AUTOMOTIVE TACHOMETER (the schematic is named tach.jpg)

Circuit for an automobile tachometer using the LM2907 or LM2917 frequency to
voltage converters. The LM2917 has a regulator onboard. 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 circuit can be adopted for use
with a diesel engine. On the diesel engine, tap into the varying A.C. voltage
coming off the alternator. The equations used with this chip are:
Vo=Vcc x Fin x R1 x C1: where Vo=output voltage, Vcc= supply voltage,
Fin=input frequency in hertz, R1=resistance of R1, C1=capacitance of C1
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 Io(uA)
1000 33.3 17
2000 66.6 33
3000 100 50
4000 133 67
5000 166 83
6000 200 100

Using a 100uA meter movement and the 14 pin LM2917 package, the following
component values are used. For the LM2917 Vcc=7.5V when the voltage
regulator is used.

Calculate the value of R1 x C1, and pick a common value for the capacitor. In this
case, .01uF was picked. This makes the value for R1=360K. The capacitor(.1uF) in
parallel with R1 is used to damp the meter. This may have to be changed
depending on the meter movement. The full scale meter deflection(100uA in this
case) is set using the 10K pot in series with the 39K resistor connected to
pin 5. The input frequency is divided and filtered prior to input to pin 1. These
values may have to be changed dependent on the specific automobile engine. All
resistors are half watt. Capacitors are non-polarized disc, mylar or tantalum. The
diode is 1N4001.

Construct the circuit on a vector board. Ensure the potentiometer is placed for easy
access. The entire circuit can be fitted to the input terminals of the meter. This
makes for a clean, compact tachometer that can be mounted anywhere.

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 100uA meter reading. Check all frequencies for the correct output
current.

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