LM555: two signls with phase shift

Discussion in 'The Projects Forum' started by DedeHai, Jul 7, 2009.

  1. DedeHai

    Thread Starter Active Member

    Jan 22, 2009
    39
    0
    Hi there

    I want to connect two 555-timers, such that I get two signals with 50% duty cycle that have about 90° phase shift to each other. The Frequency should be between 50 and 100Hz.
    The first signal is easy to create by using the timer in astable mode configured such that the output is at the desired frequency and with a duty cycle of 50% (I already have that).
    But how do I get a second timer chip to copy the signal of the first one but with a time delay?
    The circuit can operate at a fixed frequency, although adjustable frequency would be a nice to have.

    thanks in advance
     
  2. simeonz11

    Active Member

    Apr 29, 2008
    98
    0
    you can use a xor gate I believe .

    run a d flip flop divider faster frequency and a slow freq and xor both ?

    You will get that slowe one to be lagged by 90% I believe . Or is that 45 degress ?
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You need 2 timers for the 90 degree shift, one for a delay, the second for pulse.

    Simeonz11 is close, though a straight gate won't work. Think 2 flip flops counting. Then two AND gates feeding off them would give you want you want (or two NOR gates).
     
  4. DedeHai

    Thread Starter Active Member

    Jan 22, 2009
    39
    0
    can i not use a certain threshold voltage (since the 555-timer has a thresholt input, but i don't really understand how it works) and run the square wave signal of the first timer through a low pass filter and connect the filter output to the input of the second timer?
    so instead of using another timer as a delay, just use a low pass instead`?
    can you give me an example of how you would connect the second timer in such a way (if there is one)?
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    A couple of suggestions:
    1) Remember that the 555 is triggered by a negative going iput. Thus, you will need an inverter after you delay the signal with the filter. A small-signal transistor (e.g., 2N2222 or 2N3904) in common emitter (emitter grounded) configuration should work.
    2) I have found that using a coupling capacitor (0.01 uF) between 555 stages helps .
    3) This method will not give a fixed offset in phase angle (e.g., 90°) as you vary frequency. The offset will be in units of time, which is determined by the RC constant.
    4) Since you want them at the same frequency, you might want the second 555 configured as a one-shot that gets re-triggered from the first 555 with each cycle.

    John
     
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