Discussion in 'General Electronics Chat' started by ramcaress, Mar 5, 2010.

  1. ramcaress

    Thread Starter New Member

    Mar 1, 2010
    hello guys, a would like to ask if a cmos type 555 of 7555 can handle a timing period of up to 2.5hours basing on the equation T=1.1(R)(C), i want to use it in timing irrigation periods, thanks a lot:)
  2. JDT

    Well-Known Member

    Feb 12, 2009
    Only by using a very large value of resistor and/or capacitor.

    Trouble is, your large value capacitor will have to be an electrolytic type which are not very accurate value. Also these capacitors have high leakage current. As your resistor will also be a high value, leakage is a problem.

    Better to use your 555 as an oscillator driving a logic counter chain. When the last stage of the counter goes to logic 1 your time is up.

    See the HEF4060B CMOS 14-stage counter with oscillator built in. Or the HEF4521 21-stage counter (no oscillator).
  3. Wendy


    Mar 24, 2008
    Look at it this way, if you have a 10MΩ resistor for 9000 seconds (2.5 Hrs) you would need a capacitance of 818µF.

    I would try it, but you need to be prepared for the idea it will not work. A newer cap has a better chance, don't try a junk box part.

    BTW, if you have a 1000µF cap, you need a resistor of 8.2MΩ. Drop the resistance to 820KΩ and you would need a 10000µF cap.