Circuit for a very dim and efficient night light

Discussion in 'General Electronics Chat' started by goingalong, Oct 7, 2015.

  1. goingalong

    Thread Starter New Member

    Oct 7, 2015
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    I have a LED flashlight/torch made by PALights called the Workman. These are no longer available but I would like to recreate it's always-on function.

    This light has been used by me for many years travelling around the globe. In its always-on low light mode the 9v pp3 battery lasts a couple of years and has sufficient light output to allow navigation around a dark hotel room and bathroom during the night. Because a member of my family (also on travels) just tumbled down the stairs of a rented house when he thought he was stepping into the bathroom I would like to make him an equivalent to my trusty Workman. Especially because he travels a lot more than I do.

    So my question is - does anyone know, or can guess what type of circuit might be inside the original light? Oh, and I don't want to smash mine open to find out.

    Firstly it is an efficient dimmer, so some form of PWM.
    Next it is running from 9v for ~2 years, so maybe a buck down converter, or just a switched constant current source.
    Also it is small, the whole thing, batteries and all, fit into the palm of my hand. So minimal capacitors or large inductors.
    And, it has been running for at least 10 years, so a conservative design.

    My thanks.
     
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  2. nigelwright7557

    Senior Member

    May 10, 2008
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    Sounds like a job for a 555 timer.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    CMOS version of course, to minimize power.

    I would look at using a couple of AAA's or AA's in series rather than a 9V, since that should give a longer battery life for a given battery volume, along with a high efficiency switching current driver circuit, such as this, which operates down to a 1.8V battery voltage (0.9V per cell for two batteries where they are essentially totally discharged).
    That circuit will keep the LED brightness constant as the battery voltage drops.
    You would adjust the output current to the minimum value required to give the brightness you need with a high brightness type LED.
     
  4. goingalong

    Thread Starter New Member

    Oct 7, 2015
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    The original is running from a 9v pp3 which I feel was a strange choice because somewhere the circuit has to lose around 6v, maybe through pwm. Nevertheless it appears to work well - however it is being done. Especially so given what 10 years or so ago is a small volume. It would be good to know how?

    Meanwhile, going for an up-converting current driver as highlighted by crutschow is probably my best way forward. I could use a 7555 timer for pwm or maybe a picaxe to keep the size down, there may not be much in it as far as power consumption goes.
     
  5. dannyf

    Well-Known Member

    Sep 13, 2015
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    A typical 9v battery has a capacity of 600mah. Over two years, that means 600/(2*360*24)=35ua.

    I am going to say that you are going to have a very difficult time to find something that runs off 9v and consumes that little current, between the controller + the led.
     
  6. goingalong

    Thread Starter New Member

    Oct 7, 2015
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    Yes, the original light is something special and I wish they were still available.

    http://flashlightsunlimited.com/workman.htm#.VhWd0bZVLRY .

    There doesn't seem to be anything like it around now. It gives around 2 years of life, not exactly, and I don't know how over-exhausted the battery gets or can become compared with specification.

    I see that the driver highlighted by crutschow has an analog control input mode so a 7555 might not be needed. Even better.
     
  7. dannyf

    Well-Known Member

    Sep 13, 2015
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    I actually think it better to run at lower voltage, like 3v or 3.7v: you could easily use a mcu here.

    If you have to run on 9v, I would use two or more leds: one in serial with the mcu's power supply, to act as a step-down "converter", and always-on indicator. And additional leds for high beam, powered through the mcu's ports.

    You normally put the mcu into sleep -> pick a mcu that has a 10-20ua sleep current and once touched, it goes back to high beam, or any other patterns.

    There are plenty of SOT23-sized mcus, or 8dip-sized mcus to choose from. Perfect for this kind of applications.
     
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  8. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I'm not sure I would get to fancy. Almost anything you use to do a constant current buck converter will waste some 10's of microamps. (or more :()
    If it lasts 2 years with a 1200 mah battery that's only 68 micro amps. So I think I would just do a constant current driver that will run down until the battery is really dead. Maybe something like this:
    You might check for a good led and test it to see if it is bright enough in sleep mode first.
     
