Tachometer for a car. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Speedometers and tachometers

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It is sometimes interesting for the driver to know how many revolutions the car engine develops. This can be determined using a simple transistor tachometer (Fig. 1), the measuring device of which, graduated in revolutions, is conveniently located near the steering.

Fig. 1

The basis of the tachometer is a controlled multivibrator on T1 transistors. T2. It gives narrow rectangular pulses, with a duration of about 0,5 microseconds, with a constant amplitude. The constancy of the amplitude of the output pulses is achieved by stabilizing the voltage supplying the multivibrator using the Zener diode D1.

Transistor T3, connected according to the emitter follower circuit, serves to match the output resistance of the multivibrator with the internal resistance of the measuring head of the IP. The indicator gives readings when pulses from the multivibrator arrive at the base of the transistor TK. Since their amplitude and duration are constant, the instrument readings are directly proportional to the repetition rate of these pulses. The more often they are, the greater the reading of the device.

The pulses controlling the multivibrator are fed to the capacitor C3 and the base of the transistor T2. They are obtained from a capacitive sensor made in the form of several turns (5-10) of PELSHO 0,31 wire wound on a common wire supplying power to the distributor (Fig. 2). To prevent the sensor from moving along the wire, its beginning and end are reinforced with sticky vinyl chloride insulating tape.

Fig. 2

The instrument is calibrated according to the scheme in Fig. 3 using a square wave generator. The voltage at the output of the generator is set to about two volts. When the tachometer is operating, it receives 4, 6 or 8 pulses (from the ignition system) per revolution of the engine shaft, depending on the number of its cylinders. Thus, the number of revolutions will be equal to the number of pulses received divided by the number of cylinders. For example, if the frequency of impulses from a 4-cylinder engine is 10 per minute, then its number of revolutions is 000: 10 \u000d 4 rpm.

Fig. 3

The readings of the device may be unstable or incorrect if pulses with an excessively large amplitude are supplied from the speed sensor. In this case, it is necessary to reduce the number of turns of the sensor.

If it is not possible to use a measuring signal generator, then with sufficient accuracy for practice, you can calibrate the tachometer using a conventional ohmmeter. In this case, you need to select the total resistance of the resistors R7 and R8 according to the table.

The design of the device can be any. The easiest way is to attach the circuit board, which carries the main elements, to the back of the pointer device, to its output terminals.

In the tachometer, you can use any low-frequency, low-power transistors, for example, MP40 and a Zener diode D808 or D809.

Literature

1. "Practical Wireless", 1972, February.
2. Radio 9-72, p.59

Publication: N. Bolshakov, rf.atnn.ru

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