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.
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.
Questions should be directed via E-mail to: email@example.com