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  9. goingalong

    Thread Starter New Member

    Oct 7, 2015
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    This is getting interesting and something of a challenge from way back when. The original light is pre 2002 and in one version (not mine) also had a SOS flash mode so I guess it had a mcu. Maybe it dropped that to a deep sleep and then used a constant current source.

    In low light mode I can comfortably look directly into the lens at the single led. I cannot do that when it is full on. So my guess would be that from an efficiency viewpoint they would be using pwm to heavily dim. But then a mcu would need to be awake in my experience.

    I have never tried dimming a led by throttling its current.
     
  10. TheButtonThief

    Active Member

    Feb 26, 2011
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    I'm confused.

    2 years? You mean to say that it's turned on constantly for the entire 2 years? During this 2 year draining period, the LED is emitting enough light to see your way in a dark room?
     
  11. goingalong

    Thread Starter New Member

    Oct 7, 2015
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    Yes two years. See the link I gave.
     
  12. TheButtonThief

    Active Member

    Feb 26, 2011
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    ...And in this mode, emits enough light to see your way in a dark room for the entire 2 years?
     
  13. goingalong

    Thread Starter New Member

    Oct 7, 2015
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    Again yes, check the link. With dark adapted eyes I can see the beam shining along the carpet for over 3 metre. Up closer I can read with it.

    There is no off switch and it definitely lasts at least two years on a pp3 Duracell.

    It exists, how does it work?
     
  14. TheButtonThief

    Active Member

    Feb 26, 2011
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    I've noticed that on these forums, some (allot) of people like to jump into these "I need a solution" threads with over complicated ideas. I like to keep things simple as I find that the best solution to anything is often the most simple and the most elegant.

    Here, I'm thinking "could it be as simple as limiting the current to the LED by means of a humble (yet proportionally large) resistor?"
     
  15. crutschow

    Expert

    Mar 14, 2008
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    Not if you want the best efficiency and longest battery life.
    For that you need a (constant) current output switching regulator with low quiescent current.
     
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  16. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The trouble with the resistor is that the current goes down with the battery voltage. That's why I used the constant current circuit.
    Hard to tell about the brightness and life at such low currents. @goingalong could check it. Hmm, maybe I have some laying around.
    The battery may think it is just sitting on the shelf at those currents, so may have longer life than specified in the AH curves.
    @crutschow , I haven't been able to find any really low current drivers. Do you have a lead?
    I guess we could have a do it yourself LED driver contest.:D
     
  17. crutschow

    Expert

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    The TPS61042 part I referenced in my Post #3 has an operating current of about 45μA, which is quite low but still significant in this application.
    Depending upon the LED current needed, its overall efficiency may not be that high, perhaps not much better than a resistor in series or a linear constant-current source. :oops:
     
  18. goingalong

    Thread Starter New Member

    Oct 7, 2015
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    A few facts.

    • When I replaced a dead battery it had 1.1v off-load, ie down from ~9v. Trouble is I don't know when it died and whether the circuit continued taking residual charge from it.
    • With a fresh battery and in low output mode it is drawing around 0.2mA. I can't get any more precise than that with my digital multimeter.
    • In high output mode it is drawing 62mA.
    • The circuit remembers which mode it was in when the battery was disconnected. If it was in low mode it comes on briefly in high mode then drops back to low - even after a 10 min delay.
    So I am concluding that there is a simple mcu in there along with a buck type regulator.

    I think what I will do now is grab a few of the regulators crutschow indicated and build a prototype.
     
  19. crutschow

    Expert

    Mar 14, 2008
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    Let us know how it works. :)
     
  20. goingalong

    Thread Starter New Member

    Oct 7, 2015
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    More ... for the record. The circuit works normally down to 3v after which high output mode drops away, which would be consistent with ~3v led. At 1.5v it is dead.

    So it looks like there is just a down converter, ie no up converter.
     
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