Maplin auto electronics projects P6

Maplin auto electronics projects

engine s p e e d would t h e r e f o r e not allow enough time between pulses for triggering the m o n o s t a b l e This de-

sign is for 4-cylinder c a r s only and anyone using it on a 6

or 8-cylinder c a r would have to modify the count period accordingly, or use a c o m p e n s a t i o n factor on the read-ings — not e a s y while driving!

T h e output pulses from ICla, pin 12, are fed to the count input, pin 11, of IC2 This is a 4 digit c o u n t e r with both latch and r e s e t It drives a multiplexed 2 digit display directly, with transistors T R I and TR2 selecting the digit, and r e s i s t o r s R 4 - R 1 0 limiting the segment current T h e

c o u n t e r requires latch pulses in order to give s e n s i b l e readings and t h e s e are provided by IC3, TR3 and their

a s s o c i a t e d c o m p o n e n t s IC3 is the ever useful 5 5 5 , used

as an o s c i l l a t o r w h o s e frequency is c o n t r o l l e d by RV1 The oscillator output waveform, arranged such that there

is a long high and a short low period, is inverted by TR3

s o that a s h o r t high period is a c h i e v e d This s h o r t pulse

is used to c o n t r o l the latch of the c o u n t e r d e v i c e IC2, s o that when this input goes high the information in t h e counter is transferred to the internal latch and displayed

T h e trailing edge of this s h o r t pulse is used to trigger the m o n o s t a b l e I C l b , w h o s e output pulse is used to re-

set the counter so that it will start counting from 00 again

T h e use of a s e p a r a t e m o n o s t a b l e to r e s e t the c o u n t e r

ensures that the reset pulse always o c c u r s after the latch

pulse and a true reading displayed

B e c a u s e the supply voltage of a car, nominally 1 2 - 1 4 V, varies between t h e s e 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 o s c i l l a t o r (IC3) Diode D2 and c a p a c i t o r C5 r e m o v e any n o i s e on the supply

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Construction

T h e Digital T a c h o m e t e r is c o n s t r u c t e d on two PCBs; t h e main board and the display board T h e display board is mounted at 90° to the main board by being s o l d e r e d to Veropins, and this holds the display s o that it can be

viewed through the cut-out display window at the end of

the c a s e Figure 5.2 shows the c o n s t r u c t i o n a l details First j o b , however, is to build up the main printed cir-cuit board Referring to Figure 5.3, begin by fitting the smallest c o m p o n e n t s first Check the polarity of C3, C5, and the direction of Dl, D2 before fitting, then work your way through the c o m p o n e n t s by size fitting C5, the larg-

est, last Insert the ICs into the appropriate s o c k e t s only

Hain Board

Figure 5.2 Preparing the veropins for attaching main and

display PCBs together

Figure 5.3 Digital tachometer main PCB and legend layout

after all o t h e r c o n s t r u c t i o n of the t a c h o m e t e r module is

c o m p l e t e d , taking the usual p r e c a u t i o n s with CMOS

de-v i c e s Note that the negatide-ve end of C5 must be c l o s e to the PCB or you may find that adjusting RV1 is difficult during calibration!

Display board

Refer to Figure 5.4 First mount r e s i s t o r s R4, R5, R9, RIO,

and the veropins from the component side, being

care-ful not to strip the pads off t h e PCB in t h e p r o c e s s ! A hot soldering iron will help to push the pins home Don't

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forget to fit the wire link (this can be made from an cut from a r e s i s t o r , or with single-strand w i r e ) Solder and c r o p the r e s i s t o r s and the wire link Next fit the dis-play to the PCB; pins 1 to 9 are on t h e side where the decimal point will be found, and pins 1 and 18 are marked

off-on the PCB Solder and c r o p pins 1 to 18 Now m e a s u r e the required length for the display board pins by offer-ing t h e display b o a r d up to t h e main PCB, 3 - 4 mm is about right; s e e Figure 5.2 If t h e pins are t o o short, the

c o n n e c t i o n s won't be m e c h a n i c a l l y strong After ming the pins down you can s o l d e r the display b o a r d to the main PCB T h e pins on the display board also require soldering, and if this has already been done, you may find that the two b o a r d s do not marry snugly to e a c h

to earth, preferably at the HT coil end Label the tion of e a c h wire at the end that will c o n n e c t to the c a r

func-e l func-e c t r i c s If you arfunc-e going to usfunc-e thfunc-e optional box, thfunc-e front panel of the c a s e is r e p l a c e d by a p i e c e of red filter

cut to size (using the original panel as a t e m p l a t e ) with a pair of s c i s s o r s or craft knife This s l o t s neatly into the

c a s e , which is moulded in two s e c t i o n s As you may have

n o t i c e d , t h e r e is no m e t h o d of mounting the t a c h o ule into the suggested box, s o t h e alternative is to use

mod-q u i c k s t i c k pads T h e s u g g e s t e d b o x a l s o n e e d s to b e modified by removing part of t h e b a t t e r y c o m p a r t m e n t ; only the top and front partitions of this need to be re-moved, the sides will help to keep the PCB c e n t r a l in the box, and the s c r e w holes must remain intact or e l s e the box c a n n o t be fastened t o g e t h e r ( s e e 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

