Toggle Circuit with LMC555 on less than 5V

Discussion in 'General Electronics Chat' started by TomTommer, Mar 25, 2008.

  1. TomTommer

    Thread Starter New Member

    Mar 16, 2008
    4
    0
    Hello all,

    I'm trying to build the following pushbutton flip-flop circuit:
    http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm#555-T.gif
    (or like described on http://www.cpemma.co.uk/flipflop.html )

    I have a source of between 3.5 en 4.5v (3 batteries),
    so I thought I could use the low-power CMOS LMC555CN.
    Problem is that when I press the button once, the circuit switches on, but stays on. It doesn't switch off when I press again. :confused:

    Can anyone tell me how to get this circuit working with a LMC555?
    Thanks in advance (a lot!).
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    An LMC555 with a supply voltage as low as 3.5V has a very low amount of output high current, maybe only 1mA. A higher load would cause its output voltage to be too low to flip the circuit to turn off. So its output must drive a darlington transistor through a high value resistor.
     
  3. TomTommer

    Thread Starter New Member

    Mar 16, 2008
    4
    0
    Truly a guru Audioguru,

    That was the solution, it's working now! :D
    Thanks-a-bundle.
     
  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Change the xstr to an NTD3055 FET (ON Semi), which has a gate threshold of 2V max.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The threshold voltage of a Mosfet is when it barely turns on. Its current is only 250uA.
    The threshold voltage of the NTD3055 Mosfet is the same as nearly every other ordinary Mosfet, minimum 2.0V and maximum 4.0V.
    It won't work.
     
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