Hysteresis Issue

Discussion in 'The Projects Forum' started by Codee, Feb 27, 2013.

  1. Codee

    Thread Starter New Member

    Feb 27, 2013
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    I am trying to discharge a capacitor through a LED however what I want is the capacitor to start recharging once it has discharged below a certain voltage. I need to achieve this hysteresis circuit without using additional power supplies as it is for a stand alone energy harvesting module and therefore I am using silicon diodes and NPN transistors to create this effect. However I think I'm going wrong somewhere because, well, my LED doesn't flash.

    Please see attached JPEG and get back to me!
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
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    Have you thought about a Unijunction Transistor (UJT) to make a relaxation oscillator?

    UJT LED Flasher
     
  3. #12

    Expert

    Nov 30, 2010
    16,257
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    I did, but I had to work today and didn't have time to design one for him.
     
  4. Codee

    Thread Starter New Member

    Feb 27, 2013
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    I haven used a ujt before so I'm going to try understand its operation today. In the mean time if you can provide a schematic then that will speed up my understanding process a lot. I believe you are referring to this type of operation

    http://www.circuitstoday.com/ujt-relaxation-oscillator
     
  5. edwardholmes91

    Member

    Feb 25, 2013
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  6. Codee

    Thread Starter New Member

    Feb 27, 2013
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    I have realised that I don't want an oscillator. I want a voltage controlled charge\ discharge therefore I'm still looking at my first schematic. I did find an issue because the diodes were completely redundant , I've attached a new schematic if anyone can help me try to figure out its operation and resistor values il much appreciate it.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Can you state what your end goal is?

    What are the inputs?
    What are the outputs?
    What are the timing requirements?
    What are the power sources?
     
  8. Codee

    Thread Starter New Member

    Feb 27, 2013
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    ultimately i am charging up a large capacitor(mF) to approximately 4V by collecting rf energy.

    I am converting the ac input into dc and then storing it in my capacitor. What I now want is a cyclic charge/ discharge of the capacitor through a 1.7V LED once the capacitor has charged to a certain level say 2.5V(because this should make the led flash). The circuit I have posted earlier is done with a view of trying to achieve hysteresis so that until the diodes toward voltage is overcome by the capacitor the LED will not flash.

    Does this make sense to you? Also can I just thank you in advance for taking the time.
     
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