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.
What is a Tachometer
A tachometer is a device which is used for measuring vehicle engine RPM. Thus, it is basically used for checking the performance of the engine and helps an auto mechanic to understand the condition of the engine so that it can be corrected or optimized as per the desired specs.
Generally a tachometer may be considered an expensive equipment as these are highly accurate and intended for obtaining correct RPM rates of the concerned engine under test.
The conventional units are therefore very sophisticated and generate highly accurate results while testing.
However it doesn't mean that a simpler version cannot be built at home. With electronics at its best today, making a tachometer circuit at home isn't at all difficult. What's more the results obtained from such circuits are fairly accurate and provides the required data for assessing the overall working condition of the system.
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 = 1uF,
- C2 = 100n,
- C3 = 100n,
- C4 = 22uF/25V,
- C5 = 2.2uF/25V
- T1 = BC547
- IC1 = 555,
- D1, D2, D3 = 1N4148
Using only LM3915
A closer inspection of the above circuit reveals that the IC 555 stage is actually not required and seems like an overkill for the purpose.
The main concept here is to convert the frequencies into an average DC whose level would be proportionate to the input frequency level. This implies that a simple diode, resistor, capacitor network would be enough to accomplish this action.
Also called an integrator, this small circuit network could be integrated with the LM3915 for ensuring that the voltage level stored in the capacitor is proportionately varied depending on the frequency levels.
Faster frequencies would allow the capacitor to charge and hold the DC proportionately better resulting in a higher average DC output and vice versa. This would in turn produce an equivalent level of LED illuminations on the LEDs attached with LM3915 output.
Here's the simplified version of the 10 LED tachometer using just a single IC M3915.
A Video Demo for the above Circuit can be witnessed below:
My Conclusion is not Correct
It's indeed very silly of me, since I completely missed the point that the above circuit was only interpreting the voltage generated by the motor, so it is not representing the frequency or the RPM, rather only the generated voltage levels.
Although this may be also proportionate to the RPM, it is technically NOT a tachometer circuit.
Therefore I confess that the first circuit shown using the IC 555 circuit the actual and true tachometer design.
Simple Tachometer Circuit
So far we studied a 10 LED version of a tachometer, however the idea could be much simplified using a moving coil meter as explained below. Here we learn how to build a simple IC 555 based tachometer circuit which can be used for directly measuring any frequency over an analogue volt meter.
Circuit Operation
The circuit diagram shows a simple configuration utilizing the IC 555. The IC is basically configured as a monstable multivibrator.
The pulse is derived from the spark plug and fed to the end of R6.
The transistor responds to the pulses and conduct in accordance with triggers.
The transistor activates the monostable with every rising pulse of the input.
The monostable stays ON for a particular moment each time its triggered and generates an average ON time at the output which is directly proportional to the average trigger rate.
The capacitor and the resistor at the output of the IC integrate the result so that it can be directly read over a 10V FSD voltmeter.
The pot R3 should be adjusted such that the output generates the exact interpretations of the fed RPM rates.
The above setting up must be done with the help of a good conventional tachometer unit.
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 = 1uF/25V,
C2 = 100nF,
C3 = 100n,
C4 = 33uF/25V,
T1 = BC547
IC1 = 555,
M1 = 10V FSD meter,
D1, D2 = 1N4148
Video Demo shows the testing of the above circuit
Hi Sir. Thanks for this project
As i want to use it to read rpm with a microcontroler… do you know to tell me if the output voltage is pwm ?
i have 2 options..
one is to use 2 resistors at the output as a voltage divider and read rpm with an analog input (but i want to use my analog inputs for other purposes)
and the second one if it is pwm output to use microcrocontrolers digital inputs for pwm
thanks in advance
Hi Caslor,
If you are using a microcontroller then the IC 555 circuit will not be required. You can directly feed the pulses from the hall effect sensor attached on the rotating shaft.
In the above design the output from the IC 555 will be a PPM not PWM, meaning it will be pulse position modulation. The position of the pulses will closer or wide apart depending on the RPM frequency.
Hi
Unfortunately my car is from 1970’s and the distributor doesn’t have any hall effect sensor.
i tried to use a single NPN transistor circuit to take pulses from the spark-plug cable but it was very noisy signal .
that is why i thought to use your circuit to get more cleared signal.
I will use an Arduino as a microcontroller and i saw that can read also PPM signals with interrupts
so maybe i can use your circuit with an additional 2 resistors at the output for voltage divider and drop the voltage to 5v max
In that case you can use the first circuit, just make sure to remove the resistor and capacitor network at the pin3 of the IC, and then you can connect your resistive divider.
Thanks again for the help
So i have to remove R7 & R8 , C4 & C5 .
what about D3 diode? should i remove it also or keep it ?
You can remove the diode also, it has no significance.
I am a auto electrician in sri lanka . I have found this circuit diagram when I am trying to find a circuit digram in internet to convert deisel pump rpm sensor signal to square wave puls .But I have changed my idea and I intend to make this circuit with led display to fit to the relavent car directly .Thank you for this unexpected help I got from your website .
Glad you found this circuit helpful, wish you all the best!
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.
Rohit, R2 and R9 are not there, they were included for some other application, you can ignore them…D1, D2, D3 are all 1N4148
Sir why R6 (resistance) is having two wire where to connect it .
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
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
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.
What is voltage of c1,c2,c3
disc caps are always rated at 50V or higher
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
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.
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.
Thank you J. Laparaon for sharing your views, I am glad the design helped you and served the purpose. Please keep posting!
sir is it possible to change lm3915 to lm3914?
yes that’s possible!
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?
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.
sir can i use 22uf for c4 because i could not get 33uf thanks for your quick reply sir
yes that will do…
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
thanks abioye, yes you can do that….
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!