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# Speedometer For Model Cars(SFH9201)

2017-01-18 16:52
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This article describes the Speedometer For Model Cars (SFH9201). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components: SFH9201.

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 systems.

#### Figure 1

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.

#### Figure 2

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.

#### Figure 3

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.

#### Figure 4

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.

THEPRACTICALCIRCUIT:

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, theLEDwas 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 theLEDwith a 10-kHz AC current early on in the design.

#### Figure 5

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 theLEDis produced by the oscillator built around IC1a. Gate IC1b acts as a buffer that drives the sensorLEDvia 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.

#### Figure 6

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.

POWERSUPPLY:

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 theLEDin 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.

CONSTRUCTION:

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 usesSMDs. 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.

Resistors:
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

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

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