I have a very basic Volkswagen Polo which does not have a tachometer and decided to build one using LED’s for the display. Car tachometers typically display the engine speed up to about 7000 rpm with the redline starting around 6000 rpm :
A tachometer measures the frequency of the electrical pulses at the ignition coil in a petrol engine and in a four cylinder engine, a speed of 30 rpm will produce a frequency of 1Hz. Searching the internet for suitable circuits, I found several which converted the frequency to voltage using a LM2907 and then display this on a number of LED’s using a LM3914 and this type of circuit will need to be calibrated. I preferred to measure the frequency directly without the need for any calibration, so I designed my own tachometer using a PIC processor to count the pulses from the ignition coil in a suitable time period. I also found the Micrel M5451 which is a LED display driver with outputs for up to 35 LED’s and is controlled with two serial data inputs. If the M5451 is used for a display range up to 7000 rpm, then each LED will represent 200 rpm and I decided to use 30 green LED’s for the range 0 to 6000 rpm and 5 red LED’s for the range 6000 to 7000 rpm. The M5451 also has the advantage of being able to easily set the brightness of all the LED’s. If the LED’s are arranged in the arc of a circle, then the display will resemble a tachometer as shown in the pictures above. In my car, it was more convenient to arrange the LED’s in a bargraph and these were built into an enclosure with the following label attached.
The LED’s are connected to a separate enclosure containing the electronic hardware.
This video shows the tachometer in operation. When the tachometer is first turned on, there is a start up test which turns on the LED’s in sequence and then sequentially turns them off. The latter part of the video shows the testing of the tachometer during which the input signal frequency is gradually increased to turn on all the LED’s.
The input is connected to the ignition coil (refer to your car's wiring diagram).
Most modern cars have an Electronic Control Unit (ECU) which is used for the engine control and these units will usually have an output for a tachometer. If it available, the ECU output is preferable to the ignition coil connection.