Digital Multimeter for Your Car

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?

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)

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)

Testing the speedometer





Testing the tachometer




Testing the thermometer

See photos