This simple LED driver circuit allows us to drive up to seven LEDs by using a single NiMH (Nickel Metal Hydride) AA cell. 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 the LEDs are connected, the voltage pulse level will be 23V.
The LED driver uses a CMOS 555 timer since it operated with low voltages and can work for about 190 hours when using a single NiMH battery cell rated at 2000 mAh. The 555 timer drives the transistor at 222 kHz rate.
The seven LED groups can be connected paralelly if their forward voltages match. If not, the LED group with the lowest forward voltage value will dim out the other group(s). This parallel connection will not affect the total current drawn from the battery but it will reduce the brightness of the LEDs.
When a single 1.25V cell is used, the seven LED group will draw about 8mA from the battery. When the input value increases to 2.5V, the total drawn current will be 20mA.