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.