Digital tachometer-watch. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Digital tachometer-watch. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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This on-board device is designed to measure the crankshaft speed of a four-stroke gasoline internal combustion engine (main mode) and display the current time (secondary mode). The display of the device shows the rotation frequency with a resolution of 1 min-1 and seconds, minutes and hours. The tachometer is powered by the vehicle's on-board network and consumes a current of about 0,012 A.

Schematic diagram of the tachometer is shown in fig. 1. The basis of the device is the DD1 PIC16F628 microcontroller from Microchip. It is also possible to use another microcontroller of this company, but minor adjustments to the program and board will be required. Other components of the device: input pulse shaper (resistors R1-R3, zener diode VD1 and transistor VT1); voltage regulator (zener diode VD2, capacitors C3 - C6, DA1 chip); liquid crystal indicator (HG1); voltage divider on resistor R10 and LED HL1 to power indicator HG1 (about 1,7 V); controls: switch SA1 "Mode" to select the operating mode of the device ("Tachometer" - "Clock") and buttons SB1 "HB" to set the time and SB2 "SW" to set the time in clock mode.

The controller is reset automatically when the power is turned on. With a high level at the MCLR input, the controller is in the operating mode. The program also provides for the automatic transition of the controller to its initial state when it freezes, for which the built-in watchdog timer is used.

The device uses a ten-digit liquid crystal module - an indicator equipped with a Holtek HT1613 controller with sequential loading of information via the DI line and synchronization via the CLK line. Information is fed to the DI input (pin 4), it is fixed by the fall of the clock pulses at the CLK input (pin 3).

The module is a printed circuit board measuring 67x36 mm, on which the actual indicator and controller are located. Dimensions of the visible field of the indicator - 35x12 mm, symbol height - 10 mm. Module supply voltage - 1,2...1,7 V, current consumption - no more than 10 μA.

In addition to the function of indicating the measured value of the speed, the module performs the function of a clock and a timer with the output of this information to the indicator in real time. To work in the tachometer mode, the NC input (pin 5) of the module must be connected to a common wire, and the inputs S1, RST and S2 (pins 6-8) should be left free.

The indicator can display 16 different characters, each of which is encoded with a four-digit binary number. When loading the first of them into the module, it is displayed in the rightmost position of the scoreboard. When the second character is loaded, the first is shifted to the left, and so on.

The timing diagram for loading symbol codes into the indicator is shown in fig. 2. Minimum time parameters: ta=1 µs, tv=2 µs, tc=5 µs. At the same time, it takes approximately 170 μs to completely update the indicator readings. The refresh period of the displayed information should not be less than one second.

Digital tachometer-watch

In a car equipped with a standard ignition system, the tachometer input is connected to the primary winding of the ignition coil. If the breaker is built on a Hall sensor, then the tachometer input is connected to the sensor output (usually to its middle output). It is also possible to connect the input through a capacitive sensor installed on the high-voltage output of the ignition coil. The tachometer power wires are best connected directly to the battery.

All parts of the tachometer, except for the indicator module, are mounted on a single-sided printed circuit board measuring 85x54 mm made of foil fiberglass 1 mm thick. The drawing of the board is shown in fig. 3. The indicator module board is connected to the tachometer board with short flexible insulated wires. The module board can be mounted on racks parallel to the main board (it has appropriate mounting holes) or at an angle.

(click to enlarge)

The tachometer is not critical to the choice of parts used. Resistors and capacitors may have a tolerance of ±10%. The voltage stabilizer KR142EN5A (KR142EN5V or imported 7805 is also suitable) does not need a heat sink. The KT315B transistor can be replaced with a KT3102 with any letter index, the KS133A zener diode with KS139A, and the KS515A with KS518A or another voltage of 15 ... 19 V (you can also use a special automotive SIOV S10K14AUTO varistor from Siemens Matsushita Components).

We will replace the AL307B or AL307BM LED (only "red" is suitable!), Working as a low-voltage stabistor, with a KS113A (and KS115A) stabistor, but at the same time, the R10 resistor will need to be selected according to the stabilization operating current. Capacitors C1, C2, C4 and C5 - KM-5, KM-6; C3, C6 - imported oxide. Resistors - MLT, S2-33. Switch SA1 - PD9-2; buttons SB1, SB2 - MP12. The indicator module can be replaced by any other with the HT1613 controller

The program in Intel HEX format, which must be entered into the DD1 controller, is presented in the table.

(click to enlarge)

Correctly assembled from serviceable parts, the device does not need to be adjusted and starts working immediately after power is applied. The accuracy of the readings of the device depends on the frequency of the quartz resonator ZQ1.

For those who want to repeat the described design, I recommend that you familiarize yourself with the publications indicated in the bibliography.

The program of the microcontroller DD1 in assembly language MPASM V2.50.02

Literature

Novozhilov B. On-board tachometer on PIC16C84. - Radio, 1999, - 3, p. 40-42.
Dolgiy A. Development and debugging of devices on the MK. - Radio, 2001, - 5, p. 17-19; - 6, p. 24-26; - 7, p. 19-21; - 8, p. 28-31; - 9.S.22-25; - 10.S.14-16; - 11, p. 19-21; - 12, p. 23-25; 2002, - 1, p. 18,19.

Author: A. Ulyanov, Velikie Luki, Pskov region.

See other articles Section Clocks, timers, relays, load switches.

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