Speedometer For Model Cars

Avid model car fans are
naturally enough always interested in the technology and performance of
their cars. They would like to know as exactly as possible how fast
their model cars actually go, for example so that they can select the
final gear ratio for the best performance. Other factors can also be of
interest, such as the total distance that the car has travelled, since
it is worth knowing how long a car can run on one battery charge or one
tank of gas. There are impressive instruments available in the shops for
making these measurements, ranging all the way up to complete telemetry

However, they vary in price from expensive to frightfully expensive.
This is reason enough for model builders with modest budgets to look
for possible alternatives. The designer of the speedometer described
here has worked out such an alternative, and it is as simple as it is
inexpensive. He developed an adapter circuit that allows a perfectly
ordinary bicycle computer to be used as a speedometer. These devices
only cost around £10, and they have the advantage that they can display
not only the speed but also elapsed driving time, average speed and the
total distance travelled. It’s hard to imagine anything better.


Most likely everyone knows how a bicycle computer gets its speed
pulses. They are generated by a pickup that registers the rotations of
the front wheel. This pickup consists of two components. One of these is
a magnet that is clamped to a spoke, while the other is a magnetic reed
switch that is fixed to the front fork. The reed switch is connected by
a thin cable to the computer, which is mounted on the handlebars. Each
time the magnet passes the reed switch, it causes the switch contacts to
close, and the computer receives a count pulse.

This pickup cannot be used with a model car. Even if you could
somehow attach the magnet to a wheel, the wheel would then be so out of
balance that the car could not be driven. Some other kind of pickup is
thus needed. An optical sensor is an obvious solution. It is a
non-contacting and frictionless sensor, just like the magnet and reed
switch combination, but with the extra advantage that no additional
moving mass is required. The magnet is replaced in this case by a highly
reflective stripe on the side of the tyre, and the reed switch is
replaced by an infrared reflective sensor.

The most satisfactory solution for the reflective stripe turns out
to be white or silver-coloured paint. From practical experience, the
stripe should be around 1 cm wide, but in any case it should not be any
wider than one tenth of the width of the non-painted portion of the
tyre. The reflective sensor should naturally be mounted on the car in a
way that allows it to properly detect the difference between the
reflective and non-reflective areas of the tyre.


The only other thing that the new sensor needs is a circuit that
converts the signal from the reflective sensor into pulses that can be
used by the bicycle computer. There are two things that have to be done:
first, to convert optical pulses into sufficiently strong electrical
pulses, and second to adapt the frequency of the pulses. The first of
these points probably does not need any further explanation. The second
has to do with the difference between the circumference of a bicycle
wheel and that of a model car wheel.

Smaller wheels rotate faster for the same vehicle speed, so they
produce pulses at a higher rate. Although the circumference of the
bicycle wheel can be set in the computer, there are naturally limits to
the range of possible settings. It is not possible to deal with a wheel
diameter ratio of ten using the circumference setting alone. This means
that the number of pulses must be reduced by a suitable factor.


As can be seen in Figure 1, a relatively simple bit of electronics
can adequately realise the requirements just described. The heart of the
circuit is the reflective sensor (OPTO1). A Siemens SFH9201 IC is used
for this. It is available from Conrad Electronics, among other sources.
In the first version of the circuit, the LED
was simply driven by a DC current. This proved to be unsatisfactory,
since the sensor also reacted to ambient light. This produced so many
erroneous pulses that the accuracy of the speedometer suffered greatly.
We thus switched over to driving the LED with a 10-kHz AC current early on in the design.

This has the advantage that an AC amplifier can be used for the
detector circuit, which largely eliminates the effects of ambient light
variations. The 10 kHz signal for the LED is produced by the oscillator built around IC1a. Gate IC1b acts as a buffer that drives the sensor LED
via transistor T1. Whenever the white stripe on the tyre passes in
front of the sensor, the phototransistor in the sensor will briefly
conduct at a 10 kHz rate. A pulse train with a frequency of 10 kHz is
thus produced across resistor R4. This signal is coupled out by
capacitor C6 and then amplified by an AC amplifier formed by transistors
T3 and T4.

This results in a 10 kHz pulse waveform across resistor R15. This is
buffered by gate IC1c and then applied to a detector circuit consisting
of the diode D2, resistors R6 and R7 and capacitor C7. The job of the
detector circuit is to convert the short series of pulses into a logical
‘1’. The component values are rather critical, since capacitor C7
should be charged before the stripe has passed completely by the sensor,
but it should also be fully discharged via resistor R7 before the
stripe again appears in front of the sensor and a new pulse train
arrives. The output signal of the detector is buffered by gate IC1d and
finally ends up at the last part of the circuit, the divide-by-ten
counter IC2. This allows only every tenth pulse from the detector to be
passed on to transistor T2. The open collector of this transistor is
connected to the input of the bicycle computer.


The circuit runs with a supply voltage of 5 V. This can usually be
derived from the receiver module in the car. In the author ’s prototype,
a 6V supply voltage was available for the receiver. Capacitor C1
provides extra filtering for this voltage, which is then used directly
to supply the LED in the optocoupler (U+). The
supply voltage for the rest of the circuit is stabilised at around 5 V
by resistor R1 and the Zener diode D1. Capacitor C2 acts as a reservoir
capacitor, while C3 and C4 provide local decoupling for IC1 and IC2.


The circuit is not particularly critical, and given the small number
of components, it is also not difficult to build. The best way to build
it depends in part on the shape of the model car in question. The most
important factor is naturally that the sensor OPTO1 must have an
unobstructed view of the reflective stripe on the tyre. Since space is
always a consideration in model building, the author has designed a
printed circuit board for the speedometer that largely uses SMDs.
Figure 2 shows the track and component layouts of this board. Although
this board worked well in the prototype, we must emphasise that it has
not been tested in the Elektor Electronics lab.

It should thus be seen as a suggestion, in the sense of ‘this is a
possible solution.’ In addition to the exact construction of the adapter
circuit, the manner in which the bicycle computer itself is mounted
will naturally be largely determined by the specific features of the
model car in question. We leave this question to the inventiveness all
of those who build the speedometer circuit. Connecting the circuit is
dead easy. Wire the 6 V supply to the electrolytic capacitor C1 (with
the right polarity!), and connect the two leads of the computer cable to
resistor R10 and earth, respectively.

On the prototype board shown in Figure 2, the supply connections can
be made out with a bit of effort next to the labels TP1 and TP2, while
the output connections are labelled TP3 and TP4. Finally, there is one
last remark regarding setting the value of the wheel circumference in
the bicycle computer: don’t forget the factor of 10 provided by the
divider in the adapter circuit! For example, if the tire of the model
car has a circumference of 21cm, a circumference of 210 cm must be set
in the bicycle computer.

R1 = 220kΩ
R2 = 120kΩ
R3.R9 = 10kΩ
R4,R14,R15,R16 = 1kΩ
R5 = 33kΩ
R6 = 3kΩ9
R7 = 270kΩ
R8 = 100Ω
R10 = 470Ω
R11 = 8kΩ2
R12 = 180Ω
R13 = 1kΩ8

C1 = 100µF 16V
C2 = 100 µF 10V
C3,C4 = 100nF
C5 = 1nF
C6 = 10nF
C7 = 22nF
C8 = 1µF 10V

D1 = zener diode 5V6 1W3
D2 = 1N4148
T1,T2,T3 = BC547B
T4 = BC557B
IC1 = 74HC132SO
IC2 = 4017SO
OPTO1 = SFH9201 (Siemens)

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