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Setting up

One advantage of a digital t a c h o m e t e r over an analogue type is the e a s e of setting-up and calibration Only one adjustment to RV1 n e e d s to be made and, barring acci-dents, will prevail for t h e life of the unit This setting

e n s u r e s that the o s c i l l a t o r runs at the c o r r e c t frequency and the method of calibration depends on the equipment available Calibration against a n o t h e r t a c h o m e t e r is pos-

s i b l e , s e t t i n g RV1 t o give a d i s p l a y of 3 0 w h e n t h e standard t a c h o m e t e r reads 3000 r.p.m If you have ac-

c e s s to a signal g e n e r a t o r , set the frequency to 100 Hz (sine or s q u a r e w a v e ) and t h e output level to maximum ( m o r e than 4.7 V ) C o n n e c t 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 c a r r i e d 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

and a b a t t e r y c h a r g e r is very effective in this role In either c a s e RV1 is adjusted to give a display reading of

30 Calibration should include a t e s t run for up to half an

hour or s o 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 b e fitted to the car

It is i m p o s s i b l e to give detailed i n s t r u c t i o n s for every car but the following n o t e s may b e helpful

• it is a good idea to try the unit in various positions for b e s t readability, using adhesive tape, until you are satisfied,

• having decided on the b e s t position use sided tape, adhesive pads or two p i e c e s of velcro-tape, one glued to the unit and one glued to the c a r dashboard All of t h e s e methods, of c o u r s e , mean that the unit can

double-be removed easily and the d a s h b o a r d c l e a n e d and left unmarked,

• alternatively, use self-tapping s c r e w s through one half of the c a s e into the d a s h b o a r d This works well, but unless you can utilise existing s c r e w holes you will be left with holes in the d a s h b o a r d if you decide to remove the unit

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T h e t h r e e leads must pass into the engine c o m p a r t m e n t and it is important that t h e y are p r o t e c t e d by a r u b b e r

or plastic grommet It may be p o s s i b l e to s q u e e z e them through an existing c a b l e entry or you may have to drill

a new hole, but either way make sure they are p r o t e c t e d Connection to the c a r e l e c t r i c s is fairly straightforward; the t a c h o input lead is c o n n e c t e d 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 tact breaker, often marked with a ( - ) minus sign T h e

con-supply would b e s t be taken from the ignition switch via

a 100 mA fuse, s o that the unit is switched off when the

c a r is not running T h e e a s i e s t way of doing this would

be to follow the o t h e r lead from the HT coil (marked with

a (+) plus sign) up to the ballast r e s i s t o r (if fitted), and make the c o n n e c t i o n to the o t h e r side of it, s e e 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

To starter

motor solenoid switch

Tachometer CB connection here

Ballast resistor

Figure 5.7 Connecting tachometer to a typical ignition system with contact breaker

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Car digital tachometer parts list

Resistors — All 0.6 W 1% metal film

car digital t a c h o m e t e r kit 1 (LK79L)

Optional (not in kit)

small r e m o t e c o n t r o l b o x 1 (LH90X) in-line fuse holder 1 (RX51F)

1 74 in 100 mA fuse 1 ( W R 0 8 J )

v e l c r o mounts as reqd (FE45Y)

c a b l e tie-wrap 100 as reqd ( B F 9 1 Y )

1 3 2

indicator

If your c a r is not fitted with s o m e kind of lights-on ing, t h e c h a n c e s are that you will at s o m e time (if you have not already done so!) leave your lights s w i t c h e d

warn-on Murphy's law d i c t a t e s that when you do s o , your

a b s e n c e from t h e c a r will b e of sufficient duration to

e n s u r e that t h e b a t t e r y will b e well and truly flat Of

c o u r s e Murphy, not c o n t e n t to do things by halves, will

e n s u r e that it h a p p e n s when you are late for s o m e portant o c c a s i o n and that t h e r e is no one e l s e around to give you a push or a jump start!

im-Modern c a r s further aggravate t h e situation as many of them, 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 management s y s t e m s , just plain refuse to be push-started!

