Maplin auto electronics projects
Auto electronics projects engine speed would therefore not allow enough time between pulses for triggering the monostable. This de- sign is for 4-cylinder cars only and anyone using it on a 6 or 8-cylinder car would have to modify the count period accordingly, or use a compensation factor on the read- ings — not easy while driving! The output pulses from ICla, pin 12, are fed to the count input, pin 11, of IC2. This is a 4 digit counter with both latch and reset. It drives a multiplexed 2 digit display directly, with transistors TRI and TR2 selecting the digit, and resistors R4-R10 limiting the segment current. The counter requires latch pulses in order to give sensible readings and these are provided by IC3, TR3 and their associated components. IC3 is the ever useful 555, used as an oscillator whose frequency is controlled by RV1. The oscillator output waveform, arranged such that there is a long high and a short low period, is inverted by TR3 so that a short high period is achieved. This short pulse is used to control the latch of the counter device IC2, so that when this input goes high the information in the counter is transferred to the internal latch and displayed. The trailing edge of this short pulse is used to trigger the monostable IClb, whose output pulse is used to re- set the counter so that it will start counting from 00 again. The use of a separate monostable to reset the counter ensures that the reset pulse always occurs after the latch pulse and a true reading displayed. Because the supply voltage of a car, nominally 12-14 V, varies between these limits with engine speed, integrated circuit IC4 is used to derive a regulated 5 V supply from this, which is then used to supply ICI, IC2 and IC3 and is important for the stability of the oscillator (IC3). Diode D2 and capacitor C5 remove any noise on the supply. 122 Car digital tachometer Construction The Digital Tachometer is constructed on two PCBs; the main board and the display board. The display board is mounted at 90° to the main board by being soldered to Veropins, and this holds the display so that it can be viewed through the cut-out display window at the end of the case. Figure 5.2 shows the constructional details. First job, however, is to build up the main printed cir- cuit board. Referring to Figure 5.3, begin by fitting the smallest components first. Check the polarity of C3, C5, and the direction of Dl, D2 before fitting, then work your way through the components by size fitting C5, the larg- est, last. Insert the ICs into the appropriate sockets only Hain Board Figure 5.2 Preparing the veropins for attaching main and display PCBs together 123 Auto electronics projects Figure 5.3 Digital tachometer main PCB and legend layout after all other construction of the tachometer module is completed, taking the usual precautions with CMOS de- vices. Note that the negative end of C5 must be close to the PCB or you may find that adjusting RV1 is difficult during calibration! Display board Refer to Figure 5.4. First mount resistors R4, R5, R9, RIO, and the veropins from the component side, being care- ful not to strip the pads off the PCB in the process! A hot soldering iron will help to push the pins home. Don't 124 Car digital tachometer forget to fit the wire link (this can be made from an off- cut from a resistor, or with single-strand wire). Solder and crop the resistors and the wire link. Next fit the dis- play to the PCB; pins 1 to 9 are on the side where the decimal point will be found, and pins 1 and 18 are marked on the PCB. Solder and crop pins 1 to 18. Now measure the required length for the display board pins by offer- ing the display board up to the main PCB, 3-4 mm is about right; see Figure 5.2. If the pins are too short, the connections won't be mechanically strong. After trim- ming the pins down you can solder the display board to the main PCB. The pins on the display board also require soldering, and if this has already been done, you may find that the two boards do not marry snugly to each other. Figure 5.4 Display board layout and legend Use mains connection wire for power supply and screened wire for the input signal soldered to the three pins. Screened cable is used to stop the emission of RF interference, and the outer screening must be connected to earth, preferably at the HT coil end. Label the func- tion of each wire at the end that will connect to the car electrics. If you are going to use the optional box, the front panel of the case is replaced by a piece of red filter 125 Auto electronics projects cut to size (using the original panel as a template) with a pair of scissors or craft knife. This slots neatly into the case, which is moulded in two sections. As you may have noticed, there is no method of mounting the tacho mod- ule into the suggested box, so the alternative is to use quickstick pads. The suggested box also needs to be modified by removing part of the battery compartment; only the top and front partitions of this need to be re- moved, the sides will help to keep the PCB central in the box, and the screw holes must remain intact or else the box cannot be fastened together (see Figure 5.5). 