Creative way of turning a timer off

Discussion in 'The Projects Forum' started by poopscoop, Nov 27, 2013.

  1. poopscoop

    Thread Starter Member

    Dec 12, 2012
    139
    16
    In a previous thread I asked for suggestions on a refrigerator door open alarm, and while I appreciate the advice, none was applicable. I can't detect a light because it doesnt turn on until the door is open beyond a significant point, and I don't know how I'd build something to mount a snap switch without making it look terrible.

    I want to take advantage of the fact that the fridge face has almost no resistance, and use this and the absence of it to power the 555.

    The schematic is attached. R4 would be the fridge door. When closed, wires would contact it and complete the circuit with a moderately high resistance. The LED is subbed for the 555 timer.

    Intent is that, when the door is closed, electricity flows through the door rather than into the base of the BJT, and prevents the 555 from receiving power. When the door is opened C1 charges, Q1 conducts, and Q2 powers the 555.

    Questions:

    Will this work?
    What resistor values should I use? I made these up on the fly.
    How do I ensure that once ON, a transistor will turn off when desired. I can't seem to get that part to work on the breadboard.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    Why cant you just put a microswitch on the inside of your fridge, this will connect power to the 555 timer?
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,395
    1,607
    That's fine, but I would remove the unnecessary components, those being R2, R3, R5, C1, Q1 and Q2.

    Just put R4 in parallel with the LED.

    Done.
     
  4. poopscoop

    Thread Starter Member

    Dec 12, 2012
    139
    16
    I completed the design, built it, and installed it. As I've described, a switch is not an option because there is no clean way to hold it in position. Since it's battery powered, I can't parallel a resistor with an LED because the value would be too low, and would drain the battery when off.

    My design is attached. R6, R4, and a ground are in contact with the door when closed. When open, R4 and R6 cease conducting, C1 fills after about 2 minutes, and Q1 and Q2 conduct. D1 is a piezo buzzer.

    When the doors are closed the circuit consumes 8.8 microamps + whatever leakage.
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    How do you know that?
     
  6. poopscoop

    Thread Starter Member

    Dec 12, 2012
    139
    16
    Ohm's law, the effective resistance when the transistors are off is just over 1meg ohm.

    My meter reads to 1mA and doesn't register anything when the circuit is in the "off" state.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I'll argue that.

    Hot melt glue, or silastic, will be plenty strong enough to "cleanly" hold the microswitch and a little plastic battery box in position. And can be removed at a later date with no damage to the appliance.
     
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    I would love to see a picture of the OP's final solution. I wonder how it's held in place if not with some temporary adhesive such as hot melt glue, or with double sided tape as I suggested in post #9 of his first thread on this subject.
     
    Last edited: Nov 30, 2013
  9. dumpystig

    New Member

    Apr 10, 2013
    23
    0
    Just prop a chair against the door.
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,073
    3,846
    The sloped bushings on the bottom of the door that should induce it to close are either worn, damaged, installed wrong or dirty. No fridge should hold a door position that keeps the light out but remains slightly ajar. If it is new and functioning 'as designed', I would like to learn which manufacturer designed this mess so I can avoid them in the future.
     
  11. #12

    Expert

    Nov 30, 2010
    16,348
    6,836
    I'm with Gopher. I have set up many refrigerators, new and old. If you do it right, it is almost impossible for the door to hang slightly open.
     
  12. poopscoop

    Thread Starter Member

    Dec 12, 2012
    139
    16
    It's Jenn-Air, and I don't recommend it. It has a double door fridge with a drawer type freezer. The seal on the door is rigid and rolls inside a slot, but the action of rolling pushes the other door slightly open. Because it's stainless steel it's difficult to tell the second door popped open if you aren't specifically looking for it. Also, my roommate smokes a solid amount of weed and usually doesn't check if its closed.

    In fact, I don't recommend any Jenn-air appliances. They're form over function. The fridge has a filtered water spout inside the door, so you must open it and use two hands in an awkward position to fill a glass.

    My design works, did not require tools beyond a drill and a soldering iron, looks fine, and did not require me to spend 10 bucks buying and shipping 2 switches (One for each double door).
     
    #12 likes this.
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Picture, please.
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,073
    3,846
    Try adjusting the front feet per instructions in manual. If the fridge is leaning forward, the door may not close on its own. Adjusting the height of the fron feet to tip the unit back slightly will help the doors close on their own.

    https://jennair.custhelp.com/app/answers/detail/a_id/2825
     
Loading...