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SPEED ALARM Had a speeding fine lately? Painful, isn’t it? And how many more demerit points before you lose your licence? Bit of a worry, eh? Well, this Speed Alarm will help you avoid these worries and make you a safer driver too. By JOHN CLARKE In most Australian States, speeding fines are getting to be a real pain in the wallet. In New South Wales for example, ex­ceeding the speed limit by 10km/h presently means a fine of $112 and two demerit points while exceeding it by 15km/h whacks you for $179 and three demerit points. Get a few fines like these over a few 24  Silicon Chip months and it starts to run into real money and your licence is looking decidedly shaky too. And you don’t have to be a speed demon either. It’s all too easy to let the speed creep up gradually when you are on a long drive and then when you come into a low speed zone, you can be way over the limit. Even if your car has a cruise control you can still inad­ vertently exceed the speed limit. On long downhill stretch­es your car will gradually pick up speed and if you are caught it is no good claiming that you had your cruise control set. The police have heard that story before. When you consider the amount of money involved in a couple of speeding fines, it is equivalent to quite a few electronic projects you won’t be able to build. So think seriously about this speed alarm. It will cost less than being caught for exceed­ing the speed limit by 15km/h and it could save you lots. Features The SILICON CHIP Speed Alarm comprises a small control box with a 3-digit display, a LED to indicate over speed and two buttons for setting the speed and turning the alarm on or off. One button increases the speed setting in 5km/h steps while the other reduces it in 5km/h steps. Pressing both buttons at once turns the alarm on or off. In fact, the operating concept is exactly the same as the Speed Alarm using in current model Holden Com­ modores; we copied it, the operating concept that is, not the circuit! A separate larger box contains most of the circuitry. This can be located under the instrument panel. It connects to a Hall Effect pickup on the drive shaft. Calibration is simple: just tweak one trimpot after the system is installed. Block diagram Fig.1 shows the basic arrangement of the Speed Alarm. A small magnet is attached to the car’s drive shaft and as it whizzes past the Hall Effect speed sensor it produces one pulse per shaft revolution. A frequency to voltage converter converts the resulting pulse frequency to a voltage and this is applied to one input of a comparator. The second input of the comparator is fed with a voltage proportional to the Speed Alarm setting. If the voltage produced by the vehicle’s speed is greater than the voltage for the Speed Alarm setting, then the comparator switches on the alarm buzzer and Fig.1: this is the concept of the Speed Alarm. The speed signal from a Hall Effect pickup is converted to a voltage and compared with a speed setting derived from an up/down counter and D-A converter. lights the alarm LED. The Speed Alarm setting is obtained from an up/down counter which feeds a digital to analog (D-A) converter. While the block diagram of Fig.1 shows the basic concept of the Speed Alarm, the actual circuit arrangement is a good deal more complex. Instead of using one up/down counter we have had to use two. One is a BCD (binary coded decimal) type and the other a straight binary type. Fig.2 illustrates the arrangement of these up/down counters and some of the ancillary functions. Whenever one of the switches is pressed, a diode OR gate (D1, D2) The Speed Alarm consists of three main units: a control box with a 3-digit LED display, a larger box which contains most of the circuitry, and a Hall Effect pickup. December 1997  25 Specifications •  Overspeed detection accuracy ......................................................... <2% •  Hysteresis (alarm on to alarm off) ..................................................3km/h •  Standby current drain (ignition off or switched off) ............... 