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Avoid traffic fines: fit this speed alarm Fit this speed alarm to your car and avoid the possibility of heavy fines and possibly even losing your driver's licence. It will give you a brief audible warning each time you exceed a pre set speed so you can ease off the pressure on the accelerator. Design by PETER GRAY Every licenced driver exceeds the speed limit at some time or other but, depending on which state you live in, there could be a heavy financial cost for doing so. It is bad . enough when you knowingly exexceed the speed limit, because at least you know you are taking a risk. It is when you unknowingly exceed the speed limit that you could be in real trouble. 60 SILICON CHIP Maybe the stress of the journey has caused you to press down imperceptibly harder on the accelerator or maybe a long downhill slope has let the speed build up. This can happen easily if the vehicle you are travelling in is new and particularly quiet. In these circumstances you need an overspeed alarm to bring you back to reality. This kit is designed to monitor vehicle speed and give an audible warning if one of three preset speeds are exceeded. The circuit has the following features: (1). It monitors three user settable speeds; eg, 60, 80 & 100km/h. (2). Once installed and set the unit requires no further adjustment by the driver. You don't want to be fiddling with knobs every time you enter a different speed zone. (3). )'he unit can give an audible or visible warning. (4). As each preset speed is reached, the buzzer sounds briefly. If you drive within 1 to 2km/h of the preset speed, the buzzer sounds continuously. If you ignore the warning and accelerate, the unit will not sound again, until you reach the next preset speed. (5). The device can be fitted to any car - front or rear wheel drive. (6). The circuit uses a Hall Effect sensor to monitor true vehicle speed, not engine RPM. Therefore you don't get false warnings caused by high revs in low gears. The kit is essentially just a small printed circuit board with external connections to the car's 12V supply, to a buzzer and to the speed sensor. It can be mounted in a convenient spot underneath the dash. How it works As noted above, the circuit (Fig.1) uses a Hall Effect device and this senses magnets which need to be fitted to the tailshaft or transaxle. If the vehicle has a tailshaft, four magnets are required while for cars with a transaxle (all front wheel drives and VW s ), one half shaft is fitted with two magnets. The Hall Effect sensor is mounted close to the tailshaft (or half shaft) and each time a magnet passes, the sensor pulls its "open collector" output low. Essentially, the Hall Effect sensor generates a train of pulses which are directly proportional to the speed of the vehicle. These pulses are fed to pin 1 of IC1, an LM2917N frequency to voltage converter. This gives a DC voltage output at pin 5 which is directly proportional The speed of the vehicle is monitored by a Hall Effect device which senses magnets fitted to the tailshaft or transaxle. Each time a magnet passes, the sensor pulls its open collector output low. to the repetition rate (ie, frequency) of the pulses at its input. The component values at pins 2 and 3 of the LM2917 give a linear DC voltage at pin 5 for a range of input frequencies from zero up to about 80Hz. This should be sufficient for most vehicles for speeds up to 120km/h. The varying DC output of IC1 is fed to the non-inverting ( + ) inputs of the three comparators in ICZ, an LM339 quad comparator IC. Each of these three comparators, ICZa, Zb and 2c, has its inverting input connected to the wiper of a 1Ok!J trimpot, VRl , VRZ and VR3 respectively. These trimpots provide the three adjustable speed settings. Each of the three comparator outputs is connected to the gate of a silicon controlled rectifier (SCR) via a 10k!J resistor and .OlµF capacitor. If the DC output voltage from ICl exceeds the preset voltage at pin 4 of ICZa, pin 2 will switch high 470 Q 4.7k 10k +9V .,. 10 ,. IC1 LM2917N ,. + 11 + 9V D1 1N914 1M 470k A + .,. OI .,. 100k 1J .,. .,. 1M 1M SCR3 C203B 100 1W OUT +12V FROM IGNITION 2D1 16V j GND+ lW + .,. 01! .,. .,. +9V 10k 100! A .,. B .,. VR3 10k .,. AGDK ,~oo, EIO C VIEWED FROM BELOW GN0 OVERSPEED ALARM Fig.1: the Hall Effect sensor pulses frequency to voltage converter ICl. This produces a DC voltage at pin 5 which is monitored by comparators IC2a-IC2b. When a comparator output switches high, its corresponding SCR briefly turns on and turns on Qt & the buzzer via Schmitt trigger IC2d. JUNE 1990 61 I :::Jll'llS!r.l: IWsm: +12V FROMIIGNITION 1O11 1 W ZD1 CHASSIS SENSOR EARTH __:,....,...a.)_ ---BUZZER + +9V TO SENSOR-'..-4-4;.ilsiii~,_,;;;-,l".J:~ ~ ~ ~I,...!.-... BUZZER - Fig.2: take care to ensure that all polarised parts are correctly oriented when assembling the PC board. These parts include the electrolytic capacitors, the SCRs, Qt and the ICs. PC stakes are used to terminate the external wiring connections. 