Introduction: Digital Multimeter for Your Car

Picture of 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?

Step 1: Video

Step 2: List of Components

Picture of 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

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

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

Picture of 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)

Step 4: Testing

Picture of Testing
Testing the speedometer

Testing the tachometer

Testing the thermometer

Step 5: Mounting to the Dashboard

Picture of Mounting to the Dashboard

See photos


Ahmed mostafa 2015 (author)2017-08-01

ooممكن اعرف انتا منين

كنت عايز اسئلك علي حاجه

mark28 (author)2016-02-14

hello Hebo. how you can get the the scales on the photo paper , do you use a printer?

angcarn.gongngern (author)2015-04-29

supertramp78 (author)2014-03-14

For the rpm-signal :

Why is it -High Voltage- ?
Break contact is a digital 12 voltage,
or where do you get it ?

abo_hosni (author)supertramp782014-03-17

It's a frequency signal, not a voltage. Do not worry about it, it goes through a 470nF capacitor

elsonero (author)2013-06-23


rkrz (author)2013-05-07

shall i use 10uf for 7905 3pin regulator ... please tell me how to connect ..?

spikepickstock (author)2013-01-18

looks awesome, i'm going to try and build one myself only make it slightly different, put it all within one unit (its for a race car so doesn't need to be ergonomic) something like a racepack and others
basically bright leds on the top of the unit, maybe in a semi circle haven't designed properly it yet, different colors to indicate rev limits with a speed display underneath and the ability to switch between vitals (water temp, volts, oil pressure) its going to be hard but it should fit my needs perfect

SoulFury (author)2012-10-20

Why is the tachometer with only 3 displays on?

matsurambo (author)2012-09-11

awesome project...
can i use this scematic for pc fan?
i want to make rpm meter from cpu fan sensor..


kill_them_now (author)2012-03-12

nice work..what should if i want to modify this circuit using lcd interface instead of

fernandohdz (author)2011-06-27

Hi Again From Merida Mexico
After a lot of searching on the web i found that you are the best answer
Question: This i a Pic system? Im very new on this and i dont know if the icl7101 IC have to be programed??? and how to do it...
If so the signal of ech sensor (lm35, step motor and lm2917) is "translated" to the leds and 7 segments display?
Thanks and sorry for the rocky questions

abo_hosni (author)fernandohdz2011-06-28

this is not PIC system :)
ICL7107 is analog to digital converter and not to be programmed

fernandohdz (author)2011-06-25

I have an old motorcycle that im rebuiling and dont have all the clocks this is exactly what i need but im not to god in electronics, any wat i´m going to try
hope you can reach me and give me a help at
thanks FH

bega (author)2011-05-20

hi man, sorry my question but what does it mean dp in the bridge of 21 and 30 pins of the ci that goes to a resist. of 220 and later to the DP but wich pin of the DP goes it or what does it means? i appreciate your soon answer.

abo_hosni (author)bega2011-05-21

DP= decimal point in the seven segment display
if you want a DP to be lit, connect it through 220R. I connected mine to the first (right) DP. It's not necessary at all

bega (author)abo_hosni2011-05-23

thanks a lot man and sorry for questions soo sillies, i really want to make it but here in mexico its a little difficult to find the ci, thanks again.

aussa (author)2011-04-26

Its nice to see cool projects like this from Egypt!! Try to upgrade to LCD interface and microcontrollers.

opipharma (author)2011-01-23

Dear sir
i think u are in egypt
i'd like to ask you about the LM35 where did you buy it and for how much??

abo_hosni (author)opipharma2011-01-23

I bought the LM35 from "El Nekhely" in "Bab El louk" :)
It's for 8 L.E.

opipharma (author)abo_hosni2011-01-23

thanks alot
as i need to build this thermometer for my suzuki swift but i can't find it at all any where in el gharbiya gov
i was wondering if there's any substitute for it??

abo_hosni (author)opipharma2011-01-25

Some circuits use regular diodes or transistors as temperature sensors, but you can't make a direct substition in this circuit. Look for another circuit, instead

opipharma (author)abo_hosni2011-01-25

Thanks so much

MOMJE9 (author)2011-01-17

this car, it´s a Renault?

abo_hosni (author)MOMJE92011-01-21

It's Peugeot 504 :)

mafinstructables (author)2010-11-27

Hello, why you made changes in the filter of coil signal, respect the led-tacho proyect ??. which one is better ??

what changes do you mean, please?

I prefer the 7 Segment speedo with the LED tacho.

In fact, I don't like LED speedo & the 7 segment tacho too much.

hi, compare this proyect (Digital multimeter for your car) circuit diagram with the DIY LED Tachomter (RPM gauge) diagram. The filter of the input signal of the LM2917 from the coil (negative) are diferent....Nice proyects.
Regards from URUGUAY

Sorry for this mistake. I edited the "parts list" & the schematic.
The capacitor I used as a filter for LM2917 signal is 470nF.
In fact I don't have a reason for this change, but it works :)

Another funny thing about the 100pF cap of Pin No38 of ICL7107. I forgot to buy one!!! I didn't accept to delay the project. So, I connected 10x 1nF caps.(code 102) in series = 100pF lol
This capacitor together with connections to pin 39 & 40 determine the clock speed of ICL7107

Nice resources for If you check out to the first proyect, you take off the 100nf in parallel to ground also, between 10k resistor and 12v zener....sorry for my
If the second filter for signal for lm2917 works fine in field.... the first can be modified.. less components and more cheap... je..
Regards from Uruguay

dared (author)2010-11-22


VadimS (author)2010-11-21

I love the look. I'd probably use a vfd, I kind of have a thing for them, but otherwise great.

Oh in step 5, image 5 you have a text box inside of a text box.

abo_hosni (author)VadimS2010-11-21

are there VFD available of suitable size to such project? If so, what about the circuit used.

I edited the text box by the way :) Thanks

VadimS (author)abo_hosni2010-11-22

Hmm I actually couldn't find any 3 or 4 digit displays.
Digi-Key has a nice 6 digit unit:

Or you can get single digit tubes off of eBay (sometimes called nixie tubes but there is a difference).

The single digit tube can usually be run right off 12v.

The 6-digit unit I linked to has 3 digits ganged together.
So you would power grid one (34v) power the segments to display the first number then cut power to that grid and power the second grid etc.
If run fast enough it will look continues.
If testing on a workbench by hand 12v should.
The filament needs to bee powered by 4.4v a lm317 should do the trick.

A micocontroller or dedicated driver ic would be needed.

VadimS (author)VadimS2010-11-22

You could probably just use a 3 ohm resistor from that 5v regulator for the
That would get you to 4.4v (200ma).

rimar2000 (author)2010-11-22

WOW, you have done an awesome work!


About This Instructable




More by abo_hosni:Using a stepper motor as automtive speed sensorTransform a trimmer potentiometer into a multiposition rotary switchDigital multimeter for your car
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