Delayed Pwr Off w/ 1 Sec Momentary Switch

Discussion in 'General Electronics Chat' started by radiohead, Jan 16, 2010.

  1. radiohead

    Thread Starter Active Member

    May 28, 2009
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    31
    Hello,

    I am trying to find a solution to a power problem. I have a mini DVR that operates between 3.3-5 VDC and draws about 300mA but no more than 500mA. It is programmed to begin recording video when power is applied. The problem is, when power is removed, the DVR doesn't have any time to write data to the SD card. To turn it off manually, there is a spring loaded (momentary) switch that must be closed for a period of no less than one second, then the DVR will go into its shutdown sequence which takes about 10 seconds (this is when data is being written to the SD card). I am requesting help with a circuit that, when power (3.3 to 5 VDC) is removed, it will allow the unit to stay on for an additional 10 seconds and activate a switch (solid state will work) for one second (to simulate the momentary switch being pushed). There is a 12-14 VDC source (vehicle battery) available to power a circuit. Thank you in advance.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    OK, I designed this for a 11 Sec Delay, and the second monostable keeps it shut off for approx 50 sec.

    [​IMG]

    I'm a bit uncomfortable with the power injection, since I don't know how the power is being turned off in this device.

    Adjust U3 for around 5.6VDC. I needed a couple of inverters, so I used the spare comparators on U1 for the purpose.

    I was going to use a MOSFET, but in my experience P channel logic level MOSFETs are hard to find, and I was trying for off the shelf parts.

    If the internal power drops 50mv then the comparator U1d triggers the first monostable U2a, which turns on Q1 via U1C. When U2a times out the second monostable U2b keeps the sucker from switching back and forth by disabling U2a.
     
    Last edited: Jan 16, 2010
  3. radiohead

    Thread Starter Active Member

    May 28, 2009
    474
    31
    Bill,

    This is exactly what I need, and I happen to have all the parts except for the darlington pair which is easy to get. Thank you very much.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    You can home brew a darlington out of two PNP transistors, just make sure the power unit can handle the wattage and current, and put a 100KΩ resistor between base and second emitter.
     
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