10mA-15mA •  Operating current ............................350mA with all possible segments lit clocks flipflop IC4a and its Q output drives LED display DISP1 (via IC5c, Q6 & Q7). DISP1 shows either “0” or “5”, depend­ing on how the buttons are pressed. The Up and Down switches also drive the up/down detector along with the Q and Q-bar outputs from flipflop IC4a. The re­sulting detector outputs drive the clock inputs of both BCD and binary up/down counters. Clocking only occurs when DISP1 goes from “5” to “0” when counting up and from “0” to “5” when count­ing down. That makes sense because BCD counter IC1 drives the “tens” display, DISP2, via the 7-segment decoder IC2. BCD counter IC1 counts from “0” up to “9” before returning to “0”. The carry output (when counting beyond from “9” to “0”) drives flipflop IC4b via a second diode OR gate (D5, D6). When counting down from “0” to “9” the borrow output also drives flipflop IC4b via the same OR gate. Flipflop IC4b drives display DISP3 via Q4. DISP3 shows “1” for speed readings of 100 and above and is blank below 100. Why two counters? So why do we need the second binary counter, IC3? As far as the 3-digit display is concerned, the composite BCD counter (ie, IC4a, IC1 & IC4b) goes from “00”, “05”, “10”, “15” etc up to “95”, “100”, “110” etc. However, in binary form the count becomes disjointed at the count of “100”. This is because BCD counter IC1 returns to “0” after “9”. By contrast, if IC1 was a 4-bit binary counter it would continue beyond “9” (1001) to 10 (1010), 11, 12, 13 ,14 and 15 (1111) before returning to “0” (0000). Since we want the counter to provide a voltage output via a D-A converter, we require a consecutive count from “0” up to “15” for the 4-bit output. Thus we have used a second up/down counter IC3 which counts 26  Silicon Chip in binary, effectively in parallel with the BCD counter, IC1. The 5-bit D-A converter uses the four bits from binary counter IC3 plus the output from flipflop IC4a as the least significant bit. The resulting 5 bits are converted to a voltage to be presented to the speed comparator. Since we have two counters operating in parallel, there must be safeguards to ensure that the both have the same value at any time. In other words both counters must track and count up or down together. To do this, the counters are both preloaded to a “3” at power up. If counter IC3 is taken beyond its 15 count (155km/h on the display), the carry out signal returns both counters to “3” at the preload input via the over/under range detect block. If the counters are taken to below “0”, the under range detect section is triggered via the borrow output of IC3 and the counters are again preloaded to a “3”. Hence, when the Speed Alarm is Main Features •  Overspeed indication range from 0-155km/h •  Speed settings in 5km/h increments •  Audible and visual overspeed alarms •  Visual alarm stays on during overspeed •  Audible alarm sounds every 10 seconds during overspeed •  3-digit LED display •  Display dims when headlights are on •  Illuminated Up and Down speed set switches •  Single trimpot speed calibration first turned on, 30km/h is the initial speed setting. Circuit description Fig.3 shows the circuit diagram for the Speed Alarm. It uses 11 low-cost ICs and three 7-segment displays plus several transistors, diodes, resistors and capacitors. IC1 is the 74HC192 BCD counter driving the 4511 7-segment decoder driver, IC2. IC2 drives the 7-segment LED dis­play, DISP2. We have added a little refinement to the decoder to improve the display of digits 6 and 9. This adds the “d” segment when the “9” is displayed and the “a” segment for the “6”. This is achieved as follows. When “6” is displayed, the “d” segment output is high and this also drives the “a” segment via D12. Diode D13 is there to prevent D12 driving the low “a” output at pin 13. Note that the “d” segment is lit for the “0”, “2”, “3”, “5” and “8” counts as well but in this case the “a” segment is also lit and so the additional drive