Using a CMOS 555 timer and a single NPN transistor, you can drive as many as seven LEDs using a minimal amount of voltage and power from a single NiMH AA cell.
The circuit uses a CMOS timer because it functions on low voltages—in this case, as low as 1V. A single white LED rated at 9300 mcd maintains its brilliance down to this low voltage. The circuit works for 192 hours using a 2000-mAHr-rated NiMH cell. The output of the timer is a 4.5-µsec pulse repeating at a 222-kHz rate. Although you can use the circuit to power any LED, it works best using high-brightness, high-power LEDs rated at 3000 mcd or higher. Obviously, the higher the millicandela rating, the brighter the LED will appear.
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The advantage of connecting the LEDs in series—which is possible because of the high pulse voltage they produce—is equal brilliance of all LEDs, regardless of their individual forward-voltage drops and millicandela ratings. Each additional LED decreases additional voltage and lowers the resulting current into the series string of LEDs, lowering their brilliance. Using seven LEDs with a single 1.25V cell draws a current of only 8 mA. By adding a 1.25V cell to the power input, the LEDs become so brilliant that it is difficult to look at them. With a 2.5V supply, the peak voltage pulses increase to 70V with no connected LEDs. With the LEDs connected, the output voltage peaks at 25V. Current draw at 2.5V is 20 mA.
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