12mm, 30mm Cut out shaded area of battery compartment wall to leave box fixing holes. This will allow the PCB, pins and large capacitor to pass through. Figure 5.5 Box modification details 126 Car digital tachometer Setting up One advantage of a digital tachometer over an analogue type is the ease of setting-up and calibration. Only one adjustment to RV1 needs to be made and, barring acci- dents, will prevail for the life of the unit. This setting ensures that the oscillator runs at the correct frequency and the method of calibration depends on the equipment available. Calibration against another tachometer is pos- sible, setting RV1 to give a display of 30 when the standard tachometer reads 3000 r.p.m. If you have ac- cess to a signal generator, set the frequency to 100 Hz (sine or squarewave) and the output level to maximum (more than 4.7 V). Connect this signal to the I/P pin on the PCB, and again this will simulate an ignition pulse train input of 3000 r.p.m. Alternatively calibration can be carried out against the mains frequency by using a transformer and bridge rec- tifier to provide a 100 Hz signal as shown in Figure 5.6, Signal to Tachometer OV Figure 5.6 Mains frequency doubler for calibration 127 Auto electronics projects and a battery charger is very effective in this role. In either case RV1 is adjusted to give a display reading of 30. Calibration should include a test run for up to half an hour or so for warming up and stabilisation, whereafter it might be noted that RV1 requires further fine tuning. When you are satisfied with the calibration of the coun- ter, RV1 should be fixed in position with wax, paint or nail varnish. Fitting the unit into the car After calibration, the unit is ready to be fitted to the car. It is impossible to give detailed instructions for every car but the following notes may be helpful. • it is a good idea to try the unit in various positions for best readability, using adhesive tape, until you are satisfied, • having decided on the best position use double- sided tape, adhesive pads or two pieces of velcro-tape, one glued to the unit and one glued to the car dashboard. All of these methods, of course, mean that the unit can be removed easily and the dashboard cleaned and left unmarked, • alternatively, use self-tapping screws through one half of the case into the dashboard. This works well, but unless you can utilise existing screw holes you will be left with holes in the dashboard if you decide to remove the unit. 128 Car digital tachometer The three leads must pass into the engine compartment and it is important that they are protected by a rubber or plastic grommet. It may be possible to squeeze them through an existing cable entry or you may have to drill a new hole, but either way make sure they are protected. Connection to the car electrics is fairly straightforward; the tacho input lead is connected to the CB terminal of the HT coil, which can be identified by the lead from the points and distributor to the HT coil. CB stands for con- tact breaker, often marked with a (-) minus sign. The supply would best be taken from the ignition switch via a 100 mA fuse, so that the unit is switched off when the car is not running. The easiest way of doing this would be to follow the other lead from the HT coil (marked with a (+) plus sign) up to the ballast resistor (if fitted), and make the connection to the other side of it, see Figure 5.7. Take note — Take note — Take note — Take note Not all ignition systems are the same so consult your workshop manual before attempting to fit the tacho. Also please remember that a car en- gine compartment is a hazardous area — never attempt to fit the tacho f or anything else for that matter, while the engine is running! Also, secure all cables away from hot or moving parts ! Anchor them to existing wiring looms using ca- ble ties. 129 Auto electronics projects To starter motor solenoid switch Γ2 ί Tachometer +/N connection here Contact To breaker distributor points Tachometer CB connection here Ballast resistor Figure 5.7 Connecting tachometer to a typical ignition system with contact breaker 130 Car digital tachometer Car digital tachometer parts list Resistors — All 0.6 W 1% metal film Rl,2, 560 Ω R3 100 k R4-10 150 Ω RH 390 k R12 1 k R13-15 10 k RV1 100 k vert encl preset 2 (M560R) 1 (M100K) 7 (M150R) 1 (M390K) 1 (M1K) 3 (Ml OK) 1 (UH19V) Capacitors CI 100 nF polyester 1 (BX76H) C2 47 nF poly layer 1 (WW37S) C3 1 μΡ 35 V tantalum 1 (WW60Q) C4 2n2F mylar 1 (WW16S) C5 1000 μΡ 16 V axial 1 (FB82D) C6 10 nF 50 V disc 1 (ΒΧ00Α) Semiconductors ICI 74LS221 IC2 74C925 IC3 NE555 IC4 LM78L05ACZ TR1-3 BC549 ZD1 BZY88C4V7 D2 1N4001 DY3 DD display type C 1 (YF86T) 1 (QY08J) 1 (QH66W) 1 (QL26D) 3 (QQ15R) 1 (QH06G) 1 (QL73Q) 1 (BY68Y) 131 [...]