circuit does not affect other numbers. When “9” is displayed, the D input at pin 6 of IC2 is high (it is low for counts from 0-7). This high drives transistor Q5 and its emitter drives the “d” segment of the display. Note that the D (most significant bit) input is also high for a count of “8” but since the “a” and “d” segments are also lit it does not matter that Q5 also drives the “d” segment. Diode D11 prevents the low “d” output at pin 10 being driven high via Q5 when dis­playing “9”. LED display DISP1 is driven via transistors Q6 or Q7. The a, c, d and f segments are hard wired via 270Ω resistors to the 5V supply. These segments are lit for both “0” and “5”. PNP transistor Q6 is switched on when the Q output of flipflop IC4a is low and this drives the “g” segment when displaying “5”. When the Q output of IC4a is high, IC5c’s output is low and this drives Q7 and so the “b” and “e” segments are lit to display “0”. IC4a is a flipflop which is connected as a divide-by-two counter with its D input connected to the Q-bar output. On each positive edge of the clock input, the Q and Q-bar outputs toggle from a high to a low or vice versa. The clock signal to IC4a comes via diodes D1 or D2 from Schmitt trigger inverters IC5b & and IC5a which are wired as switch debouncers for the Up and Fig.2: this block diagram illustrates the parallel operation of the binary up/down counter and the BCD up/down counter. The binary up/down counter is needed for the D-A converter while the BCD counters is needed for the 3-digit display. Down buttons. So whichever button is pressed, IC4a is clocked. So the circuit so far has no way of knowing which button was pressed. Up/Down detection The outputs of IC5a and IC5b connect to NAND gates IC6a and IC6b respectively, at their pin 2 and pin 6 inputs. Meanwhile, the Q and Q-bar outputs of IC4a connect to pin 1 of IC6a and pin 5 input of IC6b, via 0.1µF capacitors. So IC6a detects when the Up button is pushed and IC6b detects when the Down button is pushed. If the Q output of IC4a was high when the Up switch was pressed, corresponding to “0” being displayed by DISP1, then the resulting low Q output upon clocking would prevent IC6a’s output going low. Thus no up counting will occur. This allows IC4a to produce a “5” on DISP1 without DISP2 changing. DISP2 will only change to the next up count when the “5” displayed on DISP1 goes to a “0”. When the Down switch is pressed, the opposite sequence happens compared to the Up count. The difference is that the down count only occurs when DISP1 goes from “0” to “5” (when IC4a’s Q-bar output goes from low to high). Borrow & carry Our circuit for the BCD up/down counter IC1 and the binary counter IC3 is a little unusual in that we are using both the “Borrow” and “Carry” outputs. These terms Borrow and Carry may seem at little confusing but they are quite straightforward. The term “Carry” comes from the familiar process of addition: when you add up a column of figures, you “carry” the sum over to the next column. Similarly, when you subtract one row of figures from another, you often have to “borrow” from the next column in order to do the operation. In an up/down counter, the carry output goes low when the count goes over “9” when counting up and the borrow output goes low when counting down, below “0”. We use the borrow and carry outputs of IC1 to determine whether the third digit, DISP3, dis­plays “1” or is blanked. The borrow and carry outputs of IC1 are coupled to the clock input of flipflop IC4b via diodes D5 and D6. When it is low, the Q output of IC4b drives PNP transistor Q4 to switch on the “b” and “c” segments of DISP3 to display a “1”. As noted above, binary counter IC3 tracks IC1. When IC3 counts up past “15” or down below “0”, the carry or borrow out­puts respectively will go low and produce a low on the load inputs of IC1 and IC3 via the two inverters IC5d and IC5e. The A and B preload inputs of IC1 and IC3 are tied high while the C and D preload inputs are tied low. This sets a count of “3” on both counters, IC1 & IC3. At the same