RC circuit and Reed switch

Discussion in 'The Projects Forum' started by yakuut, Jul 7, 2010.

  1. yakuut

    Thread Starter New Member

    Sep 21, 2009
    Hey All,
    I need help with RC circuit tuning.The Problem- I am using a Reed switch which detect an magnetic object passing just above it. One side of the switch is hooked to 24V supply from a controller and the other is grounded.When the waveform is observed on the oscilloscope(when the switch closes),the pulse width for the signal is found to be 3ms with an amplitude of 24V.But the controller has a scanning time of 50ms and only detects the signal if the signal is above 10V for 50ms.Now I need to design the RC circuit that can charge the capacitor in 3ms and remains above 10V for more than 50ms.Also I can't have a very small resistor since the reed switch has a max. current limit of 0.5A.

    Can anyone help me with this.I have to do it using RC circuit only.

  2. SgtWookie


    Jul 17, 2007
    Gee, this sounds like a homework problem that would be assigned by a professor/teacher/instructor.

    Is it?

    What do you do for a living?
  3. Syniva

    New Member

    Jun 21, 2010
    You don't mean that it is a SPDT switch, do you? I've never heard of such a thing, but anyway...

    If it is SPDT, disconnect the grounded side, so that it is used as a SPST switch, conducting 24 volts when closed and nothing at all when open. This way, you can use a large value resistor connected to ground to discharge your capacitor, giving it a large discharge time, but use a smaller value resistor connected to the switch, giving it a small charge time. To compensate for the current limits of the switch, just use a smaller capacitor to shorten the charge/discharge times, then use a larger discharge resistor to increase the discharge time as needed.

    Alternately, if you can use a diode as well, leave the switch in SPDT mode, but attach the larger resistor in parallel with the smaller resistor and a diode in series, so that both resistors are used to charge the capacitor but only the larger value resistor is used to discharge it. This way you can have asymmetric charge/discharge curves.