Elektor Electronics 10/98

Like many low-priced cars and motor-cycles, mopeds and (motor) scooterstend not to have a rev counter fittedby the manufacturer, presumably ongrounds of economy. However, suchan instrument is relatively inexpensiveand may be very useful, particularlyon vehicles with manual gear change.For instance, the combined readings ofthe speedometer and tachometer givea good indication of whether the rightgear has been selected. A falling read-ing on the rev counter is a sign tochange down, while a rising onepoints to the need of changing up.Many riders who do not have the con-

venience of a rev counter argue thatgear changing is done by ear, but thecompulsory safety helmet does notalways allow this: the sound insulationof some helmets is very good indeed!Best is, of course, to have an automaticgearbox, fortunately chosen by moreand more riders. Second best is tobuild and fit the present tachometer.

The combined readings of speedo-meter and rev counter may also beuseful in improving fuel consumption,but this implies that the power curveof the engine is known.

There are people whofeel that every moped

and motor scootershould be fitted with

a tachometer (revcounter) as standard.There are others whofind it a dodgy instru-ment since it tends to

distract the rider’sattention from the

road. If you belong tothe first category and

have a scooter ormoped without a revcounter, this article is

for you. It describes astraightforward

design of such aninstrument that can

be fitted to any modelof moped or scooter.

34

Design by L. Lemmens

tachometerfor mopeds and(motor) scooters

GENERAL INTEREST

D E S I G NThere are various waysof constructing atachometer, that is, themanner of its readout.Basically, there are threeways of achieving this:in figures via a seven-segment display,via an analogue scale consisting oflight-emitting diodes (LEDs), or via atraditional moving coil meter withpointer.

The moving-coil type is the sim-plest construction, but is also vulnera-ble to shocks and vibrations. Thismakes it not really suitable for use ona moving vehicle.

A readout via a seven-segment dis-play is highly accurate, but perhapstoo sophisticated for use on a moped.The high accuracy is not needed andwould make the design more complexthan necessary.

An analogue (LED) readout is bothsimple and robust. It can make use ofseveral types of control IC that enablean analogue voltage to be displayedon a bar of LEDs with only a fewexternal components. If the bar con-sists of, say, 20 diodes, the readout issufficiently accurate for most purposes.

The only other itemthat is needed is anelectronic circuit with asensor that providespulses in proportion tothe number of enginerevolutions. These

pulses are converted by the electroniccircuit into an analogue direct voltageto drive the LED bar.

S E N S O RIdeally, the sensor should produce apulse for each engine revolution andthis is most easily achieved with theaid of an inductor (coil) to pick up theignition pulses inductively. Since thevoltage in the ignition pulses is fairlyhigh, it suffices to construct the coilfrom 10–20 turns of insulated circuitwire around the spark-plug cable.

While the voltage level of the igni-tion pulses is fairly high, their shapevaries appreciably. Therefore, the sen-sor is followed by a pulse shaper totransform the ignition pulses into sta-ble, uniform count pulses. This ensuresthat random variations in the widthand amplitude of the ignition pulsesdo not affect the readout.

C I R C U I T D E S C R I P T I O NThe complete circuit diagram of thetachometer is shown in Figure 1. Thepick-up coil (sensor) is linked to capac-itor C3. This capacitor, in conjunctionwith resistors R3 and R4, forms a dif-ferentiating circuit that narrows theignition pulses into usable triggerpulses – an arrangement that preventsdouble triggering of the rev counter.The reshaped pulses are applied to thetrigger input of monostable (multivi-brator) IC3. This circuit outputs pulseswhose width can be preset with P1.

The pulses output by IC3 are inte-grated by a simple low-pass filterformed by R6 and C1. This filter alsoremoves any short-duration variationsof the output which otherwise mightmake the readout unstable.

The LED readout is driven by twodisplay drivers, IC1 and IC2. These cir-cuits are specially designed for thispurpose and contain a reference volt-age source and an accurate decadescaler.

Each of the drivers can control amaximum of ten LEDs, so that thetachometer can use up to 20 diodeswhich gives a sufficiently accuratereadout. Each of the LEDs representsabout 500 engine revolutions. The

35Elektor Electronics 10/98

D5

D6

D3

D4

D2

D10

D1

D8

D9

D7

R1

22

k

D15

D16

D13

D14

D12

D20

D11

D18

D19

D17

R2

2k

2

REFOUT

REFADJ

LM3914

IC1

MODE

SIG

RHI

RLOL10

17

16

15

14

13

12

11

10L9

L8

L7

L6

L5

L4

L3

L118

L2

9

5

8

4

6

7

3

2

1

REFOUT

REFADJ

LM3914

IC2

MODE

SIG

RHI

RLOL10

17

16

15

14

13

12

11

10L9

L8

L7

L6

L5

L4

L3

L118

L2

9

5

8

4

6

7

3

2

1

R7

22

k

JP1

TLC555

IC3DIS

THR

OUTTR

CV

2

7

6

4

R

3

5

8

1

R6

10

0k

C2

10n

C4

100n

C1

10µ16V

R5

22

k

R3

22M

R4

15M

50k

P1

C3

10n

5V

5V

5V

5V

5V

980077 - 11

groen

geel

rood

green

yellow

red

grün

gelb

rot

4805IC4

C5

10µ25V

C6

10µ10V

C7

100n

5V6...7V

6000 rpm

8000 rpm

10000 rpm

Figure 1. The circuit ofthe tachometer con-sists of a pulseshaper, an integratingcircuit, and a readout.The sensor is placedaround the spark plugcable.

