Timer and logic query

Discussion in 'The Projects Forum' started by hhoncho, Apr 20, 2006.

  1. hhoncho

    Thread Starter Member

    Mar 18, 2006
    I have been working on a circuit which utilises an H-bridge to drive a latching solenoid using a touch sensor, and which is very similar to this circuit by Ron_H:


    This circuit sends a pulse in one direction when first activated and then a pulse in the other direction when deactivated.

    The difference between this design and my own requirements is that I need the solenoid to be on for a greater amount of time (approx 2 seconds) when activated and then only pulsed in the opposite direction when deactivated. I have modified the circuit as follows which (hopefully) will energise the solenoid in one direction for as long as contact is detected by the touch sensor and then activate it in the other direction for a fraction of a second when contact is removed from the sensor.

    That circular stripey thing denotes the sensor and that reads Vcc in the top left where I lopped it off when scanning.


    My question is as follows: What is the best method to limit the on-time of the touch sensor to two seconds?

    I could use a higher value capacitor and resistor prior to the one already in the circuit and another logic gate or two for the purpose. My concern with this solution is that my space envelope is extremely limited and that the capacitor required for this time duration may be too large. Any idea of a ballpark figure for what value capacitor i would require for a 2 second on time? The benefit of this solution is that if i socketed the capacitor I would be able to easily switch values to test varying on times (useful for me for experimentation when limited by the solenoid's relatively low duty cycle).

    I could also use a 555 timer, maybe a cmos type after the level shifter. I'm using a QT110 touch sensor IC which operates at 5v while my solenoid operates at 11V, hence the need for the level shifter.

    Any recommendations or suggestions for alternative solutions would be greatly appreciated.
  2. Spoggles

    Well-Known Member

    Dec 2, 2005

    The time is equal to RC x LN * (Vcc/(Difference between V at time 0 and V threshold) As the voltage switching threshold for CMOS operating at 10V is 3 volts (30% of Vcc), the time delay 'going up' would be LN (10/3) or 0.356 * RC. The delay going the other way would be LN(10/7) or 1.203 * RC.

    Hope this helps.

    To control the requirement for different RC times, just use a diode across one of your timing resistors. When the control input goes one way the resistor is in the circuit, when it goes the other way it is a "0" ohm resistor.

  3. hhoncho

    Thread Starter Member

    Mar 18, 2006
    Thanks very much spoggles,

    That diode trick worked a treat.