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Timing circuit (lm555)

  1. Nov 25, 2008 #1

    JO3

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    Hello all,

    I'm trying to make a circuit with the following properties:

    1) output at +V for a time T1
    2) output at 0 for a time T2
    repeat indefinitely
    **also, whenever contact is made between two points, I would like the circuit to jump to state 2 (contact would only be made when the system is in state 1)

    It would be good if I can adjust T1 and T2 continuously, i.e. with a variable capacitor and/or potentiometer. T1 should be about 15 minutes and T2 should be about 45 minutes, but I want to understand the circuit enough to be able to make adjustments myself. Equations would be very helpful:)

    I think I might be able to use the lm555 or lm556 chip, but I'm not exactly sure how to set these up. Voltage should be between 5V-15V or so.

    Thanks for your help,
    Joe
     
  2. jcsd
  3. Nov 25, 2008 #2

    dlgoff

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    Science Advisor
    Gold Member

  4. Nov 25, 2008 #3

    JO3

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    Thanks, that site looks very helpful. I'm reading through it now.

    Joe
     
  5. Nov 26, 2008 #4

    berkeman

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    Staff: Mentor

    Those are pretty long times to be trying to use a 555 analog timer circuit. A better solution would be to use a digital oscillator (based on a 32kHz watch crystal, for example), with divider circuits and a little logic to give you the variability you want. Check out the 74HC4060 IC and its app notes, for example.
     
  6. Nov 26, 2008 #5
    WOW! I thought I was pushing the envelope timing out to around 10 minutes with a 555.
     
  7. Nov 26, 2008 #6

    Redbelly98

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    I got one to go to 40 minutes. It's in a coffee-cup heater, the heater shuts off after the 555 says it's time.

    The heater circuit originally timed out at 20 minutes. I opened it up, changed a 5M resistor to 10M, and now it's 40 minutes.
     
  8. Nov 26, 2008 #7

    dlgoff

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    I've had them work for long times also. Nice big caps.
    I think that the 555 is a good way to get your feet wet in circuit building.
     
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