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How to set a Programmable UJT Transistor's threshold value?

Discussion in 'Electronics Homework Help' started by herts, Jul 1, 2013.

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  1. herts


    Jul 1, 2013
    hello everybody,

    This is my target:
    A timer 555 is providing a short frecuency wave (10 minutes high value, 1 minute low value). My project requires a reset functionality to avoid the timer 555 to get the low value state, so, every time the user pushes reset, the timer should go back to its initial position (10 minutes to get the low state again). It won't get the low value state if the user keeps pushing reset at least once each 10 minutes maximun.

    This is my problem:
    Timer 555's pin 4 (reset) makes the wave come back to the initial position (I hope so!) but the timer output shows a low value state for a couple of seconds.
    I would like to have a low value output only when the timer gets the low value state at the end of the first state (10 minutes), so it should keep showing a high value on the output when reset is being used (low voltage on pin 4).

    My solution:
    I've placed a capacitor on time's output that keeps some voltage for several seconds, time enough to wait for timer's reset and get a hight value again. And a 2N6027 Programmable UJT Transistor Trigger connected to the capacitor that would check wether the voltage is under a certain value (1.5v in my case). If I understood well the Programmable UJT Transistor working, it should give me a high binary value output if input voltage is above 1.5v and a low value if it's under 1.5v. Is that correct?

    My solution's question:
    How can I set my transistor threshold to 1.5v? is there a calculator or formula somewhere?

    Thanks in advance
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
  3. herts


    Jul 1, 2013
    That is a great solution, thanks!
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