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  1. #1
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    how to use a photocell

    Hello everyone. I have this project but I cannot figure out how to get the photocell to do what I need. Any suggestions will be greatly appreciated.

    CHALLENGE: Integrate a battery-operated consumer (retail) smoke detector into a proprietary wireless security system, as economically as possible.
    COMPONENTS: The first component is a 9V DC smoke detector WITH an ultra brilliant white escape light (LED) that turns on, along with the horn, during a smoke/fire alarm. The second component is a 3V DC battery-operated wireless transmitter, having a normally closed circuit (2.4V DC circulating), and featuring a 15” inch two-wire cable (usually connected to the leads of a magnetic contact). Finally, there is security console that receives an RF signal from the transmitter(s) and activates a security alarm.
    METHOD: Utilize a single photocell soldered to the two wire leads of the wireless transmitter and, then, clear silicon the photocell (facing inwards) to the clear cover of the escape light of the smoke detector. Cost: $2.50.
    PROBLEM: A photocell is a resistor that allows current to flow freely through it in the presence of light and restricts (blocks) current flow in the absence of light. Connected to my transmitter leads this would act as a Normally Open circuit in the absence of light--I need the photocell to do the opposite. In darkness, I need the 2.4 V DC passing through the wires of the wireless transmitter to loop (closed circuit) back to the transmitter. Then, when a smoke/fire alarm occurs, the escape light will light and the photocell will react by opening the circuit. The transmitter will sense the open and send a radio signal to the security console. The console will then react with a security alarm.
    QUESTION: How do I reverse the action of the photocell? I have a suspicion I need to use a 5V DC SPDT (single-pole, double-throw) relay, but I don’t know what to wire to the coil and what to wire to the circuit (and it seems like I would need another power source to switch the relay). Any help would be greatly appreciated! Thank you for your consideration.

    •   Alt4th December 2005, 05:27

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  2. #2
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    photocell circuit

    Have a look at thiese two circuits (see picture below): dark activated relay and light activated relay.
    In fact in your application you will not need a relay, but you can use open collector output from transistor and its emiter to "close transmitter contact" ..
    And, you will need extra power to supply photo-resistor - adjustable resistor circuit ..
    Regards,
    IanP


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    •   Alt4th December 2005, 05:59

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  3. #3
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    how to use photocell

    Hi Steve,

    Doesn't the manufacturer of the wireless system make a wireless smoke detector to go with the system? The problem with using a light sensor is if the battery on the smoke detector is weak will there be enough light to trigger the sensor? What if the light doesn't go off?

    IanP's idea is on the right track but with the open collector idea it won't work because you are looking to open a circuit NOT close a circuit.

    Perhaps what might work is a sensitive type relay soldered to the light circuit on the smoke detector with the normally closed contacts then connected to your transmitter.

    No matter how you do this, jury rigging something to a smoke detector to get a signal to trigger an alarm is never a good idea. In fact it's a terrible idea. You are risking your family's lives and your own for a few bucks. Get real and get out of this cheap state of mind and spend some money and do the job right!

    BobK


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    •   Alt4th December 2005, 18:10

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  4. #4
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    photocell normally closed

    IanP and BobK,

    Thank you very much for taking your time to assist me in resolving my dilema. I greatly appreciate your efforts on my behalf.
    I live in Mexico...where the per capita income is less than 1/6 of the US. Sometimes when you don't have much you try to do the best you can with what you have. The smoke detector modification is an attempt to add enhanced security. If the battery is low...well, that is an issue with all battery-operated devices...but better to have tried than to have done nothing at all. I don't want to open or modify the smoke detector internally so as not to jepordize its function. But if an external modification was possible, it may save my apartment from burning while I am not at home (because interior and exterior siren and dial out).
    I have already been able to open the smoke detector and wire the LED light leads to the coil of a 5V DC SPDT relay. Next, I solder the transmitter wire leads to the normally closed leads of the relay. When the smoke detector is activated and sends current to the LED...the relay switches to open and the transmitter sends an RF off to the console.
    But I would best like to use an external switch. The photocell is perfect if it was only normally closed instead of normally open (in the absence of light).
    I am sorry IanP but I could not see your pictures...as my electronics skills are not very good, the pictures would be very good indeed. Do you agree with BobK that your proposed circuit is a normally open. Well anyway...I cannot use another power source. Thank you very much anyway.
    And thank you very much BobK for your recommendations. Sorry but there is no smoke detector sold for the security system (Chinese).


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