compact strobe light with delay on/off

Discussion in 'The Projects Forum' started by Chaz1782, Apr 12, 2014.

  1. Chaz1782

    Thread Starter New Member

    Apr 12, 2014
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    So I am building a small sphere with high output LEDs in it that I need to be able to strobe.
    The unit will have an on button in it. Once pushed 5 seconds should pass before the unit starts to strobe. Once 30 seconds has passed I want the device to turn off until the on button is pushed again.

    looking for some help on what is needed for this.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Have you chosen the LEDs yet? Specs? How many? Power source?

    A microprocessor would be helpful with your timing but can we assume you have no prior experience with such thing? I believe you can achieve the timing you want with 2 555 timer ICs and a crude RC timer. One 555 will deliver your strobe frequency and duty cycle (please define what you want for this, maybe 3 flashes per second at 50ms each?). A second 555 will be a one-shot (monostable) to give 30 seconds of run time. The latter 555 might be triggered by delaying the button press by ~5 seconds with the RC tank. I haven't thought out all the details of that but I'm confident it could be worked out.

    Your strobing LEDs will then be controlled by a MOSFET that is in turn controlled by the timers.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The nice thing about the CMOS 555s is the super low static current. Wayne's approach is good, but I'd go with 3 555's for stability and consistancy. The on button triggers the first one thaat is a 5 second monostable. Its output triggers the second one, a 30 second monostable. That output gates the third one, the strobe freq. After 35 seconds, the only battery drain is the static current of 3 CMOS chips.

    But - a 30 second timer with a 555 will not be very accurate or stable unless you use a relatively expensive capacitor with very low leakage current.

    Another approach is to use two counters in series, like two CD4060's being discussed in another thread, or one 4060 plus some external gates. Much easier timing R and C components to get. Either way, a selection of strobe frequencies falls out without a separate oscillator chip.

    ak
     
  4. Chaz1782

    Thread Starter New Member

    Apr 12, 2014
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    I have ordered 5mm Piranha LED's. I plan on squeezing 20-40 of these into the device.
    Your assumptions above are true, I have never done anything like this before and 555 timers are totally new for me.
    I am looking for cost effective if possible.

    Basically as stated above I want to push a button, have 5 secs count down then the LED's strobe until 30 secs is up then the unit shuts down until the the button is pushed again.

    OK. Lets make this simple for now. How would I go about having it setup as follows:
    Push button, 5 seconds passes and the LED's strobe. Unit does NOT power off until the push button is pushed again (Lets assume its a toggle switch or self lock push button).
     
  5. wayneh

    Expert

    Sep 9, 2010
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    That doesn't really simplify things all that much, although you only need two timers and therefore you could use a 556 chip, which contains 2 individual 555 timers.

    But if you want the turn-off function, I'd go ahead and keep it in. You need to learn about the 555 timer, and once you do, the 30 second timer will be easy.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    2.7 M and 10 uF gets you 30 seconds, but the max charging current is 4.4 uA at 12 V. Leakage could be an issue.

    ak
     
  7. Chaz1782

    Thread Starter New Member

    Apr 12, 2014
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    OK. Remove the turn off after 30 seconds.
    Want to run 36 to 48 Piranha LED's.
    Looking to run it off of a single 556 timer. 5 second start delay with strobing.
    Question: Is it possible to run such a beast off of low voltage like 1.2v?
    I know I could wire them in parallel so they would each get 1.2v. The mA requirements for the 36 would be 720 and the battery in question is 1600. So again, my main question is if 1.2v would be enough for this setup.
     
    Last edited: Apr 18, 2014
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you know that the typical operating current is 20 mA, then you know that the typical forward voltage is.....?

    ak
     
  9. Chaz1782

    Thread Starter New Member

    Apr 12, 2014
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    3.2v
    Would running it on 1.2v cause any issues other then perhaps it not being as bright?
     
  10. Chaz1782

    Thread Starter New Member

    Apr 12, 2014
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    Did some research and it looks like I will need a voltage multiplier in my setup. This circuit is getting bigger and bigger.
    Is it safe to recharge a battery that is connected to a multiplier?
     
  11. wayneh

    Expert

    Sep 9, 2010
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    You should not target 20mA for each LED, since that is the absolute maximum rating. They'll burn out soon at that level. I'd shoot for ~15mA to ensure long life.

    You could certainly power these off a single cell at 1.2V, at least in principle. That's how those solar landscape lights work off a single AAA battery. But your problem is that you need a constant current circuit for each LED. It might not be hard to find a DC-DC boost converter that can handle the total current you need. (I'd want a converter rated to at least 20mA each, even though I expect to use only 15mA each.)

    Trouble is, you'll need a current limiting resistor on each LED. Otherwise some LEDs will hog all the current and burn out prematurely.
     
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