1

Setting upThe pulse width of the output of the pulse shaper, and thus the drive voltagefor the readout, can be set within a wide band with P1.

Calibrating the scale may be done in a number of ways: with anothertachometer as reference, with a pulse generator, and also without any specialequipment. This is possible by using the pick-up coil to sense the frequencyof the mains voltage (in a safe manner!). This very stable signal at 50 Hz isexcellent for calibration purposes, since it corresponds to 50¥60=3000 rev/min.So, if the proposed maximum of 10000 rev/min is adopted, P1 should beadjusted at 50 Hz so that D6 (3000 rev/min) lights.

An ideal source for the 50 Hz mains frequency is a demagnetizer for acassette deck. The electric field radiated by this is readily picked up by thetachometer sensor. Never, never connect the input of the rev counter sirectlyto the mains: this may be lethal and, even if you’re lucky to survive, will destroythe tachometer.

means that the circuit cannot be con-nected directly to the battery terminalsof the moped or scooter. A stable sup-ply is obtained by the use of a 5 V reg-ulator between the battery and the revcounter as shown in Figure 1. Sincethe voltage at the battery terminals isonly about 6–7 V, the regulator must bea low-drop type such as the 4805: astandard 7805 will not do!

It is also possible to power thetachometer indepen-dently by a pack of

36 Elektor Electronics 10/98

Parts list

Resistors: R1, R5, R7 = 22 kΩR2 = 2.2 kΩR3 = 22 MΩR4 = 15 MΩR6 = 100 kΩP1 = 47 kΩ (50 kΩ) preset

Capacitors:C1 = 10 µF, 16 V, radialC2, C3 = 0.01 µF, pitch 5 mmC4 = 0.1 µF, pitch 5 mm

Semiconductors:D1–D13 = low-current LED, greenD14–D16 = low-current LED, yellowD17–D20 = low-current LED, red

Integrated circuits:IC1, IC2 = LM3914IC3 = TLC555

Miscellaneous:JP1 = 2-terminal 2.54 mm pin strip

and pin jumper (Maplin)Enclosure: Conrad Type 842230-55

(see text)Sensor: see textPCB Order No. 980077 (see Readers’

services towards the end of thisissue).

(C) ELEKTOR

980077-1

(C) ELEKTOR

980077-1

C1C2

C3

C4

D1

IC1

IC2

IC3

P1

R1

R2

R3

R4R5

R6

R7

T

+0

980077-1

LEDs may be of different colours tocreate, say, a safe (green) range of rev-olutions of 500–6000 rev/min (D1–D12);a caution (yellow) range of 6000–8000rev/min (D13–D16); and a danger (red)range above 8000 rev/min (D17–D20).Different ranges may, of course, bechosen to individual requirements.

Comparators are driven via each ofthe junctions of the scaler in the dis-play drivers in such a way that everytime the input voltage to the displaydriver increases the next comparator isenabled. The comparator outputs arecapable of driving an LED directly.

The LED bar may be operated inthe dot or bar mode. In the dot mode,pin 9 of the IC must be left open, andin the bar mode it should be linked tothe positive supply rail. In the presentapplication the bar mode is used.

Figure 2. Constructionof the tachometer isstraightforward whenthis printed-circuitboard is used. It isavailable through ourReaders’ services.

2

3

P O W E R S U P P L YThe tachometer needsa power supply of5–6 V. The supply railsshould be stable, which

Figure 3. The com-pleted board in theConrad enclosurementioned in the text.

four series-connected chargeable or dry1.5 V batteries (AA=HP7=LR6 orC=HP11=LR14). A regulator is then,of course, not needed. The life of suchbatteries is lengthened by using thedisplay drivers in the dot mode (inwhich pin 9 of the devices is left open).

C O N S T R U C T I O NThe electronics is best built on theprinted-circuit board shown in Fig-

ure 2. It is generally agreed that a cir-cular readout is to be preferred andthis is why the 20 LEDs have beenarranged in a circle on the board. Inview of the sparsity of components,populating the board is simplicity itselfif the circuit diagram and the parts listare followed carefully.

Pin strip and jumper JP1 enablesthe circuit to be checked on comple-tion of the construction. During such

a check, the jumper should beremoved.

When pulses are applied to capac-itor C3, it should be possible to varythe low direct voltage at the terminalof JP1 linked to junction R6-C1 with P1.If this is so, the pulse shaper operatescorrectly.

When a variable direct voltage at alevel of a few volts is applied to theother terminal of JP1, one of the dis-play diodes should light.

Forming the pick-up coil aroundthe spark plug cable (10–20 turns ofthin insulated circuit wire) should notpresent undue difficulties. The coilshould be linked to the input pin ofthe tachometer via insulated strandedcircuit wire.

In some areas it may be possible toobtain a round enclosure to house therev counter. A suitable one is producedby Conrad (Germany) and may beavailable from our regular advertiserStippler Elektronik via another regularadvertiser, Viewcom Electronics. Themodel number of the enclosure isgiven in the parts list.

[980077]

37Elektor Electronics 10/98

Figure 4. This photoclearly shows how thepick-up coil is wound(25 turns) auround theignition cable.

4