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5370 Views 6 Replies Latest reply: Apr 21, 2014 12:08 PM by MikeWeed RSS
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Apr 18, 2014 11:16 AM

Timing circuit

Hey all, I'm thinking of building a timing circuit using a momentary switch that will start a circuit (a couple LEDs) and time out after 12 to 15 seconds.   I'd rather not use a microcontroller for such a small project and have some ideas of implementation but am looking for some feedback.  Does anybody have any good ideas of how to do this on the cheap?

 

Thanks, Rob

  • papalyle Novice 55 posts since
    Apr 6, 2014
    Currently Being Moderated
    Apr 18, 2014 8:01 PM (in response to boborrob)
    Timing circuit

    You are in luck, I just finished buildin this project for my grandson. I just used an analog comparator to compare a capacitor charging through a series resistor to tthe center of two resistors in series. Made the little fella's eyes light up! The LED was on the output of the comparator. I hope this works.for you.

     

    papalyle

  • WildBill Novice 3 posts since
    Apr 20, 2014
    Currently Being Moderated
    Apr 20, 2014 10:32 PM (in response to boborrob)
    Timing circuit

    Google -- 555 timer circuits and you'll find no shortage of ciruits like this you can make with bascially a few parts, 555 timer, electrolytic cap, var resistor, and your 2 LED's, if you want to run more than a couple low power leds you can toss in a 2N2222 transistor drive some bright ones. You might check our quickkits or JamesCo or ElectronicGoldMine hobby kits, you can get a kit with the pcb for a few bucks, probably cheaper than you can by the parts for.

    • WildBill Novice 3 posts since
      Apr 20, 2014
      Currently Being Moderated
      Apr 21, 2014 9:47 AM (in response to boborrob)
      Couple Easy Timers

      Here is a simple 2 LED flasher: (You can swap the 220k resistor for a pot and make the flash rate adjustable.)

       

      555-led-flasher.jpg

      Here is a very simple 555 flasher circuit and you can use the table below it to get your frequency and part values: (This will work from 3 - 12VDC)

       

      555ast.gif

       

       

      555 astable frequencies

      C1

      R2 = 10k
      R1 = 1k

      R2 = 100k
      R1 = 10k

      R2 = 1M
      R1 = 100k

      0.001µF

      68kHz

      6.8kHz

      680Hz

      0.01µF

      6.8kHz

      680Hz

      68Hz

      0.1µF

      680Hz

      68Hz

      6.8Hz

      1µF

      68Hz

      6.8Hz

      0.68Hz

      10µF

      6.8Hz

      0.68Hz
      (41 per min.)

      0.068Hz
      (4 per min.)

  • MikeWeed Novice 62 posts since
    Oct 21, 2011
    Currently Being Moderated
    Apr 21, 2014 12:08 PM (in response to boborrob)
    Couple Easy Timers

    Use a LMC555 and the circuit of Fig. 5 at www.ti.com/lit/ds/symlink/lmc555.pdf for a single pulse each time the switch is pressed (monostable operation). Be sure to connect the reset pin to the + power supply. The trigger input also needs a pullup resistor to the + supply, and the switch should take the trigger input to ground. If the switch might be held down longer than the output pulse, you will need to couple the switch through a capacitor and provide another resistor to discharge the capacitor when the switch is released. A LMC555 is better than the older LM555 because the LMC555 has an output that is essentially rail-to-rail. A square wave oscillator (astable operation) with a LMC555 is simpler than shown in Fig. 8. of the TI document. All  it takes is one resistor from the output to the threshold and trigger inputs, tied together, and a capacitor from this point to ground. The discharge input is not connected to anything.

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