Simple 12v DC automatic switch help needed...

Discussion in 'The Projects Forum' started by bondorust, Jul 3, 2011.

  1. bondorust

    Thread Starter New Member

    Jul 3, 2011
    3
    0
    I'm basically looking for the cheapest photocell or replacement since I will be making many of these.

    I will be using a 12v 10watt solar panel with a 12v battery and 12v LED lights, so this should be simple.

    I want the lights to run during the night so can I make a switch with a diode or something that senses the loss of charging from the panel and closes the circuit?

    I've looked everywhere to find how to do this but can't find the information. I'm needing a list of materials, where to get them, and maybe a schematic of how to do it. Any help is greatly appreciated!
     
  2. simo_x

    Member

    Dec 23, 2010
    200
    6
    So you want turn on lights only when the night comes, rights?
     
  3. bondorust

    Thread Starter New Member

    Jul 3, 2011
    3
    0
    Correct. Turn on when sun goes away. Then turn off when the sun comes back.
     
  4. simo_x

    Member

    Dec 23, 2010
    200
    6
    Ok. I would use an operational amplifier to work as a comparator, possible with single power supply.. LM358 would be fine.
    In this case, you have to connect the solar cell output to the inverting input,

    As I know, depending on the light incidence on the cell, you can have less than 12V output. Can you confirm that? If solar panel behave this way, you should establish a threshold voltage to make the operational amplifier to switch the output from 0V to approximately Vcc when output voltage of the solar panel will be greater than threshold voltage reference. With this reference you choose how much light is needed to turn off the LEDs.

    Threshold voltage reference should be connected to the non inverting input.

    Current source for LM358 is typically between 20 & 40mA, just enough to drive some LED or NPN transistor.. You should also tell us how much current will consume your LEDs (each one).. Range from 5mA to 10mA I think is enough for each one..
     
    Last edited: Jul 3, 2011
  5. #12

    Expert

    Nov 30, 2010
    16,281
    6,792
    653-G5LE-1-VDDC12 is the part number at www.mouser.com for a 12 volt relay. $2.56 + shipping

    It has a "must operate" voltage of 9 volts and will stay connected until solar panel input falls to 1.2 volts. You can tune the switching levels by altering these settings with resistors. That's a simple approch. Not deadly accurate, but simple.

    datasheet attached.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Wow, if this was a little reed relay I wouldn't be surprised. Since it can handle 10A though... a 1.2V holding voltage is very impressive! ;)
     
  7. bondorust

    Thread Starter New Member

    Jul 3, 2011
    3
    0
    The LED's will be 180mA total so it sounds like the relay is exactly what I'm looking for. But will this relay open the circuit when it falls below 1.2v or will it close it?
     
  8. #12

    Expert

    Nov 30, 2010
    16,281
    6,792
    The relay stops having sufficient magnetism when the voltage falls, but since it's a type C contact arrangement (spdt), you can wire it up either way.

    The 1.2 volt spec is on the second page, first chart, listed as "must release". It probably won't hold all the way down to 1.2 volts. In other words, I'm correcting myself, but it's a good spec that indicates a 10% "must release" point. Most relays can't even get near that point.
     
    Last edited: Jul 3, 2011
  9. simo_x

    Member

    Dec 23, 2010
    200
    6
    Depending on how much transistors you want to use & how many LEDs for each one (for maximum power dissipation), I think that maybe a relay is not necessary for that load, and you should not hear the contact noise (I don't like relays). If you use the relay, don't forget to use a protection diode.
     
  10. #12

    Expert

    Nov 30, 2010
    16,281
    6,792
    I didn't suggest the relay as "necessary for that load". I suggested it because it can be used as the logic stage. It will require fiddling with resistors to keep it from activating at 9 volts, and that will alter the release point to higher than 1.2 volts, but the stimulus (the sun) is very slow and very dependable. You can pretty much bet that when the solar panels drop below 5 or 6 volts, the sun is going down.
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Here's what the circuit would look like using the relay that #12 linked to. Admittedly, I fudged a few things, like max solar panel output voltage. I show it as 0 to 15V. If your panel puts out less than this you won't get these results.

    I couldn't find a definitive DC contacts rating for that model so you may have to add RC snubbers across the contacts.

    Edit: D2 can be replaced by a Schottky diode that has a lower voltage drop.
     
    #12 likes this.
  12. #12

    Expert

    Nov 30, 2010
    16,281
    6,792
    Excellent drawing! D2 eliminates fiddling with resistors to adjust the "activate" voltage.

    The solar panel is rated at .8333 amps and the relay contacts are rated at 10 amps. I think that is plenty of safety factor and should not need snubber circuits to keep the contacts happy.

    and a gentle reminder...the solar panels must provide a higher voltage than the label on the battery in order to force current backwards through the battery. You didn't say exactly what kind of battery it is, but the circuit Cdrive made will give it every chance to be charged by the solar panels.
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    It occurred to me that hysteresis plots aren't easy to follow so I split it up into two separate plots. I think it makes things much easier to see and comprehend.
     
  14. mbxs3

    Active Member

    Oct 14, 2009
    141
    3
    Not sure what your exact application is, but you can buy solar panel "walk-way" lights from Lowes for less than that relay will cost you. I understand the learning aspect of what you are trying to do, but if you plan on "making a lot" of whatever it is you are making, you might want to see that it hasn't already been made, and available for less than the price of parts, time, and effort that you will spend.
     
  15. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    That relay is $2.56 + shipping.
     
  16. mbxs3

    Active Member

    Oct 14, 2009
    141
    3
    I read your post CD. The lights I am talking about at Lowe's are less than $4 a piece. Completely assembled, all you have to do is stick them in your yard and let the sun do it's thing.

    Like I said, I am not sure of the OP's application. So my little tidbit of info may be irrelevant.
     
  17. castley

    Member

    Jul 17, 2011
    31
    0
    A photo-transistor can be used to connect the lights to the battery, when the daylight level drops. This photo-transistor would be off in darkness and be connected
    to another base emitter junction of another transistor to enable it to feed the lights.
     
Loading...