12v timer circuit how to increase delay

Discussion in 'The Projects Forum' started by mariotech, Jan 29, 2011.

  1. mariotech

    Thread Starter New Member

    Jan 29, 2011
    2
    0
    Hi All
    I have just completed my first circuit as I needed a 12v timer and the only option was a kit from Maplins. The problem I have is that it tells me to increase the delay I have to change a component for one of a greater value. Now I know it is a capacitor but what I don't know is how much I should increase the valve by. I was hoping someone could point me in the right direction.
    The capacitor valve is 100uf 16v and is causing a 5 min time delay with the variable turned up full.
    The kit is this one from maplins
    http://www.maplin.co.uk/universal-timer-kit-3315
    I am hoping to create a 2hr + delay
    Thanks Jonathan
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Vellman usually includes a schematic in the manual, can you scan/post it as an attachment?

    It should look like This One

    If the cap is 100uF, I'm guessing the resistor is 47k (Yellow, Violet, Red).

    For a 2hr timer, you'd need to use a larger resistor as well, and accuracy won't be perfect. Time (seconds) is 1.1 * R * C , for 7200 seconds, you'd need a 500k resistor and a large 12,000 uF cap (wouldn't fit on the board). If you go with a higher resistor, the delay is less accurate (5% of 1,000,000 is a lot more than 5% of 500,000). Capacitors also are +20%-50% tolerance, so I hope you aren't looking for exactly 2 hours.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    I would say this is doable with a 555, or a CMOS 555 (longer duration). Really long times usually use counters though, then the number of counts is reached it triggers an alarm.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
  5. mariotech

    Thread Starter New Member

    Jan 29, 2011
    2
    0
    Hi yes that is the Schematic for this kit. The time does not need to be accurate at all. If I replace the cap which number resistor is the one I need to replace.
    Thanks Jonathan
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    I really don't understand what R2 and R3 do, they are the same ratio's as the internal 555 resistors, and the voltage at pin 5 would still be at the 2/3 Vcc voltage. I'm not sure why they are there. I suspect it is to lengthen the timing duration, but I don't see how it could do that. Maybe someone else would have some insight on that.

    This design should be able to reach 242 seconds. You want to reach 7200 seconds (2 Hours). Your resistance is about a large as it can get, so the cap has to change. Looking at it C2 would need to be 3,000µF. For a 555 this is about the only way. You could attach a capacitor on the bottom across the existing C2.

    Old electrolytics may not work well for this application, as the charge current is so low (about ½ µa). It is a common failure mode so I'm told. A new one should work OK though.

    I have a article for something similar, 555 Monostable.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I doubt that the chip exists any longer but many years ago (I think it was MAX) made a hybrid chip that was comprised of a 555 and two counters. It was capable of long time-outs into hours.
     
Loading...