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Cheap, Easy Way to Step Down 12-14VDC to 5VDC?

Discussion in 'General Electronics Discussion' started by wingman358, May 18, 2015.

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  1. wingman358


    May 18, 2015
    Hello all,

    I am making a custom gauge cluster for a motorcycle. The bike currently uses 12V 3.4W incandescent bulbs for the warning/indicator lights in the gauge cluster (hi-beam, water temp, oil pressure, etc). I believe the bulbs are ground switched.

    I would like to replace the entire cluster with a microcontroller so I can eventually set it up to log data. I would like to have the 12V warning/indicator signals to be connected to the microcontroller (as a 5V digital input) so I can trigger warning LEDs as well as log the occurrence. I think there are many ways I could connect the signal to the board, but I am not sure what would be best.

    I'm not very experienced with electronics, so my first thought was relays or buck converters, but I've since thought regulators, zener diodes or transistors could do the trick, though I am not sure about it. A friend told me I could do a simple voltage divider out of resistors, but I'd rather not have to dissipate 3+ Watts into heat per indicator - unless I am missing something here.

    Any tips for a simple and effective circuit?

  2. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Google buck switching regulators if your intent is to power the microcontroller. There are integrated circuits available that require minimal external components. And of course you can purchase them ready-made too.

    If you want to use the existing bulbs, and they are ground-switched, a simple resistor and zener diode at each lamp can lower the voltage to acceptable input levels for your microcontroller. Or, you can use a transistor with a base resistor connected to the ground-switched lamp, emitter to circuit common, collector to a +5 V logic supply rail through a 1 to 10 KΩ pull-up resistor. This would be my preferred implementation. If you use an NPN small-signal transistor, it will be on and saturated when the lamp is NOT illuminated because the transistor base will be forward biased through the non-illuminated light bulb with a few microamperes of current. Connecting the ground-switched side of the lamp through a switch or sensor to ground will illuminate the lamp and turn the transistor off, causing the pull-up resistor to present a logic "1" level at the collector. You connect the collector as a digital input to your microcontroller.

    It really isn't necessary, but you could also place an optical isolator's input LED in series with the transistor collector, lowering the resistor value sufficiently to turn the optocoupler's output transistor on when the transistor connected to the lamp circuit is conducting. The optocoupler transistor then replaces the transistor connected to the lamp circuit.

    You may not need a buck switching regulator to power the microcontroller, but if you plan on using a lot of LEDs or other displays, along with data logging, the current requirements will blossom upwards and a series regulator or zener diode with current-limiting resistor to drop the motorcycle supply voltage down from 12 to 14 V DC to 5 V DC (or less for some micros) will be inefficient and waste power you need to dissipate as heat.
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