Digital Speedometer, Tachometer & Engine temperature display.
4 months before, I treid to build a digital tachometer & speedometer using seven segment display, but I failed to do it the right way. The circuits I used before were too crowded with ICs & other components. Then I managed to build the LED tachomter. Later, I got a stepper motor & used it as a speed sensor to build the LED speedometer successfully, too.
I was always thinking of the Seven segment multimeter. I know it's simple to build such circuits using PICs about which I know nothing :)
The ICL7107 came to my mind. A simple, old yet reliable analog to digital converter used in digital voltmeter circuits. VOLTMETER? Why not to build a voltmeter, then calibrating it to get the car speed from my stepper motor? & get the RPM from my LM2917 voltage output? What about adding a digital thermometer using the LM35 temperature sensor?

Step 1: Video

Step 2: List of Components

Integrated Circuits:
1X ICL7107 This is an old analog to digital converter
1X 7660 This is voltage invertor
1X 7805 Voltage regulator, giving +5V ouput
1X LM35 Temperature sensor
1X LM2917 Frequency to voltage converter

Diodes:
5 x 1N4007
4X 1N4148
1X Zener diode 12V

Resistors:
1X 220 ohm
1X 470 ohm
1X 22 Kohm
2 x 100 Kohm
3 x 10 Kohm
1X 15 Kohm
2X 47 Kohm
3X 470 Kohm

Trimmer potentiometers:
1X 10 Kohm
1X 100 Kohm
1X 220 Kohm
1X 470 Kohm

Electrolytic Capacitors:
1X 470 µF
2X 10 uF
1X 2.2 uF

Polypropylene Capacitors:
1X 100 pF
1X 10nF
1X 47 nF
4X 100 nF
1X 220 nF
2X 470 nF

Seven segment display:
Either 2X two digit or 4X 1 digit display. There are also 3 digit displays but be sure you can connect them (read the datasheet)

Ic mounts
Circuit board
Rotary switch
Connector cables & pins (from old PC)
Photo paper with desried print (use two copies one over each other, in the rear one cut the yellow rectangles facing the LEDs)

Step 3: Building the Circuit

Digital voltmeter circuit:
I started with the main circuit (the ICL7107 voltmeter). The ICL7107 is an analog to digital converter interfaced to seven segment display. The 7660 provides the circuit with ( -5V) voltage from ( +5V ) input, you can use the 7905 for the same purpose (to get ( -5V ) from the ( +12V ). Other components are really few.

The voltage signal to ICL7107 goes to pin 31 through a rotary switch.

Power supply circuit:
using the 7805 voltage regulator, two 100nF pol. capacitors & one 470uF electrolytic capacitor. adding a rectifier diode 1N4007 to the 12V input (from the car battery)

Speed Signal:
From the stepper motor I mounted to my car's transmission in the previous instructable. The current generated directly from a stepper motor is AC (alternating current). So, I added a simple diode bridge to get DC (direct current) from AC using four 1N4007 rectifer diodes & a 100nF capacitor for "smoothening" the output. Adding 1.5Mohm & 470Kohm trimmer potentiometer for calibration

RPM signal:
From the LM2917 pins No, 5&10. I made a small circuit powered with the same +5V supply. The circuit is similar to the one I used in the "LED tachometer" instructable. This gets the engine revolutions signal from the car's igntion coil (high voltage input!!!). Calibration through the 220K trimpot.

Temperature signal:
I used the LM35 digital centigrade temperature sensor. It's 0.5C accuracy, gives 10mV/1C change. The LM35DZ variant operates between 0-100C only. The LM35AH operates between -55 to 150C. This is powered from the same +5V power supply. After soldering the terminals. I covered them & the wire with epoxy glue which is non-conductive to electricity & water proof. I used a 100Kohm trimpot for calibration. I put the LM35 under my tongue, waited till it gets a stable reading then calibrated it to 37C (assuming I have a normal temperature :) ). Pass it to boiling water & calibrate it too 100C.
The sensor should be fixed well in a site of good thermal conductivity to get the engine temperature. I drilled a hole in part of my engine block (steel), filled it with epoxy glue & dipped the LM35 in.
You may prefer to use this sensor to get the coolant temperature. I will add to sensors, one for ambient temperature & the other for the in-car temperature.

Switching between inputs:
I used a simple rotary switch (with 6 positions) I'm currently using only 3 positions (speed, RPM & engine temperature)
The switch is mounted in the place of an old potentiometer (used to adjust brightness of dashboard backlight)