AUTOMOTIVE TACHOMETER (the
schematic is named tach.jpg)
Circuit for an automobile tachometer using the LM2907 or LM2917
voltage converters. The LM2917 has a regulator onboard. The circuit is
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.
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.
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
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
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
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
case) is set using the 10K pot in series with the 39K resistor connected
pin 5. The input frequency is divided and filtered prior to input to pin
values may have to be changed dependent on the specific automobile
resistors are half watt. Capacitors are non-polarized disc, mylar or
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
makes for a clean, compact tachometer that can be mounted anywhere.
Calibrate the meter before installation using a pulse generator. Set the
the maximum input frequency, 1.5 mS pulse width, 10 V amplitude. Adjust
10K pot to get 100uA meter reading. Check all frequencies for the
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