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 Request
Hi,I am interested in designing a sequential LED tachometer circuit for a 12V car. how may i design this circuit, Tachometer signal, is it not in frequency? and i would want the LED's to light up sequentially as the the engine RPM gets higher?
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.
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. 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
R2 = 47E
R3 = CAN BE VARIABLE 100K POT
R4 = 3K3,
R5 = 10K,
R6 = 470K,
R7 = 1K,
R8 = 10K,
R9 = 100K,
C1 = 47n,
C2 = 100n,
C3 = 100n,
C4 = 33uF/25V,
T1 = BC547
IC1 = 555,
D1,D2 = 1N4148
Hi
Thanks for the fast reply. Do I just wrap the wire around the HT lead.
Phil
yes that will do, also attach a 12V zener across base emitter of T1
Hi there
Where does the input come from and will this work on a single cylinder 2 stroke engine.
Cheers
Phil
Hi, the input comes from your bikes pickup coil, or from the spark plug.
reduce R6 to 10K if it is from the pickup coil
How difficult to expand output from LED Bar Graphic to LED numeric output???
the above circuit cannot be modified for that, it will need a completely new design
So could you in theory change the frequency input to a magnetic pick up and have this work as an led speedometer?
yes that's possible, but the pickup output does not determine the vehicle speed, rather it's the wheel RPM which is normally considered for speed indications.
Right, but couldn't you add a variable resistor to the frequency input and calibrate the circuit so that for example: 1 led would light up when the wheel reaches 10mph (or the wheel rpm equivalent) or is there a simpler way to achieve the same result?
yes that's possible, I meant to say that speedometer is usually linked with the wheel for getting the right assessment of the RPM, the engine RPM could be accelerated even while the wheels are stationery.
Awesome. Thanks for the response and happy holidays 🙂
you are welcome!