Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I have in my car 4 LEDs which illuminate in the instrument cluster depending on the gear the vehicle is in. This is a factory standard setup which I have already run feeds from ready for the next part.
I would like to, using those 4 led inputs, build into my dash board a 7 segment LED display to show which gear the car is in, 1, 2, 3 or 4. I'm not really sure where to start with this but I'm ready to do some fast learning!
If the drivers for the original LEDs can supply enough current you can do this with a diode matrix: route the current from each of the original LEDs to the segments you want to illuminate when that LED would have been on.
In my experience a high-brightness display will be OK with ~ 1 mA per segment. Details will depend on exactly how the current LEDs are wired (to ground, to power; can you get access to the connections behind the resistors).
In general, my first impulse is to throw a micro-controller at any problem I see. But it in this case that seems ridiculous (especially given the uC-hostile environment).
As you already have the 4 (12v) signal lines you can connect up a 7 segment display using a few diodes. The resistors are a few hundred ohms and are there to limit the LED current. The circuit shown is for a common cathode display. No need for (expensive) micro controllers just 16 small signal diodes and 4 resistors. BTW you could just connect up the 4 outputs to the f (1), e (2), b(3) and c(4) segments to give you a mimic display of the gears (up, down, up across, across down). I would leave the g segment permanently powered to indicate the neutral position.
The alternative circuit
As a high-level overview, you'll need:
A way to sense the current gear that the car is in. This could easily be done by reading which of the 4 LEDs (or what combination of them) is lit.
A way to control the LEDs and 7segment. The easiest way, for me at least, would be to build in a small Arduino or micro controller. Lighting/reading one, two, three, or all 4 LEDs, along with displaying a number on the display, can easily be done with an Arduino and a BCD decoder (for the display). The nice part about Arduino is that it can handle 12V on the Vin pin, which simplifies setup. You'll need at least 7-8 digital pins; four for the LEDs as inputs and at minimum three for the BCD decoder (the most significant bit of the BCD code can be tied to 0 if you are only displaying 0-7, but four would be standard).
And, of course, you'll need to tie it all together and hide it nicely in the dash. Overall, I think this is a pretty neat project idea.
I see 2 simple solutions:
1) Get a PLD with 4 inputs and 7 or 8 outputs and program it to output the signals to light up the appropriate segments.
2) Get yourself a small 8-bit PROM/EPROM/EEPROM and program locations 1, 2, 4, and 8 as above.
Note that this is contingent on the LED signals being strong enough to run the devices, but not too strong to destroy them; if not then you may have to buffer them with a transistor each to normalize them.
