Auto shade screen

Discussion in 'The Projects Forum' started by DAK, Aug 10, 2010.

  1. DAK

    Thread Starter New Member

    Nov 25, 2009
    I’m trying to build a circuit to activate a shade screen to roll up when the sun starts to go down (dusk) and down when the sun shines on it (bright). I would like to use a light/dark comparator circuit using a LDR or a Thermistor in its place to do this operation in conjunction with a reversing circuit for the motor. The circuit and motor would be driven with the rechargeable battery that comes with the drill motor (18vdc). I would like to use a magnet that will be place on the edge of the screen at the bottom and top to activate a switch to allow the motor to shut off. Bellow is some circuit schematics I was thinking of using/modifying. My problem is how do I send a signal from the Light/Temperature circuit, to the timer, activate motor, stop motor, reverse motor, repeat the cycle etc.

    Light/Dark activated VS Temperature activated

    1. Light/Dark
    2. When the shade screen magnetic switch is activated (bright outside) it should stop the motor but also place the motor in the reverse direction to wait for the next signal to turn it on. In other words
    3. Hysteresis do to the LDR partially triggering because of the slow change in light source:
    3.1. Should a monostable 555 timer to eliminate the chatter associated with activating a relay?
    3.2. Is there another type of really that would work such as a solid-state or IC to eliminate this?
    4. Clouds coming in and out will activate the circuit prematurely because the sensitivity that need so be set (bright and dusk) will activate on the light/dark circuit.
    4.1. Should I use a 556 timer and use one half for the chatter problem and the other for the cloud problem?
    4.2. I could keep the 556 from triggering again if I set the RC to maybe an hour or so. Would this do the job?

    5. Temperature Same (Questions 1 and 2 would still apply)
    6. As far as question 3, I may be able to use block of aluminum and insert the thermistor into it. The aluminum block would retain the heat or cold causing the temperature to change slowly so the clouds moving in and out wouldn’t trigger the circuit. There may be some kind of trade of though?
    6.1. The hysteresis/chatter time would seem to be extended, but would this really matter using the 555 timer? I guess the other concern would be the amount of time the relay would chatter?
    7. Misc. Questions
    7.1. Is it a concern that one of the magnet/switch is close at all times?
    7.2. As long is one of the magnet switches is open would that work and how would that effect how the circuit is wired?

    Trying to tie each phase or major components together to accomplish my intended use is where I have the most problems. I tried to identify a few of them above and I’m sure there are many more that I haven’t thought of. Please take a look at the attached circuits/projects below and see if they may or may not work for any part of this project. Leading me in the right direction as far as schematic examples, part numbers, resources etc. would be greatly appreciated.

    Thank for Your Time
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    Sounds to me like you just want to flip and then flop a window shade.
  3. DAK

    Thread Starter New Member

    Nov 25, 2009
    I'm not sure what you mean by that?
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
  5. someonesdad

    Senior Member

    Jul 7, 2009
    Why not use a simpler approach? Find a cheap mechanical timer and use its relay contact to open and close the shade at the proper time of day? You may have to change the timing somewhat throughout the year to accommodate the changes in the length of the days, but that's pretty trivial. You could probably have a working solution in under an hour of work.
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
    Know what makes a cheap battery operated electrical timer? A common setback thermostat for a home heat/air system.
  7. nickelflipper

    Active Member

    Jun 2, 2010
    This isn't a lot different than a solar tracker circuit.
    1) You could try separate light and dark circuits run off two ldr's and a LM393 dual comparator. Put the reed or limit switches on the LM393 open collector pullups to disable further movement in that direction. Use 1MΩ resistors for hysteresis per the data sheet. Clouds, in general, should not be confused with dark or light conditions.
    2) From the LM393 outputs use 2N7000 nfets to energize the relay coils. Two SPDT relays can form a reversible h-bridge for your drill motor (check out the web for an appropriate schematic). Could be interesting to choose a coil voltage to match your battery.
    3) A large cap in parallel with the relay coil (10-47uf) eliminates the chatter on the relay contacts.
    4) Have fun on fuse protection, and determining if the shade gets stuck, else watch the 2N7000's pop:eek:.
    5) EDIT
    6) The above ideas are based on a low rpm 12V dc gear motor, and may not be appropriate for the 18V drill motor, have never played with them.
    Last edited: Aug 11, 2010