AUTOMOTIVE TACHOMETER WITH BAR-DOT DISPLAY OUTPUT
(Schematic diagram is named: tach_bd.jpg)

    Circuit for an automobile tachometer using the LM2917 frequency to voltage converters. The circuit is the same for 4,6 or 8 cylinder engines. However, the component values are different. On a gasoline powered engine, the pickup (input) comes from the distributor. Most distributor caps have a tap-off that can be used. The equations used with this chip are:
Vo=Vcc x Fin x (R4+R5) x C2: where Vo=output voltage, Fin=input frequency in hertz, R4+R5=resistance connected to pin 3 ,C2=capacitance connected to pin 2, Vcc is 7.5 volts utilizing the on-board regulator.
and RPM=(120 x freq.)/ (# of cylinders): to find the input frequency depending on the engine RPMs and the number of cylinders.

DESCRIPTION & CONSTRUCTION
Design of circuit for a four cylinder gasoline engine.
RPM Freq., Hz Vo
1000 33.3 1.0
2000 66.6 2.0
3000 100 3.0
4000 133 4.0
5000 166 5.0
6000 200 6.0

    The input frequency is divided and filtered prior to input to pin 1. The value of these components may have to be changed dependent on the specific automobile engine. The values of C2 and R4+R5 are determined first using the equations shown above. Pick a common value for C2, then calculate the value of R4+R5. Pick a common value for R4 remembering that R5 is variable. C3 is used for ripple control. If the ripple on the output is too large, increase the value of this capacitor. R3 provides current to the on-board regulator. The output from pins 4&7 is put across resistor R6.

    The values of the components are: R1=10K, R2=20K, R3=470, R4=39K, R5=10K, R6=100K, C1=.015/.02 uF, C2=.1uF and C3=.015/.02uF. All resistors are half watt unless otherwise noted. Capacitors are non-polarized disk, mylar or tantalum.

    The output of the frequency-to-voltage converter is fed into two cascaded LM3914's, Bar Dot Display Drivers. These chips are configured in the bar mode. This produces a display bar representing the engine RPMs. This mode is preferable since the dot mode outputs only one lit LED at a time which represents the engine RPMs. The dot display is very difficult for the driver to see when driving the car. The resistors, R7 & R8, form a voltage divider so half the input voltage (0 - 3V) is output by this driver. Resistors, R9 & R10, form a voltage divider for the second driver. The overflow from the first enables this chip. This driver outputs the second half of the input voltage (>3 - 6V). The twenty LED's can be any size and color. It is preferable to use different colors for the cardinal points of 1000, 2000, 3000, 4000, 5000 and redline. This will enable easy viewing and interpretation of the display.

    The values of the components are: R7=1K, R8=1K, R9=1K and R10=3K. R11 (=15@1W) is used with the voltage regulator, LM309H. This provides the voltage for the LED display.

CALIBRATION
Calibrate the meter before installation using a pulse generator. Set the generator to the maximum input frequency, 1.5 mS pulse width, 10 V amplitude. Adjust the 10K pot to get the maximum bar-dot display. Check all intermediate frequencies for the correct output display.

Bob Ausman
Questions should be directed via E-mail to: ausman@tecnet1.jcte.jcs.mil