White LEDs operate at a minimum forward voltage of 3.3 volts. This is higher than standard red, yellow or green LEDs. Like all LEDs, however, it requires a driver that can deliver a constant current at the proper voltage. You can build a simple but effective LED driver by using a linear voltage regulator to build a parallel LED driver circuit. A linear regulator is not as efficient as a switching regulator, but it requires fewer parts to build a working circuit. This circuit can drive four, or more, white LEDs in parallel.
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Things you need
- Low dropout 5V linear voltage regulator, for example LM2937-ET5.0
- .47 microfarad capacitor
- 22 microfarad electrolytic capacitor
- Four resistors
- Four white LEDs, 3.3 Volts, 25 ma
- Four 68 ohm 1/4-watt resistors
- Four C or D cell batteries and appropriate battery holder
- Solderless breadboard
- Jumper wire
Plug the regulator into the breadboard.
Connect one end of a jumper wire to the INP (left) pin of the regulator. Connect the other end of the jumper wire to the supply strip at the top of the breadboard.
Connect one end of a jumper wire to the GND (middle) pin of the regulator. Connect the other end of the jumper wire to the ground strip at the bottom of the breadboard.
Connect one lead of the 0.47 microfarad capacitor to the INP pin of the regulator. Connect the other lead to the ground strip.
Connect the positive lead of the 22 microfarad capacitor to the OUT (right) pin of the regulator. Connect the other lead to the ground strip.
Connect the negative (black) lead of the battery holder to ground strip.
Connect the positive (red) lead of the battery holder to the supply strip.
Insert the batteries in the battery holder.
Attach the positive probe of the voltmeter to the OUT pin of the regulator.
Attach the negative probe of the voltmeter to the ground strip or the grounded lead of one of the capacitors. The voltmeter display should indicate 5 volts.
Remove the positive lead of the battery holder from the supply strip for now.
Connect one lead of a 68 ohm resistor to the power strip.
Connect the other lead of the resistor to the positive lead (usually the longest of the two leads) of an LED.
Connect the negative lead of the LED to the ground strip.
Repeat steps one through three for the remaining LEDs.
Reconnect the positive lead of the battery holder to the supply strip. The four LEDs should light up.
Tips and warnings
- You should initially build this on a solderless breadboard for testing. If you are not familiar with solderless breadboards, see the link titled "The Electronics Club: Breadboard" in the Resources section for more information.
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