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LM555: Timing Circuit

Thomas Relling
Prodigy 50 points
Part Number: LM555

I am seeing if a circuit already exists before coming up with a new one. This is a timing circuit that will turn on up to eight different zones. Each zone requires 5V / 1.4A to 5V / 11A. I already have a 5V supply with the capability to power all 8 zones. I will use relays or something else to disconnect the zones as detailed below. In the example below, one zone is not used.

 

All 7 zones will turn on at the same time and then

  • Zone 1 will turn off after being on for 2 minutes
  • Zone 2 will turn off after being on for 5 minutes
  • Zone 3 will turn off after being on for 10 minutes
  • Zones 4 and 5 will turn off after being on for 20 minutes
  • Zones 6 and 7 will turn off after being on for 30 minutes

 

The times mentioned above are approximate. It is desirable to have the flexibility of scaling all the times possibly with the use of a variable resistor (pot). For example, instead of having the zones on for 2, 5, 10, 20, 20, 30, and 30 minutes it would be nice to adjust a knob so the zones are on for 1, 2.5, 5, 10, 10, 15, and 15 minutes. The longest a zone will ever be on is 30 minutes.

 

An LM555 timer may work but I wonder if I need 8 of them or if something simpler exists. I don't want to use any software.

  • 0
    TI__Guru*** 146816 points

    Hello Thomas,

    One timer can do it with the help of a CD4040 to divide the timer clock and have the benefit of smaller timing components. If the 2 minute can be 2.5 minutes then everything lines up well. Make the timer free run at 1.71 Hz and feed the timer output to clock input of the CD4040. You will also need a latching gate for each time length (5 total) The logic app support group can pick one out for you. 

    Before a start of a cycle have CD4040 RESET active high and have all the latches reset.
    At start of cycle bring CD4040 RESET inactive low and release reset on the latches.. 

    The Q9 output will go high after 2.5 minutes, use that to set 2.5 minute latch to turn off those zones.

    The Q10 output will go high after 5 minutes, use that to set 5 minute latch to turn off those zones.

    The Q11 output will go high after 10 minutes, use that to set 10 minute latch to turn off those zones.

    The Q12 output will go high after 20 minutes, use that to set 20 minute latch to turn off those zones.

    Both Q11 and Q12 will be high after 30 minutes use that AND condition to set 30 minute latch to turn off those zones. (don't be too fast is this is a ripple counter) 

    If you really need 2 minutes then you could use logical AND [Q7,Q8] for 1.88 minutes or logical AND [Q5,Q7,Q8] for 2.03 minutes.

    If this sounds good, you could start a new post for the CD4040 and reference this post to get help with AND and LATCH functions.

  • 0 in reply to Ron Michallick
    Prodigy 50 points

    Thanks Ron, I really appreciate this.  I will dig deeper into the details and let you or the logic group know if I have more questions.

    Tom Relling