time, inverter IC5f feeds a high to the set input (S) of IC4b. This causes its Q output to go high and turn off transistor Q4 and this turns off DISP3. December 1997  27 Fig.3 (right): the full circuit of the Speed Alarm operates from +5V and most of it is permanently powered. Only the 3-digit display, the Hall Effect sensor and the three LEDs are turned on or off by simultaneously pushing the Up and Down buttons. A similar preload condition occurs on power up when the 10µF capacitor at the pin 1 input to IC5d is initially low. It charges via the 100kΩ pullup resistor to provide normal count operation after about one second. D-A conversion We now come to the 5-bit D-A converter. Well, we do not have a D-A IC as such. What we do have is an R-2R ladder network comprising the 100kΩ resistors at the Q1-Q4 outputs of IC3 and the 100kΩ resistor from the Q-bar output of IC4a. This latter resistor provides the least significant bit. The 51kΩ resistors between the 100kΩ resistors complete the R-2R ladder. Note that it is called an R-2R ladder because of the fact that the resistors have a value of R (in our case 51kΩ) or 2R (100kΩ). Strictly speaking, the 51kΩ resistors should be 50kΩ or the 100kΩ values should be 102kΩ, but this circuit is not that critical. The DC output from the ladder network connects to the com­parator input at pin 10 of IC8, the LM2917 frequency-to-voltage converter. The front part of the LM2917 does the voltage to frequency conversion of the speed signal from the Hall Effect drive shaft pickup and its output is at pin 3 where it is fil­tered with a 6.8µF capacitor and then applied to the second comparator input at pin 4 via the 22kΩ resistor. Pin 5 of IC8 is the comparator output. It is fed to IC9, a 555 timer IC which we are using simply as a Schmitt trigger inverter to give a fast risetime signal. IC9 drives transistor Q3 when its pin 3 output is low and this in turn lights the over­speed LED (LED1). Audible alarm The audible alarm comprises an LM358 dual op amp IC10 and a 4017 decade counter IC11. Both op amps are configured as Schmitt trigger oscillators. When pin 3 of IC9 is high, diode D20 holds the 0.1µF capacitor at pin 6 of IC10 high and therefore 28  Silicon Chip December 1997  29 Fig.4: the component layout for the main PC board. A 16-way header is used to terminate 8-way rainbow cables to the display board. stops IC10b from oscillating. And it also keeps counter IC11 in the reset condition. IC10a is disabled by diode D16, holding the .022µF capacitor at pin 2 discharged via the 2.2kΩ resistor connecting to ground. When the car exceeds the speed setting on IC1, pin 3 of IC9 goes low, diode D20 is reversed biased and 30  Silicon Chip IC10b is allowed to oscillate at a rate of about 2Hz and it clocks counter IC11. As soon as the “1” output at pin 2 of IC11 goes high, it reverse biases D16 via D15 and IC10a starts oscillating to drive the piezo transducer, to sound the alarm. VR2 sets the frequency driving the piezo. It can be set to obtain the max- imum loudness, so that the operating frequency coincides with the piezo transducer’s resonant frequency; or you can adjust it to lower the volume. More beeps The reason why counter IC11 is included is to give you further audible warnings that you are still exceeding The main PC board is housed in a low-profile plastic instrument case which can be mounted under the dashboard or if preferred, under one of the front seats. The connections to the display board are run via ribbon cable. the set speed limit. This is necessary because you might have been dis­ tracted during a passing manoeuvre or other event. Hence, as IC10b continues to clock IC11, the “2” output goes high. IC10a now stops oscillating, with D16 holding the .022µF capacitor discharged. When IC11 is again clocked by IC10b, the “3” output goes high at pin 7 and allows IC10a to oscillate via diode D14. When IC11 is clocked again, IC10a stops as the “4” output goes high. This high “4” output of IC11 drives transistor Q8 which turns on to connect a 4.7µF capacitor at pin 6 of