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Help Googling circuit names!

Discussion in 'General Electronics Discussion' started by grim_reaper, Jun 15, 2013.

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  1. grim_reaper


    Jun 15, 2013
    Hi everyone,

    I'm trying to design a circuit to control multiple relays, but I'm running out of output pins on my microcontrollers. I'm looking for an interface IC, but I don't know the name/type/description of the circuit/gate combination that I've thought up!

    Basically, from a given input - being a digital high or low - I want two outputs. The first output should go high when the input goes high (rising edge trigger) and remain on for a time before going low again (latched timed output). The output time should be fixed (around a second - enough to pulse a relay coil) and should be unaffected by the input once it's been triggered.
    Similarly, the second output should pulse for the same time period, but when the input goes low (falling edge).

    Having just drawn a timing diagram to convince myself (attached), it's a lot easier to look at that to see what I mean! So - what's this arrangement called, and more importantly, is it available in some sort of IC?!?


    Attached Files:

  2. donkey


    Feb 26, 2011
    your relays being directly controlled by a micro is not the best idea. but anyway if you need more pins on your microcontroller try a "shift register" it uses roughly 3 pins to make 8. hope that helps with running out of pins.
  3. grim_reaper


    Jun 15, 2013
    The microcontroller outputs are already going through shift registers - using only 3 pins as you say - and I'm not driving the relays directly! The final output is through ULN2803A drivers.
    I was just going off on a tangent thinking that maybe I could drive the relays with a single output rather than two.
    Having been searching for every alternative, I think the shift registers are the only 'single IC' solution I'm going to find.
  4. grim_reaper


    Jun 15, 2013

    Never mind the reasons behind it any more - but now it's just annoying me that I can't 'name that logic'. Anyone know what a pulse-on-high/pulse-on-low circuit is called?!
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    A capacitor?
  6. GreenGiant


    Feb 9, 2012
    well that logic you have in the first post would be a combination of time-off relay with 2 flip flops reading the same input, one toggles on rising, the other on falling.

    That is one of dozens if not hundreds of ways to do what you want.
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    A circuit that generates a pulse of a specific duration is called a monostable. They can be implemented in many ways - 555, CD4528/4538/4098, 74HC123, using capacitors and gates, capacitors and discrete semiconductors, op-amps...

    Generally they are triggered from a specific edge on an input signal, but in your case you want a monostable that triggers from both edges.
  8. BobK


    Jan 5, 2010
    Actually, he want 2, one rising edge and one falling edge triggered.


    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    You never did say how many additional outputs you need. You may find that regardless of how you generate a monostable output, transistors, gates, inverters, 555's, etc, you will invariably increase your component count as compared to a second uC working as as a slave.

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