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Simple Auto Tachometer Circuit Diagram


This is the Simple Auto Tachometer Circuit Diagram. Anyone performing their own automobile tune-ups knows how important it is to know your engines speed. With this tachometer, you can measure your engines speed without any connections or annoying timing lights. 

 Simple Auto Tachometer Circuit Diagram

Simple Auto Tachometer Circuit Diagram

 PC board layout and parts placement

Notes
  1. Calibrate the unit as folows:
a.    Set up this circuit:

  
  • Turn on the Tach and allow a few minutes for temperature stabilization.
  • Set S2 to 4 cylinders and adjust R5 for a meter indication of 180 (1800 rpm).
  • Set S2 to 6 cylinders and adjust R6 for a meter indication of 120 (1200 rpm).
  • Set S2 to 8 cylinders and adjust R7 for a meter indication of 90 (900 rpm).

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.

The unit draws power from the car battery. If it is connected backwards, it will not work, but it won't be damaged.
Parts


Part

Total Qty.

Description

Substitutions
C110.47uF Capacitor
C2147uF Electrolytic Capacitor
D118V 1W Zener Diode
D2, D3, D431N914 Diode
M11200uA Meter
Q1, Q222N3391A Transistor
R1, R2, R931K 1/2 W Resistor
R3147K 1/2 W Resistor
R4110K 1/2 W Resistor
R5, R6225K Trim Pot
R7110K Trim Pot
R81200 Ohm 2 W Resistor
R10115K 1/2 W Resistor
R1112.2K 1/2 W Resistor
S11SPST Togglae Switch
S21Three Position Single Pole Rotary Switch
MISC1Telescoping Radio Antenna, Enclosure, Power Cable and Battery Connector

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