IC10b, and this greatly slows the frequency of oscillation. When IC11 is clocked again several times the 4.7µF capacitor is again placed in cir­cuit via the “8” output driving Q8. Finally, the “1” output of IC11 will go high again and allow oscillator IC10a to sound the piezo transducer again. Thus, we have a “pip pip” sound from the alarm as the “1” and “3” outputs of IC11 successively go high and then a several second pause before sounding again. The pin 3 output of IC9 goes high again, when the car’s speed drops below the set limit, and this resets IC11 and disa­bles IC10b. Power for the circuit comes from the vehicle’s 12V battery supply and is regulated to 5V with REG1. The 16V zener diode at REG1’s input gives protection against voltage spikes or wrong supply connections. Note that the circuit is powered at all times but the display is blanked until the ignition is turned on or both buttons are pressed simultaneously to bring the Speed Alarm into operation. The ignition input is monitored by NAND gate IC6c. It drives the base of Q1 and this transistor provides the 5V switched supply to the Hall sensor and LED2 & LED3. These LEDs light the Up & Down switches so they can be seen at night. Pin 9 of IC6d monitors whether the headlights are on. If they are off, pin 10 of IC6d turns Q2 on to provide the low common cathode voltage for the displays and overspeed LED (LED1). If the lights are on, IC6d oscillates and turns Q2 on and off to dim the displays, for night time driving. When the Up and Down switches are pressed simultaneously, IC5a & IC5b will both go low and diodes D3 & D4 are reverse biased. This causes the clock input to IC7 is to be pulled high via the associated 10kΩ resistor and toggles its Q output low. The re­sulting low on pin 12 of IC6c takes the pin 11 output high and Q1 is off. Diodes D17 and D18 pull both pin 8 and pin 9 of IC6d high and pin 10 is therefore low. Q2 is off and so the displays are unlit. Pressing both Up & Down switches again will toggle the Q output of IC7 high again and so IC6c can go low, driving Q1. This low also reverse biases D17 and D18 and Q2 is on and so the display will be lit. Note that pressing both the Up and Down buttons simultaneously may also change the counters depending on which switch makes contact first. So turning the speed alarm on and off may change the setting by 5km/h, meaning that the initial setting may be for example 35km/h instead of 30km/h. Construction The Speed Alarm is constructed on three PC boards. The main PC board is coded 05311971 and measures 198 x 155mm. The display PC board is coded 05311972 and measures 62 x December 1997  31 Table 1: Resistor Colour Codes ❏ No. ❏   1 ❏   1 ❏   1 ❏ 19 ❏   4 ❏   2 ❏ 18 ❏   2 ❏   3 ❏   1 ❏ 18 ❏   1 Value 10MΩ 1MΩ 220kΩ 100kΩ 51kΩ 22kΩ 10kΩ 4.7kΩ 2.2kΩ 470Ω 270Ω 2.2Ω Table 2: Capacitor Codes ❏ Value IEC Code EIA Code ❏ 0.47µF  470n   474 ❏ 0.1µF  100n   104 ❏ .047µF   47n   473 ❏ .022µF   22n   223 ❏ .001µF    1n   102 47mm, while the sensor PC board is coded 05311973 and measures 25 x 31mm. The main PC board is housed in a case measuring 225 x 40 x 165mm, while the display PC board is housed in a plastic utility case measuring 82 x 53 x 30mm. Before doing any assembly, check the PC boards for any breaks or shorts between tracks and undrilled holes. Make any repairs needed. Then start with the main board and solder in all the links as shown on the overlay diagram of Fig.4. Insert and solder in all 4-Band Code (1%) brown black blue brown brown black green brown red red yellow brown brown black yellow brown green brown orange brown red red orange brown brown black orange brown yellow violet red brown red red red brown yellow violet brown brown red violet brown brown red red gold brown the resistors using the accompanying resistor colour code table (Table 1) to select each value. The ICs can be installed next, taking care with their orienta­tion. Note that IC2 is oriented differently to all the other ICs. Then solder in the diodes, including the zeners, and take care with their orientation. Insert