Digital speedometer, tachometer & temp indicator

Digital speedometer, tachometer & temp indicator

I tried to build a digital speedometer and tachometer with a 7-segment meter, but it did not work, because the circuit was too complicated. Subsequently I built the tachometer with LEDs. Then I bought a stepper motor, used it as a speed sensor and built a speedometer with LEDs.

But I’ve always wondered about a 7-segment multimeter. It could be built on programmable PICs, but unfortunately I’m not good at it. Then I thought of the ICL7107 chip, a simple and reliable analog-to-digital converter (ADC) used in digital voltmeters.

Digital speedometer, tachometer and engine temperature indicator.

A VOLTMETER? Why not build a voltmeter and then calibrate it so that it shows the speed of the car from the speed sensor (stepper motor)? And take the voltage for the tachometer from the output of the LM2917 ? Why not add a digital thermometer using the LM35 temperature sensor ?


Digital Voltmeter Schematic

Digital speedometer, tachometer and engine temperature indicator.

I started with the main circuit (ICL7107 voltmeter). The ICL7107 is an analog to digital converter interfaced with a seven segment display.

6 5 7

The “-5V” supply is derived from the 7660 chip from the “+5V” input voltage, although “-5V” can also be obtained with the 7905 voltage regulator from +12V. To this are added the other few components.

Power supply

The +12V voltage from the battery is converted to “+5V” using a 7805 voltage regulator, two 100nF non-polar capacitors, one 470µF electrolytic capacitor, and a 1N4007 rectifier diode.

Speed signal

A stepper motor was previously attached to the transmission of my car. The current generated by the stepper motor is variable, so I added a 1N4007 diode bridge and 100NF for smoothing at the output. I added 1.5mOhm and a 470kOhm potentiometer for calibration.

Digital speedometer, tachometer and engine temperature indicator.

Tachometer signal

The LM2917 is a frequency to voltage converter. It converts the RPM signal from the ignition coil to a voltage (high input voltage . ).


The voltage corresponding to the rpm is taken from pins 5 and 10. It is calibrated by means of a 220K trimmer. Power supply from the same source +5V.

Current Electronic Load

Temperature signal

I used a digital temperature sensor LM35. It is accurate to 0.5 degrees and has a sensitivity of 10mV/degree. The LM35DZ version has an operating range of only 0-100 degrees Celsius, and the LM35AH from -55 to 150 degrees. The sensor is also powered by +5V. After connecting the wires, I filled them with epoxy resin.


The resin does not conduct current and will provide an airtight seal. I used a 100k ohm potentiometer for calibration. I put the LM35 sensor under my tongue, waited a while and used the potentiometer to set the display to 37 degrees (assume I had normal body temperature?).Then I put it in boiling water and calibrated it to 100 degrees.

The sensor must be well attached to the motor housing to show the correct temperature. I drilled a small hole in the housing (steel), inserted the sensor and filled it with epoxy. You may prefer to use such a sensor to measure coolant temperature. In the future I will add 2 more sensors, one for the outside temperature and one for the temperature inside the car.

Digital speedometer, tachometer and engine temperature indicator.10

Switching the Display

I used a simple rotary switch with 6 positions. I am currently using only 3 positions (speed, tachometer and engine temperature).

The switch is installed in place of the old potentiometer (used to adjust the brightness of the dashboard backlight).

And one more thing I want to note, if you decide to buy a truck, crane or other special equipment, I want to recommend you an excellent company that deals with this very thing. Come in, look and choose, trucks are always in stock, both new and b / w.

G92081 Car OBD+GPS External Digital Speedometer

Today we will take a look at an in-car multifunction device. This class of devices called differently, such as HUD display, I will be the old fashioned external speedometer, as there is no projection on the windshield, but only a display. The device extends the functionality of the trip computer (not for all car models), under the hood, let’s see how.

Headlights as main beam

I have LADA xray 16 year and the onboard computer capabilities in terms of displaying parameters more than modest! No display of coolant temperature, oil temperature, etc. People “hack” different options. As an option to put the external device working via the protocol OBD2.

Package and accessories

Specifications: Model: G92081 Modes: OBD + GPS Displays: 9 Color: Black Size: 109 x 86mm Material: ABS Plastic OBD mode voltage: 11-18 VDC (12V 200mA) GPS mode voltage: 5VDC

Comes with two long cables, one is only power, the other flat to the bus OBD2: If you power just from the mini USB 5 V, only the GPS mode will work. From the bus OBD2 comes its own power. And a vacuum suction cup of crappy quality on the glass: Holds badly. But the good thing at least is that there are two options for installing the device in the car: on the torpedo or windshield. Both with adjustable tilt.


The body is plastic, with a developed visor in front of the screen. The screen is divided into the main part and a small clock area in the lower right corner. There is a photocell on the back for adjusting screen brightness and a squeaker. Top-bottom: On top of the three control buttons. Bottom swivel pad 6×4,5 cm for attaching to the double-sided adhesive tape. Weight G92081: Installation in the car: Under the screen is an indicator LED bar – blue – normal, red – warning of exceeding the parameter, white – customizable. When you turn off the ignition, the device is still powered from the OBD, so you have to pull the cable: Or set the power threshold for sleep mode.

Device functionality

  • Time
  • Compass
  • Speed
  • Distance traveled
  • Trip time
  • Altitude
  • Number of active satellites

There are interesting, clearly brighter than the boring standard on the dashboard of the car. For turbocharged engines there are their own skins with pressure display. It also activates the acceleration in seconds 0-100 km/h and the braking distance test in meters from 100 km/h-0. Supported parameters for the display: Change the display of parameters:

Case for a fire extinguisher in the car

There is just a tabular form: you can configure which parameters to display. Readable parameters (PID’s):

The G92081 settings are divided into two types: instrument settings and OBD operation:

The languages are Chinese and English only. There are quite a few settings…

Working with errors (diagnostic trouble codes): more details here. I don’t have them: It is convenient, that the device can be used as OBD diagnostic. You can also remove unnecessary errors.

Internal device of the device

Result As you can see, in my case, the device allows you to display parameters which are not displayed by the on-board computer of a car (coolant temperature, on-board line voltage, etc.) Just about a hundred car parameters (PID’s) are displayed, which can help in the primary diagnosis of the auto faults. Actual GPS speed will keep you out of the penalty area.

There is a coupon BGH08101 for this product, giving a price of $49.99 until March 21:

Thank you for your consideration. Have fun shopping!

Item was submitted for review by the store. The review is published in accordance with paragraph 18 of the site rules.

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