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leds driver from nimh battery

LED Driver with 555 Timer

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.

Related Products: Drivers and Interfaces | Display Driver

LED Driver Circuit Schematic

leds driver from nimh battery

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.

13 Comments

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  • ahmed

    how to increase the amp of led string to 350ma with 18v also can i use a rectifier circuit to obtain the dc input voltage with 220ac supply

  • Al

    Approximately 28 feet number 28 wire will give you approximately 220mh on a ferocious ring. With the right transistor and an unpublished one part mod I light 100 bright leds producing 92 lux at 12 feet from a 2.5 volts to 4.95 volt input. Using Fairchild cmos 555 tested to 1.2 volts in. The unpublished part is a diode. Current draw is a Meer 11ma at 3.5vin. At this time my mod is being integrated in a new type of Led driver. Super efficient. Better than a joule thief and way more light to light yards.

  • Jim Keith

    Three parallel circuits should work OK. Key to this functioning well is the inductor parameters. Inductor saturation current rating must substantially exceed the peak output current. You may have to experiment with different inductors to obtain best results.

    • ivorcadiramen

      Dear Jim,

      Thanks for your very helpful suggestions.

      If I were to wind my own choke – say on a small diameter ferrite ring, how many turns will I need to drive a single row of say 5 white LEDs ?

      Thank you very much for the assistance you keep giving relative novices like me.

      Ivor

  • Francisco

    Thanks Mr. Keith, what about if LEDs are wired in 3 parallel strips?, each one of 6 or 7 series LEDs, this way (I think) voltage keeps same, but I don’t know if current is enough to keep a decent glow. or some component values must be changed. Again thanks in advance for your answers and patience and sorry by my weak knowledge in this matter

  • Francisco

    hi, it is possible to drive 20 leds with this circuit?, elsewhere what i need to change to do this?, thanks in advance

    • Jim Keith

      20 LEDs will require 50 to 60V so you will need a 100V transistor. Note that the inductive discharge pulse will be very narrow (low duty cycle). Total brightness will probably not increase unless the peak inductor charging current is very high. This would require a lower resistance inductor.

  • David Moran

    I am playing with a YX8018 chip from a cheap solar yard light, it also uses a 150uH to 80uH choke. it puts out 200kHz squarewave, charges the battery by solar cell. I put in an LED that flickers but it would not flicker due to being pulsed, I put a white LED in parellel (highter voltage)now both will flicker, very nice.

  • krn

    try 7555 ic

  • krn

    as per my experience timer ic won’t start untill supply volt is above 4v

  • neeraj

    thanx…for list of components

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