SAAB Blue Light Special

A few months ago I received a small key chain light that contained a blue LED. A while ago I happened to point this at the gauge cluster on my 89 SAAB 900 Turbo. I noticed that the gauges seamed to glow, almost as if a black light where pointed at it. I thought it was a neat effect. So I pondered the idea of placing my cluster lights blue LEDs. This is how I did it.

Items Needed

• 1 Saab 900 classic
• 2 blue LEDs (i used Radio Shack 276-316, 3.7v, 20mA, and 2600mcd)
• some solder
• 2 380 ohm resistors or a networked equivalent
• soldering iron
• Heat shrink
• drill
• standard SAAB needed tools

• Constants
  • Forward Bias Voltage of the LED 3.7V = V_LED
  • Forward Bias Current of the LED 20mA = I_LED
  • Maximum Forward Bias Current of the LED 30mA = I_max
  • Assumed maximum input voltage from instrument cluster 14.7V = V_max
  • Assumed minimum input voltage from instrument cluster 9.0V = V_min
• Basic circuit
  
• Figuring out R_LED
  • R_LED is in place to limit the current through LED.
  • V_max - V_LED = I_max * R_LED
    • 14.7V - 3.7V = 30mA * R_LED
    • R_LED = 11.0V / 30mA = 367ohms
  • I would use a resister that has a few more Ohms in it for R_LED. That way you do not come close to making your Blue LED release the its blue smoke. So i would make it the next higher standard resister value.
• Figuring out I_min
  • We also need to check the minimum current flow through the LED. We will call this I_min.
  • V_min - V_LED = I_min * R_LED
    • 9.0V - 3.7V = I_min * 367ohms
    • I_min = 5.3V / 367ohms = 14.4mA
    • this should be plenty to minimally light the LED

• Testing
  • Assemble the parts needed to test the circuit
  • Build the circuit. In my case i had a bunch of 270 ohm resistors that I used to make a 405 ohm resistor. 1 resister in serial with 2 parallel resistors. "click here for image"
  • If you have a multimeter test the voltages and the resistances
• Construction
  • retrieve the the dash light holders from under the drivers side speaker grill.
  • drill a hole in back of each big enough for the LED pins to fit trough. Remove the Light first. It should be able to done with out effecting the ability of the light holder to hook into the circuit or hold the light.
  • Choose a side of the light fixture as positive
  • solder the resister to the metal strip on the side you chose as positive.
  • apply some heat shrink around the resistor
  • heat shrink or use electrical tape to insulate the 2 leads of of the diode (cathode and anode). This will ensure that they do not short to any other electrical conductor.
  • solder the longer pin of the diode to the resistor
  • solder the shorter pin of the diode to the negative side of the fixture. It should look something like this
  • Repeat for the other side
  • Install the fixtures back into the SAAB. Remember Diodes only pass current in one direction. So if the LED does not light up flip the fixture over.
  • It should look something like this at night when your done

• The traces on my left hand light had pulled of the board. Therefore one side of the area was not getting lighting. I fixed this by running 2 wires between the to fixures as shown here
• I think I would use brighter LEDs next time
• I take no responsibility for anything that happens because somebody tries this out. For me this is only a fun experiment
• all photos where taken with an agfa 1280 night time photos done with an olympus 3040

since 10.04.01

page last modified 10.09.01

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