Digital tachometer. Encyclopedia of radio electronics and electrical engineering

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

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Unfortunately, many domestic and imported cars lack one very important device - a tachometer.

I offer a simple but reliable electronic tachometer that works on my Ford Escort car. The scheme published in [1] was taken as a basis. The device has a two-digit digital indicator showing the number of thousands and hundreds of revolutions per minute. The tachometer is powered by the vehicle's on-board network and consumes a current of 0,45A. The circuit diagram of the device is shown in the figure.

The tachometer consists of a control unit on the IC DD1 and elements DD2.1, DD2.3, DD2.4; two generators on transistors VT1 ... VT4; input pulse shaper - Schmitt trigger VT5, DD2.2; pulse counter on IC DD3, DD4; intermediate memory on ICs DD5, DD6; decoder on IC DD7, DD8 and indicator H1.

The operation of the control node is described in detail in [1]. The multivibrator on VT1, VT2 sets the measurement time, and the multivibrator on VT3, VT4 determines the operating cycle of the device. The input shaper and counter are assembled according to a typical scheme and do not have any special features, so they immediately start working. Intermediate memory on triggers has been introduced into the tachometer circuit to prevent flickering of the indicator digits during counting. The K155TM5 triggers are synchronized with a signal from pin 8 DD1 passing through capacitor C4 and DD9.1 to obtain the desired duration and phase. The operating cycle time is set by selecting the resistor R11, and the measurement time is set by selecting the resistor R7.

For a conventional automotive four-stroke four-cylinder engine, an inductive pickup is used. It consists of 50 ... 70 turns of PEL 1.0 wire wound turn to turn on a high-voltage wire connecting the ignition distributor to the ignition coil.

One end of the sensor coil must be insulated, and the other end must be connected to the input of the tachometer.

(click to enlarge)

The frequency of the ignition pulses and the number of revolutions of the engine are interconnected by the formula: f=2n/60 where f is the frequency of the ignition pulses; n is the number of revolutions per minute of the motor shaft.

So, the value of the number of revolutions 3000 corresponds to a pulse frequency of 100 Hz. But since the tachometer indicator should show 3.0 at this time, only 30 out of 100 pulses per second should go to the counter. Therefore, the measurement time in this case is set to 0,3 s. The working cycle time should be 10...20 times longer (3...6 s).

Construction and details. The two-digit Bulgarian-made VQE24 digital indicator can be replaced by two domestic ones - ALS324B or ALS342B. If you replace the KR514ID2 decoders with K514ID1, you can exclude resistors R13 ... R19, R21 ... R27 from the circuit and use ALS324A indicators with a common cathode. All microcircuits of the 155 series can be replaced with the corresponding series 133, 555, 1533. KT315 transistors can be replaced with KT3 12, and KT361 with KT326. Diodes VD2, VD3 - - any silicon: KD510, KD521, KD509. Capacitors C1, C2, C6 - K53-1, K53-14; C3, C5 - K73-17; the rest - KM5, KM6. IC voltage stabilizer DA1-type KR142EN5A, can be replaced by KR142EN5V.

All parts of the electronic tachometer, except for R1 and a digital indicator, are located on a double-sided printed circuit board measuring 60x115 mm. The board is placed in a polystyrene case measuring 65x145x35mm. DA1 is mounted on a small finned heatsink. About 1 V drops across resistor R5, which greatly facilitates the thermal regime of the stabilizer. If all the parts are in order, the device immediately starts working. The adjustment is as follows: a signal with a frequency of 5 Hz is applied to the VT100 base, and by selecting R7, the indicator readings are set to 3.0, which corresponds to 3000 rpm.

In conclusion, I want to note that according to this scheme, my friends assembled several tachometers that differ only in design, and all of them have been working fine on various cars for several years.

Literature

1. Shirokov B. Digital tachometer//Radio. - 1983. - N9. - P.28-29.
2. Biryukov S. Digital devices on integrated circuits. - M.: Radio and communication, 1984.

Author: V.Kravchuk, Brest; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Automobile. Speedometers and tachometers.

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