The 555 timer is used here as an idea for a car tachometer.



What is a car tachometer?

A tachometer is simply a means of counting the engine revolutions of an automobile engine. In this suggested idea a NE555 timer is configured as a monostable or one shot. The period of the 555 timer for our car tachometer is determined by the resistor from the 9V regulated supply to pin 7 (50K) and the capacitor from pin 6 and ground (0.1 uF). Time "T" = 1.1 RC  or  1.1 X 50,000 X 0.1 X10 -6  =  0.0055 or 5.5 mS (milli-seconds).

The 555 timer receives trigger pulses from the distributor points. These are limited by the 1K resistor and the 5V zener diode, these are AC coupled to the trigger input through the 0.1 uF coupling capacitor. The 50 mA meter for the car tachometer receives calibrated current through the 200K calibration potentiometer (a linear type) when the NE555 timer output is high. After the timer has timed out, the car tachometer meter movement receives no further current for that part of the duty cycle.

This image is copyright © by Ian C. Purdie VK2TIP - schematic of a car tachometer

Figure 1 - schematic of a car tachometer

Integration of the variable duty cycle by the meter movement produces a visible indication of the automobiles engine speed.

Supply is taken from the 12V automobile system and for the NE555 timer car tachometer is reduced to a regulated 9V by the 15 ohm resistor in conjunction with the 9V zener diode. Note that the 10 uF polarised capacitor must be placed physically as close as possible to the supply pin 8. It is depicted in the schematic diagram in this manner purely for clarity.

I am particularly indebted to Philips Components and Semiconductors Australia for their most generous assistance in giving me access to material presented on this page.

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Bibliography: - Philips Semiconductors - Data Handbook IC11

the author Ian C. Purdie, VK2TIP of asserts the moral right to be identified as the author of this web site and all contents herein. Copyright © 2000, all rights reserved. See copying and links. These electronic tutorials are provided for individual private use and the author assumes no liability whatsoever for the application, use, misuse, of any of these projects or electronics tutorials that may result in the direct or indirect damage or loss that comes from these projects or tutorials. All materials are provided for free private and public use.
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Updated 20th August, 2000