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Drive Multiple LEDs by Using Only One NiMH Cell

2014-12-18 00:51  
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Figure 1 

This simple circuit allows us to drive serially connected sevenLEDs by using a single NiMH (Nickel Metal Hydride) AA cell. As you can see there are a few components, aCMOS555 timer, anNPNtransistor, an inductor, three resistors and two capacitors.

The circuit produces voltage pulses at a much higher level than the input supply voltage by pulsing the 220 uH inductor. The inductor must be a high Q (Q>90) power inductor. When the input is 1.25 V and theLEDs are connected, the voltage pulse level will be 23V.

Since the circuit operates with low supply inputs (as low as 1V), it uses aCMOStimer. When a single cell rated at 2000 mAh is connected , the circuit works for about 190 hours. The 555 timer drives the transistor at 222 kHz rate. The circuit is also can be used as a high powerLEDdriver. Obviously, the greater the mCd rating, the brighter theLEDwill appear.

The sevenLEDgroups can be connected paralelly if their forward voltages match. If not, theLEDgroup with the lowest forward voltage value will dim out the other group(s). This parallel connection will not effect the total current drawn from the battery but it will reduce the brightness of theLEDs. So the total operating time will not differ.

When a single 1.25V cell is used, the sevenLEDgroup will draw about 8mA from the battery. When the input value increases to 2.5V, the total drawn current will be 20mA and theLEDs will be so brilliant that it is difficult to look at them.

Source: http://www.edn.com

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