1 1 0.47uF Capacitor C2 47uF Electrolytic Capacitor
D1 8V 1W Zener Diode
D2, D3, D4 1N914 Diode
M1 200uA Meter
Q1, Q2 2N3391A Transistor
R1, R2, R9 1K 1/2 W Resistor
R3 47K 1/2 W Resistor
R4 10K 1/2 W Resistor
R5, R6 25K Trim Pot
R7 10K Trim Pot
R8 200 Ohm 2 W Resistor
R1 15K 1/2 W Resistor
R1 2.2K 1/2 W Resistor
S1 SPST Togglae Switch
S2 Three Position Single Pole Rotary Switch
Notes:
1. Calibrate the unit as folows:
a. Set up this circuit:
b. Turn on the Tach and allow a few minutes for temperature stabilization.
c. Set S2 to 4 cylinders and adjust R5 for a meter indication of 180 (1800 rpm).
d. Set S2 to 6 cylinders and adjust R6 for a meter indication of 120 (1200 rpm).
e. Set S2 to 8 cylinders and adjust R7 for a meter indication of 90 (900 rpm).
2. To use the Tach, turn it on and let it sit for one minute to allow for temperature stabilization. Extend the antenna, select the right number of cylinders and hold the unit over the engine. If the reading is erratic or the needle jumps around, move the antenna closer to the ignition coil or spark plug wires.
3. The unit draws power from the car battery. If it is connected backwards, it will not work, but it won't be damaged.