Automobile tachometer. Free Technical Library article

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Car tachometer

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The proposed device is very simple according to the scheme, but it has good technical characteristics, it is assembled on available components. A tachometer can be very useful when adjusting operations with electronic ignition units of a car engine, when setting the economizer thresholds accurately, etc. "Radio" magazine once published an article by A. Mezhlumyan "Digital or analog?" -1986, No. 7, p. 25, 26.

The tachometer is designed to measure the crankshaft speed of a four-cylinder automobile gasoline engine. The device can be used both for adjustment work at idle, and for operational control of the engine shaft speed during movement.

The measurement cycle is 1 s, and the indication time is also 1 s, i.e. during the indication time, the next measurement takes place, the indicator readings change once per second. The maximum measurement error is 30 min~1, the number of indicator digits is 3; switching of measurement limits is not provided. The tachometer has a quartz stabilization of the clock generator, so the measurement error does not depend on the ambient temperature and changes in the supply voltage.

Schematic diagram of the tachometer is shown in fig. 1. Functionally, the device consists of a quartz oscillator assembled on a DD1 chip, an input node on a VT1 transistor, an input pulse frequency tripler on DD2.1-DD2.3 elements and a DD3 counter, DD4-DD6 counters, DD7-DD9 code converters, digital indicators HG1-HG3 and supply voltage stabilizer OA1. The signal to the input node of the tachometer comes from the contacts of the breaker.

(click to enlarge)

After applying the supply voltage trigger DD2.1, DD2.2 may be in any state (out of two possible). Suppose that at the output of element DD2.2 there is a low-level voltage that prohibits the passage of pulses with a frequency of 2.3 Hz through the element DD1024 from the output F of the counter DD1 to the counting input СР of the counters D03 and DD4.

When the contacts of the breaker open, the transistor VT1 opens, switches the trigger DD2.1, DD2.2 and opens the element DD2.3. Counters DD3 and DD4 will start counting pulses with a frequency of 1024 Hz. On the decline of the third input pulse of the counter DD3 at its output 2, a pulse will be generated that will switch the trigger DD2.1, DD2.2 to its original state, the element D02.3 will be closed again, and the counter DD3 will be reset. At the next pulse from the interrupter, the process will be repeated. Thus, with each opening of the breaker contacts, the number written into the counter circuit DD4-DD6 will increase by 3.

The recording process will continue for a second, i.e. until the moment when the next positive voltage drop appears at the output S1 of the counter DD1. At this moment, the information accumulated in the counters DD4-DD6 will be rewritten into the buffer registers of the code converters DD7-DD9, and soon the counters DD4-DD6 will be reset by the signal from the C5R9 circuit at the R input. Immediately after the high-level pulse decays at the input of the counters DD4-DD6, a new recording cycle will begin, etc. To ensure the necessary time delay between the moments of rewriting information from the counters DD4-DD6 to the buffer registers of the code converter DD7-DD9 and resetting the counters, differentiating circuits C3R6 are used , C4R8, C5R9 and element DD2.4.

Tripling the frequency of the pulses coming from the interrupter is necessary to obtain a correspondence between the indicator readings and the engine speed in min-1. Since the counting time of the input pulses is 1 s, the number 2N3 / 60 will be written to the counters, and then the indicators will be displayed, where N is the crankshaft speed in min-1, 2N is the sparking frequency. At a shaft speed of 3000 min-1, the indicator will read 3.00.

All parts of the tachometer, except for the voltage regulator DA1 and indicators HG1-HG3, are placed on a printed circuit board made of double-sided foil fiberglass. A drawing of a printed circuit board and the location of parts on it are shown in fig. 2. The tachometer is not critical to the type of parts used. The values of resistors and capacitors may differ from those indicated in the diagram by ±20%.

Resistor R1 is a CMM, but since high-resistance resistors are rather scarce, mounting pads are provided on the board for installation instead of a single resistor with a nominal value of 22 MΩ, several smaller resistances in series.

Quartz resonator ZQ1 - any, from a digital watch. The VD1 zener diode is any small-sized one for a stabilization voltage of 3 ... 5 V. The K176 series microcircuits can be replaced with the corresponding K561 series. The KR142EN8A microcircuit stabilizer is installed on a heat sink with an area of about 10 cm2.

The tachometer uses seven-element indicators AL304G (digit height - 3 mm) with high brightness and relatively low current consumption (about 5 mA per element). The brightness of the glow is quite enough for a confident reading of information in the car, even in sunny weather. The tachometer board should be covered with a dense light filter of the corresponding color.

If necessary, you can use indicators with larger numbers, for example, ALS321A, ALS321B, ALS324A, ALS324B. The current consumed by each of their elements is much higher - up to 20 mA, therefore, to ensure the reserve of the brightness of the glow, the output current of the decoders must be increased. Current amplifier circuits for indicators of the ALS321 and ALS324 series are shown in fig. 3, a and b.

Please note that when using indicators with a common cathode ALS321A and ALS324A, a low-level voltage should be applied to the input S of the DD7-DD9 code converters (pins 6 connected to a common wire).

Of course, the use of large-sign indicators will require the correction of the printed circuit board and the installation of the DA1 stabilizer on a larger heat sink (at least 30 cm2).

A tachometer correctly assembled from serviceable parts starts working immediately, and the display should show a zero reading in about 2 seconds after turning on the power. If this does not happen, you should check the presence of second pulses at the output S1 of the counter DD1. Their absence or a noticeable difference in the period from 1 s most likely means a malfunction of the quartz resonator. To check the performance of the remaining nodes of the tachometer, you can apply a signal from pin 3 of the DD1 counter (pulses with a frequency of 128 Hz) through a 10 kΩ resistor to the base of the transistor VT1. In this case, the number 3.84 should appear on the indicator.

In a car equipped with a standard battery ignition system, the tachometer input is connected to the breaker terminal. With a non-contact electronic ignition system, a tachometer can be connected to its output by increasing the resistance of resistor R3 to 200-250 kOhm, and it is advisable to install this resistor not on the board, but in a break in the wire going from the tachometer board to the ignition system output. This is due to the fact that the voltage at the output of the electronic ignition system can reach 400 V or even more, which can lead to breakdowns on the tachometer board. If the electronic ignition system is powered by a contact interrupter, then the tachometer is connected to the output of the interrupter by reducing the resistance of the resistor R3 to 12 kOhm.

Author: A. Biryukov, Moscow, Radio No. 11 1997; Publication: cxem.net

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