... n e d by e x p e r i m e n t a t i o n P6 is c o n n e c t e d t o t h e d r i v e r ' s d o o r s w i t c h , t h u s when t h e d o o r is o p e n e d , a c o m p l e t e path t o 0 V is provided b y t h e d o o r s w i t c h , allowing t h e b u z z e r t o s o u n d PI ο J L LN4001 ^ R3 ^ Γ P3 Figure 6.1 Ρ" P6 C i r c u i t diagram 135 Auto electronics projects When t h e ignition s w i t c h... t u a t i o n as many of t h e m , being fitted with e l e c t r o n i c ignition or e l e c t r o n i c engine m a n a g e m e n t s y s t e m s , j u s t plain refuse to b e pushstarted! 133 Auto electronics projects It is amazing t h a t s u c h m e c h a n i c a l l y a d v a n c e d c a r s often do not have a lights-on warning i n d i c a t o r of s o m e kind T o illustrate t h i s , t h e p.. .Auto electronics projects Miscellaneous 8-pin DIL s o c k e t 1 (BL17T) 16-pin DIL s o c k e t 2 (BL19V) dig t a c h o main PCB 1 (GA26D) dig t a c h o display PCB 1 (GA27E) 1 mm PCB pin 1 pkt (FL24B) red display... involve r e m o v i n g t h e u n d e r s i d e of t h e d a s h b o a r d , trim Drivers door switch in • Figure 6.3 line fuse Chassis connection may not be required See text PCB connections 137 Auto electronics projects panels, e t c It is advisable to refer to a w o r k s h o p manual, e.g of t h e Haynes variety; if you do not have one, e i t h e r buy o n e — as it will b e useful anyway, or b o... and c o n n e c t i o n s Ο f Ignition switched -M2V supply P.rmon.n via fuse 1 v JL // 4 ^~rt i V ^J~^'' JL _^Head lights^ Tail lights Side light Side light Car lights-on warning indicator 139 Auto electronics projects C o n n e c t P2 to a point in t h e wiring which b e c o m e s live when t h e ignition s w i t c h is turned t o accessory, i.e +V supply t o t h e radio (Figure 6.6) Alternatively,... c h p o s i t i o n s or d o o r s open or c l o s e d Assuming t h e unit is working c o r r e c t l y , refit u n d e r s i d e of d a s h b o a r d and trim p a n e l s Happy motoring 141 Auto electronics projects Lights-on warning parts list Resistors — all 0.6 W 1% metal film Rl R2 R3 3k9 10 k 100 k 1 1 1 (M3K9) (M10K) (M100K) 1 (QL80B) 1 1 1 (QL73Q) (QB66W) (YH96E) 1 1 pkt 1 1 1 1 1 strp 1 1... garage! Circuit description Figure 7.1 s h o w s t h e c i r c u i t diagram of t h e c o u r t e s y light e x t e n d e r P I and P3 c o n n e c t directly a c r o s s a d o o r switch 143 Auto electronics projects PI o- TR1 TIP122 9 D Rl 4k7 R2 560k Dl 1N4148 D2 1N4148 4k7 1 TR2 MPSA14 9 P3 cb ci •i 47uF O- Figure 7.1 C i r c u i t diagram c o n t r o l l i n g t h e i n t e r i o r light and P2,... o n e n t s a r e fitted is not c r i t i c a l ; h o w e v e r , t h e following i n s t r u c t i o n s will b e of use in making t h e s e t a s k s as straightforward as p o s s i ble 145 Auto electronics projects Insert and s o l d e r t h e PCB pins using a hot soldering iron If t h e pins a r e h e a t e d , v e r y little p r e s s u r e is r e q u i r e d to p r e s s t h e m into p o s i... electrical tape C o n n e c t P I , via a fuse and fuseholder, to a point in t h e wiring w h i c h b e c o m e s live w h e n t h e s i d e l i g h t s a r e s w i t c h e d on (Figure 6 4 ) C o n n e c t P6, to the driver's d o o r switch (Figure 6 5 ) T o p r e v e n t o t h e r d o o r s from operating the buzzer, install an MR751 diode in s e r i e s with t h e wire to the c o u r t e s y light 138... e work b e n c h than u n d e r n e a t h t h e c a r d a s h b o a r d ! Using a 9 t o 14 V supply ( i e PP3-sized b a t t e r y , b a t t e r y e l i m i n a t o r , e t c ) c o n n e c t P3 and P6 t o 0 V, t h e n c o n n e c t P I to +V, t h e b u z z e r s h o u l d s o u n d C o n n e c t P2 t o +V as well, t h i s s h o u l d s i l e n c e t h e buzzer Installation Refer t o Figures 6.3, . display type C 1 (YF86T) 1 (QY08J) 1 (QH66W) 1 (QL26D) 3 (QQ15R) 1 (QH06G) 1 (QL73Q) 1 (BY68Y) 131 Auto electronics projects Miscellaneous 8-pin DIL socket 16- pin DIL socket dig. etc.) connect P3 and P6 to 0 V, then connect PI to +V, the buzzer should sound. Connect P2 to +V as well, this should silence the buzzer. Refer to Figures 6. 3, 6. 4, 6. 5 and 6. 6. It is necessary. provide a 100 Hz signal as shown in Figure 5 .6, Signal to Tachometer OV Figure 5 .6 Mains frequency doubler for calibration 127 Auto electronics projects and a battery charger is very