the capacitors next. Table 2 shows the codes which are likely to be marked on the MKT polyester types. Take care to insert the electrolytic capacitors with the correct polarity. The 3-terminal regulator REG1 mounts horizontally with its metal face towards the PC board and a small heatsink beneath it. Next, mount the spacers, transistors and trimpots. We used a 16-way pin header for the multiple connections required to the display PC board. Fig.6 shows the component layout for the display PC board and sensor board. Before inserting any components into the dis­play board, check 5-Band Code (1%) brown black black green brown brown black black yellow brown red red black orange brown brown black black orange brown green brown black red brown red red black red brown brown black black red brown yellow violet black brown brown red red black brown brown yellow violet black black brown red violet black black brown red red black silver brown The completed sensor board and its companion button magnet. that it fits neatly into the small case. You may need to do some judicious filing to make it a neat fit. Insert the 7-segment displays with the decimal points towards the switch­es. All the resistors are mounted end-on as shown. LED1 is mounted hard against the PC board, while LEDs 2 and 3 need to lean over towards their respective switches. The two SPEED ALARM  km/h  SET  + ON/OFF +  Fig.5 (above) shows the full-size artwork for the display case, while at left is the assembled display PC board. Note how the two green LEDs are arranged. 32  Silicon Chip switches are oriented with their flat sides towards the bottom of the PC board, as shown in Fig.6. The two 8-way rainbow cables are soldered to the back of the board. The sensor board is assembled as shown in Fig.6. The sensor and capacitor mount flat on the PC board, with the labelled side of the sensor facing up. Case assembly The main PC board can be placed in its case and secured with four self-tapping screws into the integral standoffs in the base. Drill out the rear panel for the cordgrip grommet. The front panel requires two holes for the rainbow cable entry and holes to mount the piezo transducer. This is secured with two self-tapping screws. Drill a small hole for the wires. The display case is cut down to 23mm in height using a hacksaw and file. This allows the displays to sit directly under the red Perspex which replaces the front panel lid of the case. Cut the Perspex to size and cut out the display area on the front panel label with a sharp hobby knife. Affix the label to the Perspex and drill holes for the switches and securing screws at each corner. You will need to cut a slot in the base of the case for the rainbow cable to exit. Pass the rainbow cables through the slot in the case and clip the PC board in place. Secure the front panel in place with self-tappers. Pass the rainbow cables through the holes in the front panel of the main PC board case and attach the 16-way pin header socket to the wires. We used IDC (Insulation Dis­placement Connector) in-line pin headers. Fig.6: the component layouts for the display and Hall Effect sensor PC boards. Note that LEDs 2 & 3 lean towards their respective pushbutton switches. Fig.7: the mounting details for the Hall Effect speed sensor. The gap between the sensor and the magnet should be 2-3mm. Testing Apply 12V to the +12V and IGN inputs. The display should light. If not, press the two switches together to check that it turns on. If not check for supply on all the ICs. There should be +5V between pins 16 & 8 of IC1, IC2, IC3 and IC11, between pins 14 & 7 of IC4, IC5, IC6 & IC7, between pin 8 & 12 of IC8, pins 8 & 1 of IC9 and pins 4 & 8 of IC10. Most of these ICs will have additional pins tied to the +5V rail, as can be seen on the circuit of Fig.3. These can also be checked with your multimeter, as can the IC pins which are tied to 0V. Fig.8: actual size artworks for the display (right) and speed sensor boards. If the display is showing a reading, test the Up and Down switches. Now count down to 0 and