circuit design

Discussion in 'General Electronics Chat' started by Roy Ford, Aug 17, 2012.

  1. Roy Ford

    Thread Starter New Member

    Aug 17, 2012
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    I have a device that uses a DC motor operated by two solar cells in series. The device has a switch on it to run the motor from a AA battery when sunlight is not strong enough to operate the motor. There is a 5 ohm resistor in series with the battery to lower the voltage on the motor.


    Rather than manually switch from solar to battery power and back again, I would like a circuit that would sense the drop in voltage from the solar cells and automatically connect the battery then restore solar operation when the sun is bright again.


    I would also like to include an ambient light sensor circuit to disconnect the battery at night because the device only needs to operate in daylight hours.


    Please suggest circuits to meet the above objectives.
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
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    To do the first bit (sense the drop in voltage from the solar cells and automatically connect the battery then restore solar operation when the sun is bright again) all you need is two diodes. The diodes drop the voltage a bit though, so best to use Sckottky types that have a low forward voltage drop.

    See my simple circuit attached.

    For your ambient light sensing to disconnect the battery at night, you need a more complicated circuit an example of which I have also sketched (only a sketch, mind - more detail is required). Here, I am assuming the solar cells will still produce some voltage when shaded but not enough to run the motor. When dark, the voltage falls to a much lower level.

    So two comparators are used. One to compare the solar voltage with the battery voltage, The other to sense the "dark" voltage. To switch the current, MOSFETs are used because they have low voltage drop and zero drive current.

    That's it - going to do some work now!
     
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  3. Ron H

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    He said it runs on a single AA cell. This makes the problem a lot tougher.
     
  4. JDT

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    Feb 12, 2009
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    So he did! Never was very good at reading the question. Makes it tough because not sure if you can get MOSFETs that will switch on with only about 1 - 1.2V on the gate. Comparators and logic possible?

    You could power the whole thing from a micropower step-up DC-DC converter. This one runs from 1.0 to 1.6V.

    http://www.linear.com/product/LTC3525
     
  5. Ron H

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    Roy, what is the motor current when powered by the AA?
     
  6. Roy Ford

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    Aug 17, 2012
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    First, thank you both for your replies.
    I measured the voltage across the resistor and calculated the current to be 20ma. (.1 volts, 5 ohms)
     
  7. Ron H

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    That must be a tiny motor!
     
  8. Roy Ford

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    Aug 17, 2012
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    Yes it is very small. I had no problem running it with a single AA or the solar cells through the 1N5819s suggested by JDT.
     
  9. Ron H

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    Here is a circuit design I started on Friday night, but put on hold until you told us how low the motor current is.
    The comparators are surface mount, but AFAIK you have no choice when you want ones that will work at around 1V.
     
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  10. Roy Ford

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    Aug 17, 2012
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    This is for Ron H.
    I am very impressed and thank you but last night I was wondering if rather than comparing the voltages, a transistor could be put in series with the battery or motor. Could it be biased on with current from the ambient light sensor?
     
    Last edited: Aug 21, 2012
  11. Roy Ford

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    Aug 17, 2012
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    That is a very impressive circuit and I thank you. I wonder if rather than comparing voltages, a transistor in series with the battery (or motor) could be used as a switch. Could it be biased on with daylight current from an ambient light sensor?
     
  12. Ron H

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    That could work, if you can provide specs of a sensor you would use.
     
  13. Roy Ford

    Thread Starter New Member

    Aug 17, 2012
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    I could not find a light dependent resistor or ambient sensor locally but I did find a photo transister which might work. It is designed for IR but spills over into the visible. It is a Fairchild L14G1 (spec sheet attached in pdf format).

    Today is my golf day so I will try it tomorrow. I'm open to any suggestions you have and thank you again for your very knowledgeable assistance.
     
  14. Roy Ford

    Thread Starter New Member

    Aug 17, 2012
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    I tried to run the motor directly with an L14G1 photo transistor in series with it. No matter how much light I used it would not run. I then used the L14G1 to turn on an NTE123AP transistor in series with the motor. This worked very well.


    The only remaining problem I have is that the Schottky diode works well with battery power but it has a little too much foreword voltage drop for the solar cells I'm stuck with. The motor runs a little too slow. Do you have a suggestion for this problem?
     
  15. Ron H

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    You should lose less than 100mV across the two transistors with this circuit.

    EDIT: Another idea: move the 5Ω resistor, as shown in the 2nd schematic, or get rid of it altogether, as in the 3rd schematic (which is probably the best option).
     
    Last edited: Aug 24, 2012
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  16. Roy Ford

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    Aug 17, 2012
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    I used the third option you showed and the circuit works very well. Thank you again for your professional advice.
     
  17. Ron H

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    Professionals get paid. This is gratis.:D
    I'm glad it worked for you.
     
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