Introduction: LED Speedometer

My Peugeot 504 speedometer came to an end,  with a wire & wobbly needle. I don't like it anyway.
I made an LED RPM gauge before. I think an LED speedometer would be fine too. The main obstacle for me was the speed sensor. I thought of using variable reluctance magnetic pick up sensors or automotive ABS sensors. They operate on the same principle, fixed sensor over a rotating toothed ferrous wheel. As the teeth pass by the sensor, up & down electrical impulses are generated, as the speed increases, the frequency goes up too. Then we need to convert this frequency to voltage (using the LM2917 as in a tachometer), the increasing voltage will drive 30 LEDs through 3x LM3914 ICs. Good
I didn't spend much time looking for ABS sensors. I had a stepper motor from an old inkjet printer. I tought it can generate similar signal to be interpretted by LM2917. Later I found this motor can generate increasing voltage as the car speed goes up. So, I don't need a frequency to voltage converter (LM2917)

What to do:
1- remove the old speedometer with its wire
2- mount the speed sensor underneath the car to get movement from where the old wire got it
3- building the circuit
4- calibration
5-  finish

Step 1: The Speed Sensor

There are many Ideas when it comes to the speed sensors.

1- Variable reluctance magnetic pick up (ABS sensors)
2- IR LEDs & IR detectors
3- In my case, a stepper motor as a generator

I took this motor from my old ink-jet printer. It had two stepper motors, this is the stronger one in terms of size, weight, sensitivity & output voltage
The most challenging step is to mount the motor to the car transmission where the old speedometer cable used to be. So that the motor gets driven as the car move.
There should be:
1- Mechanical linkage between the motor head (gear) & the rotating speedometer rod inside the transmission
This was achieved using a small peice of copper folded upon itself. one end being rotated by the transmission, the second end to be inserted in a groove I made in the center of motor head (gear)
no welding needed :)
2- Good method for mounting the motor centrally in place. No shaking or to & fro movement is accepted
At my mechanic's, I drilled a 4mm hole in the transmission body, passed a screw & nut. Later on, I filled the gaps with epoxy resin glue. That was enough for good  fixation.

Now I have a speed sensor with a 4 wires cable :)
I took wire No 1 as a signal, wire No 2 as a ground

This motor can generate up to 48 Volts as I tested it. But When mount to my car's transmission, it rotates at near top speed generating 28 Volts. This is good. The relation betweed the car speed & voltage increase is linear. This is great.

Step 2: Building the Circuit

Attached the circuit schematics

12 Volts : from car battery
Ground :  Car chassis
Signal : from the sensor "stepper motor" 

The No 5 Pin of LM3914 can withstand up to 35 Volts input. I added a 47K ohm trimpot potentiometer in series with signal input for calibration
Calibration was done using GPS equippied cellphone to get the car speed

:::::::::::::::::::: List of components: ::::::::::::::::::::::::
Integrated Circuits:
3 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)

31 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 30 will be lit one by one as you increase the engine speed. I didn't connect the first one by the way

1 x 1N4007
2 x 1 Kohm
3 x 2.2 Kohm

Electrolytic Capacitors:
1 x 470 µFarad 25 Volts (µ = micro)
Polypropylene Capacitors:
2 x 100 nF (nano)
:::::::::::::::::: List of other things you need: ::::::::::::::::::::::::
Soldering equipment
Digital multimeter
Circuit board
Aluminium heatsink
A lot of patience

Step 3: Dashboard Work

See photos :)