ::.-:.:::=:::;: NEW SPEAK 1lw-:::::: ]1~l I-lti I,,,: %t ::.¾1~ Since the introduction of VIFA speaker kits in Australia in 1985, thousands of speakers have been built with superb results . VIFA is now proud to release four new speaker kits ranging from a mere $399 to $1199 per pair including cabinets. Never before have speaker kits been so popular in Australia than after the heavy devaluation of the dollar. Similar fully imported quality loudspeakers are today typically 2-2½ times more expensive. 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(02) 938 4690 I ./ ... VDC ... / ... VDC. OR Cut out and post this coupon to Post Office Box 740 ~ B _r_oo_k_v_a1_e,_ N._s_.w_._ 2_1_o_o___ I Please post me Name details of the fol lowing values: VDC ... / .. VDC (Please print) I Address L~~~~~- ~~~~~~J JANUARY 1989 31