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I've been trying to interface with my car's info screen, called a TID (Triple Information Display), from Opel.

From the factory, there are multiple things attached to this screen, one of those a line called SDV - Speed Dependant Volume. I've been able to write to the display using Arduino and the protocol described here: http://wiki.carluccio.de/index.php/Opel_TID.

One of the information I would like to display on the screen is the current vehicle speed, using that SDV line.

The thing is, I can't get any signal from it. I know it's connected because the display has a test mode where it briefly shows the (correct) speed, but I can't intercept it with the Arduino. I've tried measuring with a DVM but there is no voltage across this line and ground...

There are multiple circuit diagrams online for the kind of thing I'm trying to do, but since I'm just a begginer in electronics, I don't quite understand them. Could someone please try to explain to me the SDV (or vehicle speed sensor) part of these circuits, and the difference between them?

Circuit 1
(source: rolandgruber.de)

Circuit 2

Upon further reading I understand that the transistor in the first circuit is driving the RA4 port in the PIC.

I found it strange that a port in a microcontroller would be happy with +12v from the battery of the car, but it turns out that port RA4 is an "open drain", being happy with voltages up to 14v.

It seems I won't be able to replicate this circuit with arduino (at least without extra transistors)

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    \$\begingroup\$ Have you considered getting the information from your car's OBD-II interface? Here's the WikiPedia article on OBD in general. \$\endgroup\$
    – Ricardo
    Jul 28, 2015 at 12:35
  • \$\begingroup\$ The car is old, it has no OBD interface. \$\endgroup\$
    – vascoFG
    Jul 28, 2015 at 12:42
  • \$\begingroup\$ On the first circuit, wouldn't the connection on SDV from the collector of the transistor to pin 3 on the PIC instantly fry it, considering car batteries run at +12v? \$\endgroup\$
    – vascoFG
    Jul 28, 2015 at 12:44
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    \$\begingroup\$ Which signal are you really interested in - the SDV or the actual speed sensor? While SDV is speed dependent, its actual purpose is to signal your sound-system to boost its volume as you increase speed in order to compensate for greater road noise (hence its name Speed Dependant Volume) \$\endgroup\$
    – brhans
    Jul 28, 2015 at 13:00
  • \$\begingroup\$ I'm trying to use the SDV to get the speed value, since it's the easiest way to get it on the car. I understand that in the first circuit, the transistor "inverts" the pulse, letting current pass to the PIC pin if the base is Low, but I believe that in that case 12v reaches the pin, so it will fry the microcontroller? \$\endgroup\$
    – vascoFG
    Jul 28, 2015 at 13:21

1 Answer 1

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A vehicle speed sensor works by monitoring the passing of a toothed wheel or some other source of magnetic interference on the transaxle of your car (though the exact location and mechanism may depend on your model of car.)

Because of this, it won't put out an analog signal but a PWM with varying frequency depending on how rapidly the shaft is rotating (and thus how fast your vehicle is moving). Without filtering, you likely won't be able to pick it up as an analog voltage, though the Arduino should be able to measure the frequency of the PWM and allow you to determine the vehicle speed with some relatively simple calculations. This appears to be what both of the above circuits are doing.

Vehicle Speed Sensor Diagram

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  • \$\begingroup\$ I've tried hooking up the line both through a voltage divider and straight to the arduino and no impulses are detected. So what's the difference between that and the circuits shown? Could you please explain what each component is doing? \$\endgroup\$
    – vascoFG
    Jul 28, 2015 at 12:14
  • \$\begingroup\$ In both cases, the output of the speed sensor is just driving the pin on the PIC high or low through a transistor and likely doing some math with the PWM signal. It's hard to say exactly what signal you're measuring as it's unclear if the signal is coming directly from the sensor itself or through some other circuitry that is processing the signal. I would recommend looking at the sensor output in an oscilloscope to see what you get and determine whether or not it's something you'll be able to operate on. \$\endgroup\$
    – Cheibriados
    Jul 28, 2015 at 14:36

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