10 LED Tachometer

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

R3 = CAN BE VARIABLE 100K POT

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

Need Help? Please send your queries through Comments for quick replies!




Comments

Mark McDonald said…
How difficult to expand output from LED Bar Graphic to LED numeric output???
Swagatam said…
the above circuit cannot be modified for that, it will need a completely new design
Hampton Smith said…
So could you in theory change the frequency input to a magnetic pick up and have this work as an led speedometer?
Swagatam said…
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 said…
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 said…
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 said…
Awesome. Thanks for the response and happy holidays :)
Phil Ellis said…
Hi there
Where does the input come from and will this work on a single cylinder 2 stroke engine.
Cheers
Phil
Swag said…
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
Phil Ellis said…
Hi
Thanks for the fast reply. Do I just wrap the wire around the HT lead.
Phil
Swag said…
yes that will do, also attach a 12V zener across base emitter of T1
abioye said…
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 said…
thanks abioye, yes you can do that....
abioye said…
sir can i use 22uf for c4 because i could not get 33uf thanks for your quick reply sir
Swag said…
yes that will do...
Jesse said…
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 said…
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.
afanlab said…
sir is it possible to change lm3915 to lm3914?
Swag said…
yes that's possible!
J. Laparaon said…
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 said…
Thank you J. Laparaon for sharing your views, I am glad the design helped you and served the purpose. Please keep posting!
Rohit said…
Where to connect FREQ and if i m using it on scooty India what is value of C5 . What is R3 pot means. Can it use for 110 cc engine. Thank you
Rohit said…
What is voltage of c1,c2,c3
Swag said…
Frequency inut can seen at the base of BC547. R3 pot can be used for optimizing the best possible response on the LED bar for the entire frequency range. You can use this circuit for any engine.
Swag said…
disc caps are always rated at 50V or higher
Rohit said…
Sir what is used on resistance on ic Lm 3915 leg 5 is confusing resistance has only 2 legs how to make that connection with other resistance. How 10k is connected in centre . Thank you
Swag said…
Rohit, that's a preset as shown below:

https://homemade-circuits.com/wp-content/uploads/2015/09/preset-2.jpg

short the center pin with any of the outer pins together, use this shorted pin to connect with the 10K resistor and pin#5, and then connect the other pin of the preset with the ground line.
Rohit said…
Sir why R6 (resistance) is having two wire where to connect it .
Swag said…
R6 has a single end only, the other one is a square wave symbol...

make sure to make all the negative lines or the ground lines in common, otherwise the circuit will not work
Rohit said…
Sir i can't see a value of D3 and R2 and R9 are missing but listed in parts list where R2 and R9 will connect.
Swag said…
Rohit, R2 and R9 are not there, they were included for some other application, you can ignore them...D1, D2, D3 are all 1N4148

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