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
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
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J. Laparaon says
I was concerned about the tach because of the price. I needed to measure the speed of my lathe at different speed settings. I added a small piece of the reflective tape to the drive shaft and the tach worked great. It measured the speed at each setting and was stable. The readings were able to be repeated the next day. This is a great product especially for the price
Best regards.
Swag says
Thank you J. Laparaon for sharing your views, I am glad the design helped you and served the purpose. Please keep posting!
afanlab says
sir is it possible to change lm3915 to lm3914?
Swag says
yes that’s possible!
Jesse says
I don’t see the capacitance for C5 listed. If u used a 33uF in circuit lab to simulate there was no output, and I found an output of .5v – 4.5v for the estimated frequency range of my cars crank signal (66Hz idle, 755Hz rev limiter.) I still need to hook it to a function generator to test operation. I was just curious what your design calls for a value of C5?
Swag says
The output RC network works like an integrator and makes sure that the output ripples are converted into a reasonably good exponentially rising and falling DC, which can be measured, or appropriately interpreted by the subsequent stages.
C5 can be a 4.7uF/25V cap.
also you can try increasing the value of C1 to 1uF, and adjust the R3 pot carefully for getting the right calibration at the output.
abioye says
sir can i use 22uf for c4 because i could not get 33uf thanks for your quick reply sir
Swag says
yes that will do…
abioye says
sir more wisdom and knowledge in jesus name, can this work for my electric car if i tap the pwm from the motor controller to the above circiut
Swag says
thanks abioye, yes you can do that….
Phil Ellis says
Hi
Thanks for the fast reply. Do I just wrap the wire around the HT lead.
Phil
Swag says
yes that will do, also attach a 12V zener across base emitter of T1
Phil Ellis says
Hi there
Where does the input come from and will this work on a single cylinder 2 stroke engine.
Cheers
Phil
Swag says
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
Mark McDonald says
How difficult to expand output from LED Bar Graphic to LED numeric output???
Swagatam says
the above circuit cannot be modified for that, it will need a completely new design
Hampton Smith says
So could you in theory change the frequency input to a magnetic pick up and have this work as an led speedometer?
Swagatam says
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.
Hampton Smith says
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?
Swagatam says
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.
Hampton Smith says
Awesome. Thanks for the response and happy holidays š
Swagatam says
you are welcome!