Efficient 555 (CMOS 7555) circuit design - help needed

Discussion in 'The Projects Forum' started by theta, Aug 29, 2015.

  1. theta

    Thread Starter New Member

    Sep 25, 2012
    I am hoping somebody will be able to offer me some help with a 555 circuit design. I previously worked with the 555 during college, but it was the mass produced variant with a standard data sheet.

    The project that I am working on at the moment requires 2 LED panels (3v @ 20ma each) to be driven from a single CR123 lithium battery, with the ability to dim the brightness.

    The reason I was considering a 7555 is:

    1. It can provide a dimming via PWM with the inclusion of a potentiometer/variable resistor
    2. It could pulse a full output with a duty cycle that would increase battery life

    This is operating on the threshold of most 555 chips, except those mentioned in this thread: http://forum.allaboutcircuits.com/threads/555-3v-led-flasher-circuit-help.47486/

    I did not want to hijack the thread, but it is similar to my requirement.

    These are the LED backlights I am hoping to run (x2) from the CR123: https://www.coolcomponents.co.uk/white-led-backlight-module-large-45mm-x-86mm.html

    Any help appreciated...

    (If there is an existing module - like those used in torches as drivers - then links would be welcome)
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    If the LEDs have a Vf of 3V then a 3V supply leaves no headroom, so the LEDs won't be able to run at their max when the battery voltage sags.
    What colour LEDs?
    Do the panels incorporate current-limiting resistors?
    Do both panels need to be dimmed simultaneously by the same amount?
  3. theta

    Thread Starter New Member

    Sep 25, 2012
    If they could both be dimmed in a uniform manner, this would be produce an ideal output.

    I could use a combination of AA's or two CR123's max.

    I am very flexible on the design and any advice is welcome.

    Here is the datasheet: http://www.adafruit.com/datasheets/KWB-R8445W-1W.pdf

    Recommended Vf of 3.2v

    Would a joule thief also work on the output?
  4. Wendy


    Mar 24, 2008
  5. theta

    Thread Starter New Member

    Sep 25, 2012
    I am trying to make myself a cheap version of an existing product.

    It is a C2R tacpad, using a single CR123 to power a large EL panel. With dimming!

    This circuit must be bespoke and a custom job - I cannot find anything online that suggests such a panel (photos show it is in fact two thinner tapes placed in parallel along the long edge) could be powered by a CR123, let alone dipped.

    So, I turned to the LED backlight modules as the circuits is simpler, and the batteries will last longer :)

    Any thoughts on how they accomplished this? And my intention to develop an LED version for the reasons stated...


    http://soldiersystems.net/tag/c2r/ (a top article, and a video half way down)