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Ultrasonic PARKING RADAR Do you park by the “touch and go” method – touch the car behind you, go forward a bit, touch the car in front, go back a bit . . . ? Shame, shame, shame! But even if you’re not a careless parker, this little radar warning unit could help get you into tight spots! Parking a car is a real problem for many people. They can’t! Unless there is a shop window to reflect off they simply have no idea how to judge the distance between the back of their car and the front of the next. They either make life very hard for themselves, making what should be a three-point parking a ten or twenty point saga (you think we’re joking?) or, worse still, stop reversing when their tow bar has made a nice little scallop in the number plate of the car behind. Even a slight touch on a modern car can cost thousands of dollars to 38  Silicon Chip repair, especially where integral, moul-ded (or non-existent) bumper bars or too-sensitive air bag sensors are involved. So what’s the answer? Here it is – a small ultrasonic transmitter/receiver which warns you when you’re getting too close. It’s designed to fit to the back bumper (or other suitable location) of your car and sounds a buzzer or closes a relay when you’re within the range you set – anything from about a metre down to just a few centimetres. While designed for the specific purpose of parking, there are other Design by Branco Justic* applications where you might want to sense objects that come within range – security is one which springs to mind, perhaps even things like vehicles or other objects going past or through a small opening. It’s all housed on one PC board (even the ultrasonic transducers) which can all fit into a small disposals plastic case, ready for mounting on the car. Circuit description The circuit can be divided into three sections – a high gain amplifier based on transistors Q1 & Q2 and the ultrasonic transducers, a rectifier (D1&D2) and an output switch (actually two output switches) based on IC1c and IC1d. Two cascaded amplifier stages based on Q1 and Q2 form a potentially high gain amplifier, with its gain set by VR1. This amplifier has a 40kHz ultrasonic receiver transducer connected to its input and a 40kHz ultrasonic transmitter transducer connected to its output (ie, Q2’s collector). So there is a potential feedback path between output and input. Normally, the overall loop gain is set to be less than one but if an object comes into reasonably close proximity and reflects enough energy from the ultrasonic transmitter to the ultrasonic receiver, the gain increases to the point where it exceeds one and the amplifier will then break into oscillation. The loop gain includes the acoustic feedback between the transducers (loss) and the gain of the amplifier (gain). The distance at which the oscillation will first occur depends on the mechanical setup, acoustic isolation between the ultrasonic transducers and the setting of VR1. We'll look at the mechanical considerations shortly. The oscillator output is rectified by C4, D1, D2 and C5 which form a “diode pump”. The detected voltage across C5 is added to the voltage across C6, which is set by trimpot VR2 connected across the 6.2V supply. The total voltage is applied to the input of NAND gate IC1d. When this exceeds approximately 3.1V (half the supply voltage), IC1d’s output goes low which allows capacitor C7 to quickly charge via diode D3. When C7 is charged the input to IC1c goes low so its output goes high. Transistor Q3 is turned on, energising the load connected to its collector. This load could be an electro-mechanical buzzer or a relay with diode D5 connected across it. At the same time, the LED connected to Q3’s collector lights up. The high output from IC1c also enables the 3kHz oscillator based on gates IC1a and IC1b and therefore the buzzer “buzzes”. C7 will begin to discharge via R6 after the input amplifier (Q1 & Q2) stops oscillating; ie when the object that caused the oscillation is moved away from the transducers. When the Ready to mount on the bumper bar or other suitable location, the Ultrasonic Parking Radar is simple to build, automatic in use and could save you $$$! voltage falls below 3.1V the output from IC1c goes low and the LED, buzzer and/or relay turn off. There is a test link on the PC board to assist in setting the unit up. With no test link it takes C7 about 10 seconds to discharge. With the test link in place R10 is in parallel with R6 and the time is reduced to about one second. If you wish to adjust the time later, it is simply a matter of changing R6 – smaller values for shorter times, larger values for longer times. Construction Start by checking the PC board for any flaws, defects or undrilled holes. Then it is simply a matter of mounting and soldering the lowest components (resistors and diodes) first, followed by the capacitors, the trimpots and finally the transistors, IC and LED. Note that all semiconductors and electrolytic capacitors are polarised and must be inserted the way shown on the component overlay. The test link should be installed at this stage – use one of the resistor pigtail cut-offs. PC stakes can be used for external connections – the + and - power wires and the wires to the buzzer and/or relay. The ultrasonic transducers can be soldered directly to the PC board or mounted remotely via suitable lengths of shielded (coaxial) cable. If you mount them on the board, they can be on either the component side or copper side of the board. On the component side, though, you will need PC stakes as the leads will not be long enough to allow mounting on edge (ie, facing off the edge of the board). The transducers are not polarised but there is a difference between the transmitter and receiver: one is branded “S” and the other “R”. Guess which is which? (A clue: R stands for receive). The ultrasonic transmitter and receiver transducers are mounted flush with the case edge. Alternatively, they could be mounted externally to give even wider acoustic separation, thus increasing the range of the unit. FEBRUARY 2000  39 Fig. 1: the complete Ultrasonic Parking Radar. It's your choice whether the output is a relay or buzzer. Fig.2 (above): the PC board component layout. Use this in conjunction with the same-size photograph below and you should have no problems at all assembling the board. 