It is amazing that such m e c h a n i c a l l y advanced c a r s

of-ten do not have a lights-on warning indicator of s o m e

kind T o illustrate this, the p r o t o t y p e was installed in a 2.0 litre petrol-injection Ford S i e r r a E s t a t e — despite being a Ghia, there was no lights-on warning device! Various warning d e v i c e s are available, however, s o m e

b e c o m e a n u i s a n c e b e c a u s e t h e y sound c o n t i n u o u s l y when the lights are d e l i b e r a t e l y left on For i n s t a n c e , while the driver is waiting in the c a r at night, with the engine switched off

Some more s o p h i s t i c a t e d d e v i c e s will not sound if the

lights are switched on again after the ignition has been

switched off, i.e for parking lights However, this fails

to warn the driver if he inadvertently knocks the light

switch on when leaving the c a r — as could be the c a s e

with many c a r s having the light switch stalk on the

driv-e r ' s door siddriv-e of thdriv-e stdriv-edriv-ering column

This lights-on warning indicator will emit a clearly ble buzzing sound when the c a r lights are left on, the

audi-i g n audi-i t audi-i o n s w audi-i t c h audi-is t u r n e d - o f f and t h e d r audi-i v e r ' s d o o r opened In this manner the buzzer will only sound when the driver is genuinely about to leave the car

Now that you are thoroughly c o n v i n c e d that for the sake

of a few pounds, you need not be caught out in the ture, why not build this handy a c c e s s o r y (which t h e manufacturer should have included as s t a n d a r d ) and fit

fu-it into your c a r ? Enterprising r e a d e r s may wish to offer

this add-on to friends, relatives and n e i g h b o u r s for a

suitable fee (don't forget to tell the tax man!) A personal

tale of woe and the a s s u r a n c e that, " / V e got one and it has stopped me from getting caught out againF is sure

to win a few favourable r e s p o n s e s

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Circuit description and operation

T h e circuit of t h e lights-on warning indicator is very ple, as c a n b e s e e n from F i g u r e 6 1 H o w e v e r , it is worthwhile to know how t h e circuit o p e r a t e s as this will help, should p r o b l e m s o c c u r

sim-P I of t h e unit is c o n n e c t e d to t h e sidelight circuit of t h e car and provides power to the circuit only when the lights are switched on T h e sidelight circuit is live when either sidelights or headlights are s w i t c h e d on

P2 is c o n n e c t e d to the a c c e s s o r y circuit and when the ignition switch is off, P2 is pulled low via r e s i s t o r R3 (P3

is c o n n e c t e d to O V ) Diode Dl is forward b i a s e d and turns on t r a n s i s t o r T R I via r e s i s t o r R2 Note that the internal r e s i s t a n c e of a c c e s s o r i e s (i.e r a d i o - c a s s e t t e ) may b e sufficiently low to make t h e c o n n e c t i o n to P3 unnecessary; this can be determined by experimentation P6 is c o n n e c t e d to t h e driver's d o o r switch, thus when the door is opened, a c o m p l e t e path to 0 V is provided

by t h e door switch, allowing t h e buzzer to sound

When the ignition switch is on, P2 is pulled high, r e v e r s e biasing diode D l R e s i s t o r R3 ensures that t r a n s i s t o r T R I

is held in the off s t a t e T h e positive supply to buzzer BZ1 is removed and thus prevents it from sounding, re-gardless of w h e t h e r the driver's door is open or shut When the lights are off and the c a r doors are c l o s e d , the polarity of the supply to the unit is effectively r e v e r s e d Diode D2 prevents damage to the circuit under this con-dition

Construction

A s s e m b l y of the unit is simplicity itself Referring to ure 6.2, it is advised that the PCB pins are fitted first, followed by the r e s i s t o r s and the diodes and finally the

Fig-t r a n s i s Fig-t o r Make sure Fig-thaFig-t Fig-the Fig-t r a n s i s Fig-t o r is fiFig-tFig-ted fairly

c l o s e to the PCB o t h e r w i s e the PCB will not fit into the

c a s e

Next solder the buzzer's wires to the PCB pins, red (+V)

to P4, black ( - V ) to P5 Attach the c o n n e c t i n g wires to the PCB pins and label the free ends so that you can iden-tify the wires after the PCB has been fitted into the c a s e !

T h e PCB simply lies in the c a s e , the wires protruding through the aperture provided S c r e w the c a s e t o g e t h e r

Figure 6.2 PCB legend and track

1 3 6