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Thread: Reverse Photocell

  1. #1
    Join Date
    Apr 2012
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    Fort Lauderdale, FL
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    Default Reverse Photocell

    I know photocells (photoeye) close the circuit at night to turn lights on at night when the sun goes down then opens the circuit during the day. What I am really looking for is a photocell that closes the circuit during the day (turns pump on) and opens the circuit at night (pump off). This is for a day time use only solar pumping application. Is there anything like this out there in 24VDC? If not is there a device similar? Time clock would be easy, but the days become longer-shorter over time.

    Thanks in advance,

    Scott

  2. #2

    Default Re: Reverse Photocell

    Yes: photovoltaic cell.
    Since they produce electricity when exposed to light it is quite possible to adapt one to switch a relay at the appropriate illumination level so that the pump runs.

    Of course you can cut out the "middleman" and have a pump powered directly from solar. I'm not sure what size pump your looking at or what the application is or the budget, but it is technically possible.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  3. #3
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    Default Re: Reverse Photocell

    Carrib- thanks. The issue is that the pump will not gradually start as the sun comes up. Some electronics in the pump that will not let it start. So, I need a way to "switch" the pump on when the sun is a bit higher in the sky and turn it off late afternoon.

  4. #4
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    Default Re: Reverse Photocell

    Actually, a "photocell" itself conducts in sunlight and turns off in dark... So there is already a relay (or electronic switch) that is there to reverse the sense of the photo cell.

    How accurate are you looking for this thing to work? One easy way would be to get a small solar panel and simply drive a small 6 volt or 12 volt relay--And wire up the relay to "turn off" when energized and "turn on" when denergized. That would not even "waste" any electrical power...

    http://www.radioshack.com/family/ind...goryId=2032277

    Even a larger 40 amp contact relay would work with a few watt "12 volt" solar panel.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  5. #5
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    Default Re: Reverse Photocell

    Regarding the pump--You might want a "hiccup" circuit. When there is enough voltage, the relay "turns on"... If there is not enough current from the solar panel and the output voltage "collapses", then the relay opens for a short bit of time and closes again (an RC timing circuit/voltage comparator would work if you are up the the challenge).

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  6. #6
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    Default Re: Reverse Photocell

    I think I have it figured out. So, I need a designated panel only for the relay so when the motor is activated it doesn't consume the power necessary to keep the relay powered avoiding on/off/on/off until there is enough sunlight to energize both the relay and motor.

  7. #7

    Default Re: Reverse Photocell

    On the outside chance that this is to be powered by an off-grid system ...
    Some charge controllers have AUX functions that can be programmed to switch a relay on/off in accordance to some programmed state (such as batteries in Absorb).

    As Bill said, a regular photo electric eye can be used to turn a relay on or off with the presence of light; it's simply a change in circuit or relay state (use normally open contact instead of normally closed).

    Not difficult to adapt things like Voltage controlled switches to operate with PV's, if that would work.

    I think we need some more details as to the power source, Voltage levels, reasons for desired timing of operation, et cetera.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  8. #8
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    Default Re: Reverse Photocell

    Okay- 24VDC Air pump and cooling fan. Pump runs about 3A @24VDC, Fan runs .6A @ 24VDC. The pump and and fan are located in a cabinet underneath the panels. The panels are (2) 235 Watt panels 24V nominal connected in series pole mounted. The positive and negative from the panels connects to a terminal block inside the cabinet that distributes the voltage to the fan and compressor.

    The pump will not start on its own as the sun gradually rises. The cooling fan does. I need something that can sense the sunlight when the sun is at a decent height (7:30am today's time) then close the circuit to power the pump and fan.

  9. #9
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    Default Re: Reverse Photocell

    Simplest would be a solar panel and 12 volt (or whatever matches) relay... Or, look at a linear current booster--That is what is usually required to start a brushed motor (changes "high voltage"/"low current" into low voltage/high current needed by the motor to develop enough torque to begin turning.

    7 amp Linear Current Booster/Pump Driver
    (and they have others too)

    Not sure if it would start your compressor or not (kind of expensive to experiment with--but is supposed to support more hours of operation per day--albeit at a lower pump RPM.

    Obviously, time of day is not enough--clouds blowing over can cause similar issues. A relay with a capacitor and resistor to slow down its actions (assuming solar array voltaeg is low when pump is not running) so that it take a few seconds to a few minutes to "hiccup" would be a good way to start--More "accurate" than a solar array (probably) or photo cell (it is the stalled pump and collapsed Varray that is the issue, not always just the amount of sun).

    When there is "enough sun" to turn on the relay (panel voltage), then if the array voltage collapses (discharging the timing circuit), the relay opens. Voltage rises again and "charges the timing circuit" and relay turns a few minutes later to repeat the cycle until the pump "stays on".

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  10. #10
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    Default Re: Reverse Photocell

    Tried a pump driver- no dice. Once the pump is started the pump driver works great but doesn't help with the starting issue.

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