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TLC556: Trigger Astable operation to restart

Nicholas Yunker
Prodigy 50 points
Part Number: TLC556


Hello,

I am using a TLC556 duel timer to create a stable time period with the first timer and to use that to trigger the second timer in mono-stable mode to generate a pulse based on a potentiometer input and the timer period generated in the first "stage".  Here is my schematic (R2 is the pot from one end to the wiper):

I have been able to both simulate and build this circuit and it works great.  Timing is perfect (except due to the 20% tolerance of the caps it's not exactly 600s as I'd like) but there is one interesting caviat..if you adjust the pot in the middle of the cycle your adjustment won't take effect until the next cycle.  This leads to the problem where you start the board with the pot set to 0 and after 10 seconds you change it you need to wait another 590 seconds before you even see the output of the second stage do anything.  It makes the circuit look "dead" to the user.  While I wouldn't expect an immediate reaction to pot adjustments, it needs to react faster than that.  Is there a way to modify this circuit to trigger the first stage to restart when the pot is adjusted?  The obvious one I can think of is adding a push button to the reset pin of the first stage and you press that button after adjusting the pot...but that is not feasible in this application.

  • 0
    Prodigy 50 points

    the capacitors should be 680uF...I was playing with this simulation to see what would happen with real cap values vs. ideal.

  • 0
    Prodigy 50 points

    And the pot is R9, not R2

  • 0 in reply to Nicholas Yunker
    TI__Genius 11815 points

    Hi Nicholas,

    That is an interesting caveat to your design. 

    The first option that comes to mind is to use a dual potentiometer where the resistance of the second leg can be measured and connected to a circuit that will trigger the reset pin when a change in resistance occurs. This can be done by biasing the second leg of the dual pot to the supply voltage and measuring the current flowing through it. The current measuring circuit can be connected to an edge-sensitive comparator function that will pulse the reset.

    Of course, this will add quite a bit of complexity and cost to your design. There could be more efficient means of achieving this, but this is a very atypical circuit function so I'm not sure what the optimal solution is.

    Regards,

    Zach

  • 0 in reply to Zach Olson
    Prodigy 50 points

    So currently I connect the one leg of the pot to 12V and the wiper to the 10k resistor.  What your saying then is to take the other leg of the pot and connect it to 12VDC through a current monitor?

  • 0 in reply to Nicholas Yunker
    TI__Genius 11815 points

    Hi Nicholas,

    My idea is to replace your single potentiometer with a dual potentiometer. This will have 6 pins and 2 wipers that are both controlled by the same rotary knob. Each "leg" as I am referring to it has 3 pins. A duel pot schematic looks like this:

    https://www.bourns.com/docs/product-datasheets/prs14r.pdf 

    As the user adjusts the rotary knob, both legs will exhibit a similar change in resistance. This way your working circuit looks the same using the first leg, and the second leg can be monitored to indicate that a change in resistance occurred.

    Of course there could be other solutions, but a dual pot solution will allow you to monitor a user input event without affecting the performance of your current design.

    Regards,

    Zach