The post explains how an accurate 10 LED tachometer circuit can be built using ordinary parts like IC 555 and IC LM3915. The idea was requested by Mr. Munsif.

The Design

A simple 10 LED tachometer circuit can be seen in the above diagram.

The circuit basically consists of two mains stages. A monostable based tachometer using IC 555, and an LED driver stage using IC LM3915.

Referring to the figure below, the left side stage consists of a IC 555 monostable stage which triggers to the input frequencies from a given source such as an automobile engine, and causes its output to stay ON for a predetermined period as set by the R/C components at its pin6/2.

Circuit Diagram

This situation allows the user to set the response pattern of the output.

The output triggering of the IC 555 is further smoothened by an integrator stage using R7/R8 and C4/C5.

The integrated or smothened output is applied to a 10 step dot/bar LED driver LM3915 circuit stage.

The processed frequency to voltage conversion from the IC 555 tachometer circuit is appropriately displayed across the 10 LEDs associated with the LM3915 IC.

Since pin#9 of the IC is attached with the positive rail, the LED displays a bar mode pattern of the frequency level or the RPM level of the connected engine.

The 10 LED bar graph ascend or descend their illumination in response to the frequency levels from the automobile engine and allow the circuit to be used like an effective 10 LED tachometer.

Parts list for the IC 555 section

Parts List

R1 = 4K7


R4 = 3K3,

R5 = 10K,

R6 = 470K,

R7 = 1K,

R8 = 10K,

C1 = 47n,C2 = 100n,

C3 = 100n,

C4 = 22uF/25V,

C5 = 2.2uF/25V

T1 = BC547

IC1 = 555,

D1, D2, D3 = 1N4148

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