0 RESISTORS No □ □ □ □ □ □ □ □ □ 3 1 1 6 1 4 1 1 1 Value 1MO 470k0 100k0 10k0 4.7k0 3.3k0 6800 4700 100 and a brief positive pulse will be delivered to the gate of SCRl via the RC components just mentioned. This will cause SCRl to conduct and discharge the lµF capacitor at pin 10 of IC2d, the fourth comparator in the package. Discharging the lµF capacitor causes pin 10 of IC2d to be pulled low so that pin 13 goes high and turns on transistor Q 1 via the 1OkO resistor. This sounds the buzzer. Once the lµF capacitor has discharged, there is not enough current flowing through SCRl to keep it turned on and so it turns off. This allows the lµF capacitor to charge up again via the associated 470k0 resistor. This pulls pin 10 of IC2d high and so Q 1 turns off and so does the buzzer. :, 0.. .... ., + lr""' I' w AREA FRONT VIEW MOUNTING BRACKET TO BODY D D D TOP VIEW 62 SILICON CHIP 4-Band Code (5%) brown black green gold yellow violet yellow gold brown black yellow gold brown black orange gold yellow violet red gold orange orange red gold blue grey brown gold yellow violet brown gold brown black black gold 5-Band Code (111/o) brown black black yellow brown yellow violet black orange brown brown black black orange brown brown black black red brown yellow violet black brown brown orange orange black brown brown blue grey black black brown yellow violet black black brown brown black black gold brown So the net effect of exceeding one of the preset limits for IC2a, 2b or 2c is to turn on one of the three associated SCRs briefly and so sound the buzzer briefly. Furthermore, each comparator and SCR combination needs to be reset before it can sound another warning. To explain this, let's consider IC2a and SCRl. If the speed limit set by VRl is exceeded, the output at pin 2 of IC2a will go high and trigger SCRl as explained before. SCRl then cannot turn on again until it receives another gate trigger pulse via the :01µF capacitor and lOkO resistor. For this to happen, the output of IC2a must first go low, to discharge the .OlµF gate capacitor. For that to Fig.3: the Hall Effect sensor is installed on a mounting bracket attached to the car body, while the magnets are epoxied to the tailshaft [or transaxle). Note that the magnets must pass within 3mm of the sense area of the Hall sensor. The sensor is also pole sensitive so be sure to install the magnets the right way around (see text). happen, the vehicle must drop below the limit set by VRl. If you drive at close to the preset speed for IC2a, its output will randomly flick high and low so that SCRl is being retriggered all the time. This will sound the buzzer continuously. The same comment applies for the other two comparators and their SCRs. Power for the circuit is derived from the vehicle's + 12V supply via the ignition switch. The + 12V is fed to a 7809 9V 3-terminal regulator which then supplies the rest of the circuit. Protection against excessive input voltage or spikes is provided by the 100 resistor and associated 16V 1W zener diode at the input of the 7809 regulator. A feature of the circuit is that every time the ignition switch is turned on, the buzzer will sound briefly, to let you know the speed alarm is working. Board construction The PC board measures 105 x 43mm and accommodates all the components except for the small buzzer. The component layout is shown in Fig.2. We suggest you first check the board carefully for any faults such as hairline cracks in the tracks, The completed PC board and the buzzer can be housed in a plastic zippy case and hidden under the dashboard. We used sockets for the two ICs but suggest that you solder them straight in for improved reliability. Use PC stakes to terminate the external wiring connections and note that the prototype used on resistors instead of the wire links shown in Fig.3. shorts between tracks or undrilled component holes. These should be corrected before any components are installed. Begin the assembly by installing all the resistors and soldering them in. Check the value of each resistor with your multimeter before it is put into the board. You can then install the capacitors, making sure that the electrolytics are inserted the right way around. Next, fit the diode Dl and the 16V zener, ZDl. After those, fit the three SCRs, Ql and the 3-terminal r egulators. Finally, fit the three multiturn trimpots and the two ICs. Testing To test the board, you ideally need a function generator or an oscillator which can put out square waves of at least 2 volts peak-topeak or a sinewave output of larger amplitude. You also need a power supply which can deliver 12 volts DC. This will enable you to test the board on the bench before it is fitted to the car. Connect the buzzer and the 1 ZV power supply to the board, then use your multimeter to check for the + 9V rail at the output of the 3-terminal regulator, at pins 8 and and 9 of ICl, and at pin 3 of ICZ. Set trimpot VRl so that pin 4 of ICZ is at, say, + 3V; set VRZ so that pin 8 of ICZ is at, say, + 4V and set VR3 so that pin 6 of ICZ is + 5V. Now start with the oscillator set to the lowest possible frequency and increase it slowly while monitoring the output of ICZa at pin 2. It should start low and then switch high suddenly when the voltage at pin 5 exceeds that at pin 4. When this happens, SCRl should conduct briefly and pin 13 of ICZd should go high to turn on Ql and the buzzer, for a brief time. As you turn up the oscillator frequency, the buzzer should sound again, corresponding to the output of ICZb, pin 14, going high and turning on SCRZ. Finally, the buzzer should