Monostable 555 timeout to control Astable 555 pulsing LED

Discussion in 'The Projects Forum' started by Keith Lemelin, Feb 5, 2016.

  1. Keith Lemelin

    Thread Starter New Member

    Feb 5, 2016
    2
    0
    I have a small project I'd like to make for a valentine's gift. I want a pulsing LED to be inside of the gift (which would sit on a shelf, say), but I want the LED to eventually turn off before (say a minute, for example) running through the battery. So it seems like I would need an astable 555 for the pulse, but it gets triggered by a monostable 555. I haven't drawn anything out because I'm a bit out of my depth (mechanical background). I thought maybe it was as simple as wiring the output voltage of one circuit into the input voltage of the 2nd circuit, but I don't think the 1st circuit provides a high enough voltage to trigger the 2nd?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    You can use a 556 dual timer, one half flashing the led the other timing how long it stays on, it will run from 3V upwards, or use a flashing led and a single 555 timer.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,539
    1,251
    You want a circuit that, when activated, flashes an LED for one minute than shuts itself off, correct? If so, what activates the circuit and how often might the circuit be triggered?

    The problem with the 555, even the CMOS version, is that a 1 minute timer requires very large resistor and capacitor values, is not very accurate, drifts with temperature changes, etc. Another way is to start with a higher speed oscillator and divide the output down to once-per-second for the LED and once-per-minute for the shutoff. The higher frequency oscillator means smaller, more accurate, more stable timing components. All of this fits within the abilities of the CD4060. It is an oscillator and a 14-stage divider.

    1 CD4060
    2 resistors
    1 capacitor
    1 diode
    1 2N7000 MOSFET (optional)
    1 LED and resistor

    The problem is that the 4060 is a low power CMOS device, and doesn't have the 555's big fat output stage. The 4060 can drive an LED directly, but not to very high brightness. The 2N700 is a small MOSFET that makes an excellant LED driver for your application if you need more brightness.

    ak
     
    Alec_t likes this.
  4. Keith Lemelin

    Thread Starter New Member

    Feb 5, 2016
    2
    0
    Sorry for missing how it actually turns on, kind of important. It would essentially be a small box with a push button on the front. Once pressed, the led would pulse. After a set time it would turn off. The time isn't important aside from that it does turn off and isn't obnoxiously long. Less than 5 minutes at the max. Ideally it would be a slow pulse with a fade in and out, not a fast or immediate on-off. Maybe a frequency of a few seconds.
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
    1,952
    219
    How bright an LED? What kind? You can buy LEDs that flash on their own. No oscillator needed. They blink at an even rate. They fade in and out slowly. They flicker quickly in a rainbow of colors. Then all you need is the one second timer.
    Electronic Goldmine has a good assortment of such LEDs.
    http://www.goldmine-elec-products.com/prodinfo.asp?number=G16226 rainbow LED
    http://www.goldmine-elec-products.com/prodinfo.asp?number=G15257 blinking LED

    Both operate from 3 Volts and will run on a button cell CR2032 for about 11 hours, about 4,000 seconds of use..
    Another LED flasher is the LM3909. It only needs one capacitor and a 3 V battery.
    But they are getting hard to find.
     
    Last edited: Feb 5, 2016
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,539
    1,251
    I have a circuit drawn up, but a self-flashing LED is the clear winner for a *mechanical* guy.
     
  7. hp1729

    Well-Known Member

    Nov 23, 2015
    1,952
    219
Loading...