Wireless Auto Tachometer

Schematic

PC board layout and parts placement

Parts

Part Total Qty. Description Substitutions C1 1 0.47uF Capacitor C2 1 47uF Electrolytic Capacitor D1 1 8V 1W Zener Diode D2, D3, D4 3 1N914 Diode M1 1 200uA Meter Q1, Q2 2 2N3391A Transistor R1, R2, R9 3 1K 1/2 W Resistor R3 1 47K 1/2 W Resistor R4 1 10K 1/2 W Resistor R5, R6 2 25K Trim Pot R7 1 10K Trim Pot R8 1 200 Ohm 2 W Resistor R10 1 15K 1/2 W Resistor R11 1 2.2K 1/2 W Resistor S1 1 SPST Togglae Switch S2 1 Three Position Single Pole Rotary Switch MISC 1 Telescoping Radio Antenna, Enclosure, Power Cable and Battery Connector

Notes

1. Calibrate the unit as folows:
1. Set up this circuit:
   
   
   
2. Turn on the Tach and allow a few minutes for temperature stabilization.
3. Set S2 to 4 cylinders and adjust R5 for a meter indication of 180 (1800 rpm).
4. Set S2 to 6 cylinders and adjust R6 for a meter indication of 120 (1200 rpm).
5. 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.