sound a third time, corresponding to the output of ICZc, pin 1, going high and turning on SCR3. Now reduce the oscillator output frequency to the minimum and confirm that the buzzer again sounds three times, as the frequency is increased. If you monitor pin 5 of ICl you will find that the voltage rises as the input frequency is increased. A point will be reached where the voltage does not change any more as frequency increases. On the prototype, this was at about + 6V and an input frequency close to 80Hz. Installation The Hall Effect sensor in this pro- Where to buy the kit The Speed Alarm was designed by Peter Gray of Novocastrian Electronic Supplies Pty Ltd. The full kit of parts including printed board , Hall Effect sensor and magnets will be available from Novocastrian Electronic Supplies Pty Ltd, as this issue goes on sale . The full price of the kit is $49 .95 plus $3 post and packing to anywhere in Australia. The Hall Effect sensor will be available separately as Part No. KOSASENS for $14 .95, while the magnets will also be available at $1 . 56. All prices include sales tax. Contact Novocastrian Electronic Supplies Pty Ltd , 24 Broadmeadow Road, Broadmeadow, NSW 2292 ; or PO Box 87, Broadmeadow 2292 . Phone (049) 62 1358. JU NE 1990 63 PARTS LIST 1 printed circuit board, code PGOSA10/89, 105 x 43mm (copyright Novocastrian Electronics) 1 piezo buzzer 6 PC pins 4 magnets (only 2 required for transaxles) 3 1 0k0 multiturn trimpots (VR1, VR2, VR3) Semiconductors The 7809 3-terminal regulator is installed with its metal tab towards the O.lµF capacitor. The multi-turn pots are used to set the three alarm speeds, as described in the text. ject is a custom job. It is encapsulated in epoxy and has three terminals for + , - and output. Since all cars are different it is left to the constructor to decide where to mount the sensor but the following points should be noted before installation begins. (1). Hall Effect sensors do not have a high sense distance, typically 2-3mm. This unit is no different. When choosing a mounting point for the sensor ensure that it will not obstruct any other part of the car. (2). When choosing a mounting site, remember that you have to be able to mount a bracket for the Hall Effect sensor that will allow it to sit within 2-3mm of the magnet faces. You will need to make a mounting bracket, as shown in Fig.3. (3). The magnets need to be epoxied in place on the tailshaft or transaxle halfshaft, so follow the instructions carefully for the glue you use. We don't recommend 5-minute epoxy for this job - use a stronger variety which takes longer to set. Give it at least 24 hours to cure properly. (4). The Hall Effect sensor is pole sensitive so the magnets must be mounted with their poles facing the right way. If you don't do this correctly, the Hall Effect sensor won't work. Magnets supplied in the kit (from Novocastrian Electronic Supplies) will have their pole faces marked. Also don't be a wally and work 64 STLTCON CHTP under a car supported by a jack only. This is a car ramp and wheel chock job only! (5). Read 1, 2, 3 and 4 again. The Hall Effect sensor is the heart of this project. If it is not installed properly the rest is a waste of time. (6). The printed board and buzzer can be mounted in a zippy box and hidden under the dash. The buzzer is fairly loud so it need not be mounted separately. Until tested, leave the wiring and board accessible. (7). To give a visible warning, you could wire a LED and 4700 resistor in series across the piezo buzzer. Most people will probably find the buzzer is all they need. Vehicle testing When connections have been made from the sensor to the board and the piezo buzzer and supply connections have been made, you are ready for a test. Get someone else to drive the vehicle while you monitor the DC voltage on pin 5/10 of the LM2917. As the car moves you should see a DC voltage appear and increase as vehicle speed increases. If you don't, chances are the Hall Effect sensor is too far from the magnet face or you've installed the magnets back to front (very bad!). If all goes well, get your mate to drive the car (in the correct speed zone) at the speeds you wish to monitor. Set the trimpots so that the 1 Hall Effect sensor (see text) 1 LM2917 F/V converter (14-pin version, IC1) 1 LM339, GL339 quad comparator (IC2) 1 7809 9V 3-terminal regulator 1 BC547 NPN transistor (01) 3 C2038 200V SCRs (SCR1 ,2,3) 1 16V 1 W zener diode (ZD1) 1 1 N914 silicon diode (D1) Capacitors 1 2 1 3 1 00µF 16VW PC ele ctrolytic 1µ,F 1 6VW PC electrolytic 0. 1 µF metallised polyester .01 µF metallised polyester Resistors (0 .25W, 5%) 3 1 1 6 1 1MO 4 70k0 1 00k0 1 0k0 4.7k0 4 1 1 1 3.3k0 6800 4700 1 on 1 W Optional 1 red light emitting diode 1 4700 0.25W resistor 1 plastic utility box Miscellanous Epoxy adhesive, hookup wire, solder buzzer sounds as you exceed the selected speeds by, say, 5km/h. Some fine tuning may be required. With this done you should be rewarded with a beep each time you exceed any of the preset speeds. If you hover around any preset speed, the buzzer will sound more or less continuously, urging you to slow down. This is because the output of the relevant comparator (IC2a, 2b or 2c) is flicking up and down and retriggering its SCR. The buzzer should not sound when you decrease speed. I§;]