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  
