4541 Power On Reset

Discussion in 'General Electronics Chat' started by Clarence Donath, Apr 29, 2011.

  1. Clarence Donath

    Thread Starter New Member

    Apr 29, 2011
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    Hello helpful allaboutcircuits forum people,

    I have one of these kits, http://www.canakit.com/universal-1-hour-start-stop-timer-kit-ck158b-uk158b.html

    It works perfectly for my needs, except that it "starts" when power is applied. The IC is a HEF4541B.

    I observed that holding pin 6 (MR) low on power up keeps the timer from activating, but this cannot be held low all the time since triggering a start causes the power supply to short. I'm thinking what I need is a short delay in holding pin 6 low at power up. Is there a simple RC network that can do this?

    Thank you
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It's difficult to offer advice without a schematic. You might get the wrong advice and end up with your timer damaged.
     
  3. Clarence Donath

    Thread Starter New Member

    Apr 29, 2011
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    You're right, I know. I don't have a schematic as it is a kit. However, pin 6 is unconnected. Knowing that holding pin 6 (MR) low already does what I need, I just need to come up with something that'll do this at power up.
     
  4. k7elp60

    Senior Member

    Nov 4, 2008
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    69
    Looking at the data sheet, pin 6 (MR) needs to go high to reset the timer. If you want to activate the MR(pin 6) on power up. Connect a 0.1uf capacitor between pin 6 and VDD, Connect a 100KΩ resistor between pin 6 and ground (VSS) The value of the capacitor and the resistor determine the timing. With the above conditions pin 5 should be connected to VSS
     
  5. Clarence Donath

    Thread Starter New Member

    Apr 29, 2011
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    Thank you for your reply k7elp60.

    I didn't have any luck experimenting with that configuration.

    What I've found, by experimentation, is that holding pin 6 on VSS at power up cancels the start cycle. I then release the connection of pin 6 to VSS and start/stop operations function as normal. The normal configuration of pin 6 in this circuit is floating. It's not connected to anything.

    Is there a circuit I can use that will make a connection to GND for a second, more or less, at power up, then release it to an unconnected state while powered up?
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It's not recommended practice leaving CMOS inputs floating - they should be tied to an input - albeit to GND or Vcc if unused. Are you positive that pin is actually floating?
     
  7. Clarence Donath

    Thread Starter New Member

    Apr 29, 2011
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    0

    [​IMG]
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    You are absolutely correct t_n_k. Pin 6 is indeed tied to the BGR resistor straight up from pin 1, which in turn is connected to the kit's STOP header. I failed to realize there are traces under the IC socket that couldn't see with the chip installed.
     
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