# DIY LED Tachomter (RPM Gauge)

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## Introduction: DIY LED Tachomter (RPM Gauge)

This is my LED project. An LED tachometer for my 1977 PEUGEOT 504.

## Step 1: Building the Circuit

This is the circuit I used in my project. A simple one which costs me about 35 Egyptian Pounds for components (<6 US Dollars)

:::::::::::::::::::: List of components: ::::::::::::::::::::::::
Integrated Circuits:
1 x LM2917  This is the frequency to voltage converter you need to convert the signal from the car's ignition coil into voltage
2 x LM3914  This is LED display driver. Each one can drive 10LEDs in either bar mode like this project or dot mode. Switching between both modes can be done, more instructions in datasheet.
1 x 7809 Volt regulator (outputs +9 Volts)

LEDs:
21 LEDs of any color you want. I used bright white ones. You can use some green, some yellow & some red for the red zone. You MUST use bright ones if you are planning to put paper covers over the LEDs or they are going to be simply invisible
The first LED is continously lit as long as the +12V source is connected. The remaining 20 will be lit one by one as you increase the engine speed

Diodes:
1 x  1N4007
1 x  1N4148
1 x  Zener diode 12 Volts

Resistors:
1 x 220 Kohm Trimpot variable resistor (for calibration)
2 x 1 Kohm
3 x 10 Kohm
1 x 22 Kohm
1 x 2.2 Kohm
1 x 470 ohm     (I forgot I have one :) So, I connected two 1Kohm in parallel, see photos)

Electrolytic Capacitors:
1 x 470 µFarad 25 Volts (µ = micro)
1 x 2.2  µF      16 Volts

Polypropylene Capacitors:
3 x 100 nF  (nano)
1 x 47 nF
1 x 1 µF  (Instead,I used 8 x 100nF in parallel = 0.8 µF  :\$ & it works)

:::::::::::::::::: List of other things you need:  ::::::::::::::::::::::::
Soldering equipment
Digital multimeter
Circuit board
Wires
Aluminium heatsink
A lot of patience

## Step 2: Testing the Circuit

This is the circuit I've built. Tested on my car

Connections as follows:
1- Power: red wire to car battery +12 volts. Later on, I connected it to the 12V accessory line in the fuse box. This line functions only when you turn the car key on.
2- Ground: blue wire to car chassis (any metallic part of the car body)
3- Signal: white wire to -ve terminal of the Ignition coil. Refer to your car manual in case you cannot identify the ignition coil, it may vary in shape from car to another. Be careful not to manipulate connections related to ignition coil especially when the engine is running. The function of this coil is to generate high voltage (30-50 Kilo Volts) & you are not supposed to recieve such sparks by mistake. It's better to wear gloves & remove the key from the car to be safe while making your connections.

This video is before calibration. I've made a visit to a friend of mine whose car with RPM gauge where I made my calibration

Calibration was made later on my friends car with RPM gauge. I made the calibration through the 220kohm trimpot resistor so that every 3 LEDs = 1000 RPM

## Step 3: Mounting to the Instrument Cluster

First I needed to provide a place to put my project in. The best one was the old  -& non working- clock of my car. I simply removed it to find plenty of room for my circuit.

I cut my board to fit well, making a secondary circuit board for the 20 LEDs.

I modified a simple RPM gauge design to print on a photo paper. I made 2 prints above each other, so that not so much backlight will pass through. I used a scalpel to cut the yellow "windows" in front of my LEDs from the first paper only, keeping the second one intact.

662 Enrolled

## 116 Discussions

Really nice. Loved it. Bookmarked :)

hey, i followed your given circuit and made one such but i am using it in bike,then from where to get -ve of ignition coil

Anyone have the PCB designe for this?

Hi Abo!

Can you ask me where you conect the signal capble??

Thank u! My English is not good, maybe i wrote wrong something. I hope u understand me. :)

Dear abo_hosni,
Could tell me the value of resistor R6 and R7 if I use 5VDC power supply; and can I use IC 555 instead of LM 2917 to convert input signal to voltage?
Tks.

that would we make your entire circut less reliable. the voltage drop would not be absorbed easily in 555. LM2917 regulates a potential of 9v and your source is 12v. so always LM2917 is prferable.

Hi Abo,

I dont even know if somebody is following this instructable anymore. I have a honda cbr 600 and my tach broke. I googled and found your blog. I am not educated in electronics but know how to follow instructions.
I followed your diagram (From Step 1) and the circuit is almost completed.
When I wanted to test it in real life I just realised that the ignition coils are mounted inline for each spark plug in my cbr and it is like impossible to get to the -ve terminal.

I wanted to ask, can we modify this circuit in any ways so that it can pick up the signal using induction i.e. by wounding a cable around the spark plug wire externally and not the actual signal from the ignition.

I think it became too long, but any help from anybody would be appreciated.

Thanks

instead try connecting those terminals to battery and add a seperate key for the tach.. that would be simple!

is it possible to adjust led intensity as the rpm increases, as I want this for my motor cycles light graphics to to be at max brightness at full speed and off under 1k.

i reallly intested to this project， but unfortunally im not good in electrical job， may i buy complete project with you ？by paypal or else？kindly to email me if possible to buy from you：zincjunyi@gmail.com

May I ask what car did you use? Im planning to make this circuit for my car also

Yes. The only difference is that the LM2917 has an internal Zener diode that clamps the device's internal Vcc to 7.56V, so if you wonder why your input voltage is 9V (from the 7809 regulator) but your output is less than that at the max frequency, it's probably due to the internal Zener of the LM2917.

The purpose of the Zener is to keep the Vcc of the device stable with transient power supply voltages. Since the 7809 will regulate the voltage to 9V, the internal zener of the 7809 really isn't needed.

Alternatively, you could use the LM2907, delete the 7809 regulator, and use a 12V Zener rated for maybe 2W, placing it in series between 12V and pins 8 & 9. You'd probably want another Zener for each of the LM3914 LED drivers as well. The main difference with this circuit vs the one shown is that you'd have the full 12V swing on the LM2917 output, which allows a greater voltage difference between the various RPM levels. But that also means the reference voltages on the LM3914 may need adjusting. The downside is that if the 12V supply on the car drops (like the case of a failed alternator or too much electrical load on the car's system), the circuit may not operate correctly. With the 7809 regulator in place, it would absorb the voltage drop, as long as it was above 9V.

yes, when you use the NTE cross reference chart for both, it points to the same NTE placement part.

Not really sure. You can download the PDF datasheets of both & compare to see weather it's possible to replace LM2917 with LM2907 & what changes should be made to the circuit

circuit easy but putting them together on the vero seems to be quite a pain in the ass. maybe i'll try to design a compact pcb layout

hi abo can you help im fairly new to this can you tell me what voltage the polypropylene capacitors need to be ?