check that LED1 lights and that the piezo alarm sounds. You can test the dimming feature by applying 12V to the lights input. Installation The speed alarm can be installed into a vehicle using auto­motive connectors to make the connections to +12V, the ignition supply and lights. Use automotive wire for these connections. Also the ground connection can be made to the chassis with an eyelet and a self-tapping screw. Attach the main case under the dashboard on suitable brackets. Mount the display December 1997  33 The display board fits neatly inside a small plastic utility case. Take care to ensure that the LED displays are correctly oriented. The external leads emerge through a slots in the back of the case. PARTS LIST 1 PC board, code 05311971, 198 x 155mm 1 PC board, code 05311972, 62 x 47mm 1 PC board, code 05311973, 25 x 31mm 1 front panel label, 81 x 52mm 1 plastic case utility case, 82 x 53 x 30mm 1 plastic case, 225 x 40 x 165mm 1 red Perspex sheet, 81 x 52 x 3mm 1 piezo transducer 1 mini heatsink, 20 x 20 x 10mm 1 button magnet 12 PC stakes 1 16-way pin header launcher 1 16-way pin header socket (4 x 4-way, 2 x 8-way) 3 M3 x 6mm screws and nuts 6 self-tapping screws to mount main PC board and piezo 1 small cordgrip grommet 2 PC-mount click action push-on switches (white) (S1,S2) 1 800mm length of 0.8mm tinned copper wire 2 1m lengths of 8-way rainbow cable 3 2m lengths of hookup wire (+, GND and signal sensor wires) 3 2m lengths of red automotive wire (+12V, ign. & lights input) 34  Silicon Chip 1 2m length of black or green automotive wire (ground wire) 1 200kΩ horizontal trimpot (VR1) 1 22kΩ horizontal trimpot (VR2) Semiconductors 1 40192, 74HC192 4-bit BCD up/down counter (IC1) 1 4511 BCD to 7-segment decoder (IC2) 1 40193, 74HC193 4-bit binary up/down counter (IC3) 2 4013 dual D flipflops (IC4,IC7) 1 74C14, 40106 hex Schmitt trigger (IC5) 1 4093 quad Schmitt NAND gate (IC6) 1 LM2917N 14-pin frequency-tovoltage converter (IC8) 1 LMC555CN, TLC555 CMOS timer (IC9) 1 LM358 dual op amp (IC10) 1 4017 decade counter (IC11) 1 7805, LM340T5 5V 1A 3terminal regulator (REG1) 1 UGN3503 Hall Effect sensor (sensor1) 21 1N914, 1N4148 signal diodes (D1-D21) 1 16V 1W zener diode (ZD1) 2 4.7V 1W zener diodes (ZD2,3) 5 BC327 PNP transistors (Q1,Q3, Q4,Q6,Q7) 3 BC337 NPN transistors (Q2,Q5, Q8) 3 HDSP5303 common cathode 7-segment LED displays (DISP1-DISP3) 1 5mm high intensity red LED (LED1) 2 3mm red or green LEDs (LED2,LED3) Capacitors 2 100µF 16VW PC electrolytic 4 10µF 16VW PC electrolytic 1 6.8µF 16VW PC electrolytic 1 4.7µF 16VW PC electrolytic 2 1µF 16VW PC electrolytic 13 0.1µF MKT polyester 1 .047µF MKT polyester 1 .022µF MKT polyester 2 .001µF MKT polyester Resistors (0.25W, 1%) 1 10MΩ 18 10kΩ 1 1MΩ 2 4.7kΩ 1 220kΩ 3 2.2kΩ 19 100kΩ 1 470Ω 4 51kΩ 18 270Ω 2 22kΩ 1 2.2Ω 0.5W Miscellaneous Automotive connectors, bracket for sensor board, heatsh­rink tubing, etc. Fig.9: actual size artwork for the main PC board. Check your board carefully against this artwork for possible etching defects before installing any of the parts. in a convenient place on the dashboard. The sensor board should be sheathed in a piece of heatsh­ rink sleeving and then mounted near the drive shaft as shown in Fig.7. Temporarily mount the button magnet in place with a cable tie and secure the board so that the magnet will directly pass the sensor with a 2-3mm gap. Wire the sensor to the main PC board using hookup wire. Test that the speed alarm works at a low speed setting. You may need to adjust VR1 slightly so that it works at the correct speed. It is calibrated so that the alarm sounds near the set speed, as indicated on the speedo­meter. If nothing happens, remove the magnet and turn it around so that the opposite pole is facing out and test again. If the speed alarm cannot be made to work at any speed, the magnet may not be powerful enough or the gap between sensor and magnet is too great. When the speed alarm is operating satisfactorily, use epoxy resin to permanently secure the magnet to the SC drive shaft. December 1997  35