40  Silicon Chip Setting up The range of this unit depends to some degree on the acoustic separation between the transducers. With them mounted as shown in the plastic box, the circuit works quite satisfactorily but it would probably work even better with more separation. Some experimentation may be necessary to achieve maximum range. We have also found that, in this box, a small piece of polyurethane foam placed between the transducers will improve the range of the system. Before you get to that point though, you will need to set the trimpots (VR1 and VR2) to at least get the circuit operational. Turn VR1 and VR2 fully anticlockwise. These settings correspond to minimum amplifier gain (VR1) and minimum trigger threshold voltage (VR2). Rotate VR2 clockwise until the LED just lights, then back it off slightly until the LED extinguishes. This procedure sets the threshold of the trigger point. At this stage the unit still cannot be triggered by approaching objects as the amplifier gain is set to zero (VR1 is fully anticlockwise). Increase VR1 (clockwise) by small increments, checking with a solid object brought in front of the transducers at a distance of say, 500mm. You should find that a point is reached where triggering is reliable. If you want a shorter range, back VR1 off a little. Conversely, a longer range can be achieved by increasing the amplifier gain (ie, increasing VR1) but beyond a certain point the unit will be permanently triggered, even with no objects placed in front of the transducers. This is because the loop gain Parts List 1 Ultrasonic Radar PC board, 96 x 50mm 1 plastic case to suit 1 panel label to suit 1 piezo or electro-mechanical buzzer and/or 12V coil relay (see text) 2 10mm M3 screws and nuts 2 5mm M3 spacers Here’s how it all fits together in the disposals case from Oatley Electronics. The buzzer shown is mounted under the dash or similar location inside the car. is now greater than one, producing permanent oscillation. If you need higher range, the only way that this oscillation can be stopped is to introduce more acoustic separation between the transducers. When the desired settings are achieved, the test link can be removed and the unit mounted in an appropriate position on the vehicle. Note that both the case (or the transducers if mounted remotely) will need to be fairly well waterproofed if placed in a position where they can be rained on or splashed (and that’s most useful positions on the rear of the car!). Waterproof ultrasonic transducers may be available shortly but at a higher cost. Power for the unit is most sensibly taken from the reversing light circuit, so that it is powered only when you are reversing. Identifying a reversing light Where to get the kit This kit is available only from *Oatley Electronics, who hold copyright on the design and PC board. A complete kit of parts including the case, label and some cable is available for $24.00. A short-form kit, including the PC board, all on-board components and transducers, is $19.00 Oatley Electronics sell by mail, phone and email/internet. You can contact them on (02) 9548 3563, Fax (02) 9584 3561, PO Box 89, Oatley NSW 2233, or by email sales<at>oatleyelectronics.com. The website is located at www.oatleyelectronics.com shouldn’t be too difficult and you can tap into the wiring using a “Scotchlok” or similar connector. These need no soldering – they pierce the wiring insulation and make contact as you squeeze them into position with a pair of pliers. A fuse is probably unnecessary as the reversing light circuit itself is fused. The buzzer will need to be mounted within hearing range – under the dashboard seems to make sense. Ordinary (thin) figure-8 cable is quite OK for this purpose. The buzzer shown in our photographs is probably inadequate for most cars because of its limited output. However, there are plenty of piezo and electro-magnetic buzzers around which would be more than loud enough. We wouldn’t suggest using an alarm piezo though – not if you value your hearing, that is. With most of these alarms designed to make a lot of noise (around 100-110dB output), that’s just a bit too loud for comfort! SC Semiconductors 1 4093B quad 2-input NAND gate (IC1) 3 C8050 NPN transistors (Q1, Q2, Q3) 2 1N60 germanium signal diodes (D1, D2) 2 1N914 silicon signal diodes (D3, 34) 1 GIG or 1N4004 power diode (D5) 1 6.2V 400mW zener diode 1 5mm LED (any colour) 1 MA40A3S or equivalent   ultrasonic transmitter   transducer (TX1) 1 MA40A3R or equivalent   ultrasonic receiver   transducer (RX1) Resistors (0.25W, 1%) 3 1MΩ 1 100kΩ 1 12kΩ 3 10kΩ 2 2.2kΩ 1 470Ω 2 10kΩ trimpots Capacitors 1 10µF 25VW PC electrolytic 3 10µF 16VW PC electrolytic 1 0.1µF polyester (code: 104 or 100n) 4 .012µF polyester    (code: 123 or 12n) Miscellaneous Suitable lengths of hookup wire, figure-8 cable and shielded cable, “Scotchlok” or similar wiring connectors, suitable mounting nuts and bolts, solder, etc. Resistor Colour Codes       No. 3 1 1 3 2 1 Value 1MΩ 100kΩ 12kΩ 10kΩ 2.2kΩ 470Ω 4-Band Code (1%) brown black green brown brown black yellow brown brown red orange brown brown black orange brown red red red brown yellow violet brown brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown brown red black red brown brown black black red brown red red black brown brown yellow violet black black brown FEBRUARY 2000  41