Light activated timer to power motor - Design

Discussion in 'The Projects Forum' started by fz1, Dec 21, 2012.

  1. fz1

    Thread Starter New Member

    Dec 20, 2012
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    Long time reader, first time poster.
    First off, I'm not real keen on electronics beyond knowing what capacitors, resistors, and diodes are for, but I love to tinker and learn.
    My hope is that someone is willing to design a circuit and provide parts list so I can build this. (Practice for you?)

    This circuit should activate with the introduction of light, power a small DC motor for 5-10 minutes, and then shut off the motor. The motor should stay running for the full 5-10 minute cycle, even if the light is removed.
    A push-button activation would be nice, but not necessary.

    The power supply for this is 12vDC, 1A from a AC transformer.
    The motor is 12V and draws 500mA (don't have specs for the stall draw)

    Thanks in advance for any help. If you need any other information, just ask.

    Thanks!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Well, what you want is called a monostable multivibrator, or one-shot, and the 555 timer IC is usually used. Ten minutes is a bit long for the 555, so you may need to additionally use a divider or some other way to get a timing period that long.

    You should have no problem finding a 555-based photo alarm circuit to start with.
     
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  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    What tools and skills do you have?
     
  4. fz1

    Thread Starter New Member

    Dec 20, 2012
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    I have soldering irons, multi-meters, helping hands, wire-wrapping and tools, perf boards (not at the level of making my own PCB's yet...not sure what else I might need.

    As far as skills, I have steady hands, pretty quick to understand concepts. ???what else???
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Do you have a solderless breadboard and a 12 VDC power supply to use it with? And are you wanting to experiment and learn, or just build and use?
     
  6. fz1

    Thread Starter New Member

    Dec 20, 2012
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    I don't have a breadboard yet, never used one, but I'm definitely going to buy one before starting this project. I see how handy they are. I do have 12v power supply.

    I do have a particular application for this, but also want to learn how these timer circuits work. There are a couple of other applications for them (one being a soap dispenser so my kids don't use half a bottle of detergent for a load of dishes!)
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    OK, as WayneH said, the NE555 timer is the standard IC to use in an application like this, but using it for longer periods decreases the accuracy of the circuit. What is the shortest period of time that you really need the motor to run, and how accurate does that time period have to be?

    Attached is a circuit using a NE555 as a monostable multivibrator, also called a one-shot. I think trying a version of this might suffice. Also attached is a photo of how the circuit could be laid out on a solderless breadboard.

    You would need to breadboard this, and test the timing. Then, you could add a light dependent resistor or a phototransistor to provide the trigger. Then, you would add a MOSFET to power the motor.

    All that being said, there is probably a ready made timer or a kit to do what you want, which would turn out to be easier and cheaper.
     
    Last edited: Dec 21, 2012
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  8. fz1

    Thread Starter New Member

    Dec 20, 2012
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    Awesome, thanks! That gives me a great starting point. You're right, Icould brobably buy something to do this work, but if you're like me, you want to know how it works. Also, Ilike to know that I could build gadgets in a zombie apocolypse.

    Thanks again, I'll let you know how it turns out.
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You are welcome. Here are some additional facts about the circuit I posted.

    The combination of R1 and C1 is what determines how long the output (pin 3) stays high after the button is pushed. With the components shown, it stays high for 10 seconds. One of the nice things about using a 910k resistor for R1 is that the value in microfarads is approximately equal to the on time in seconds, i.e., 10 μF yields 10 seconds, and 330 μF would yield approximately 330 seconds, or 5.5 minutes, which is close to what you want.

    In addition, the combination of R4 and C1 prevents multiple button pushes from having any effect. Once SW1 is pushed, the output goes high for 10 seconds no matter how many times SW1 is pushed during that 10 seconds. And even if SW1 is held down longer than 10 seconds, the output goes low after 10 seconds.

    I am going to experiment with an LDR and a phototransistor to see what is needed to replace SW1 with a light sensitive subcircuit. I'll post what I find. I already have a MOSFET circuit to put on the output to control a motor, so we are getting there.
     
    Last edited: Dec 21, 2012
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  10. fz1

    Thread Starter New Member

    Dec 20, 2012
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    Again, thanks!
    The 330 second time will probably be fine. Sorry I didn't answer your question about the time accuracy before. I can adjust the final drive on the motor's gearbox to increase or decrease the speed to get the job done, and then fine-tune the time with different resistors.
    I just ordered some 555 timers, and an assortment of resistors and capacitors, the motor, and a few other parts to get started on this project. It will be a little while before I can start. I got the parts dirt cheap, but the downside is that they're mostly coming from China, Hong Kong, and Korea, so it will take a while.
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Here's the circuit using an LDR as a light sensor. It works well at 10 seconds, but you should probably try it with a 330 μF cap to see if the timer works for 5 1/2 minutes.

    It shouldn't be too hard to add a MOSFET to run the motor.
     
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