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Simple pulse generator

Discussion in 'General Electronics Discussion' started by theTroll527, Dec 4, 2011.

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  1. theTroll527

    theTroll527

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    Dec 4, 2011
    I have a little project that I for some reason can't seem to get right.

    I have a 3 volt power source, and an LED Laser 3v, that I need to send a pulse to (would prefer to be about to adjust the pulse length), I have a latch switch that will be on for 1 second or more, the pulse should be around a 1/10th of a second or so. Once the latch is release we need to be ready to send another pulse when the latch goes high again.

    Any ideas? Can't really adjust the power source due to the batter pack that needs to be used and the form factor I am stuck with.

    tt.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Get a Schmitt trigger inverter (40106 should work at 3V, but there are others). Connect a small capacitor between your latched output and the Schmitt trigger input. Connect a resistor from the input of the Schmitt trigger to ground. When the input goes high the output of the Schmitt trigger inverter will go low for approximately 1 RC period.

    Make the resistor a pot and you can adjust the interval. Small value capacitors and high value resistors work well because the CMOS gate has a very high input impedance, but if you keep the resistors under a meg or so you won't have trouble with stray resistances.

    If you need the output to go high rather than low, just string another inverter to the output of the first and it will go the other way (high pulse on rising edge input).

    All other inputs need to go somewhere or the chip may misbehave. Connecting them to ground is fine. Another option (especially if you are using a second inverter as describes above) is to connect all the other inverters in parallel as the second inverter. This will increase their current drive, possibly to a point where you can drive a small load directly (although, at 3V it would be quite marginal).
     
  3. theTroll527

    theTroll527

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    0
    Dec 4, 2011
    Diagram

    Any chance you can throw up a diagram with two of the Schmitt Triggers? For some reason it is not working right. Might need to check on voltages also.

    Jim.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Like this:

    [​IMG]

    The pulse will have a duration determined by R and C (about 1RC from memory).

    So 1uF and 100k should do the trick.

    On a rising edge the output will stay high for approx 0.1sec, or the duration of the input pulse (whichever is shorter)
     

    Attached Files:

  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
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    Jan 21, 2010
    I used 5 Schmitt triggers for the output because I could :)

    You could also use them for other things like an input pulse extender (to ensure that the input pulse was never less then 0.1 sec, or to condition the input if it's more analog than digital.
     
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