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How can I make a light of a sensor stay on?

Hey there! I’ve been wondering how to make a light of a sensor stay on after activated. For example, I have this light sensor which turns on a LED when there’s no light over the photo resistor, but how can I make that LED remain on even after light comes back over the photo resistor?

Same thing for proximity sensors and IR sensor.

10 Answers

  • 3 weeks ago

    Use a latching relay. and a reset switch to shut it off.

  • Anonymous
    1 month ago

    You want a latching circuit.

    Then you must include a reset switch.

    What is the point of your circuit?

  • Mr. P
    Lv 7
    1 month ago

    You have to decide what you want it to do. After that you can make up a truth table.

    Do you want it to stay on until cancelled? Or turned off? Do you want it to stay on for a set time then turn off?

    If it is to stay on until you do something - then use a latch. Either transistor or logic based or just use a latching relay.

    If it is a time delay you want then look at circuits like the car interior light time delay circuit or use a light -off delay timer or relay.

    It's basically a capacitor that holds a charge and will slowly discharge until it can no longer keep a transistor turned on.

    You will learn to copy bits of other circuits to make your own ;) That is what data sheets are all about.

  • 1 month ago

    Put black tape over the sensor

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  • 1 month ago

    You could use a relay to turn on the LED. When power goes to the LED, that same power directly or indirectly goes to the relay coil to keep it on. Or you could use an SCR to turn on the LED as an SCR will stay on once triggered until the current through it drops below a certain threshold .

  • ?
    Lv 6
    1 month ago

    Most motion detectors have a built-in override:

    Normally the switch is kept on all the time. ...

    If you turn the switch off and on within a second or so, the light will stay on, and this overrides motion detection.

    To go back to normal operation, turn the switch off and wait ~10 seconds, then turn back on.

  • ?
    Lv 7
    1 month ago

    Instead of turning on the LED you turn on a latching relay and use the relay to turn on the LED. You would also need to add a momentary switch to reset the relay when the LED is on in the light and you want to reinitialise the circuit. 

    You could do this without a mechanical relay and just using electronic components but the details of how best to do it would depend on things like the power supplies available, the space available, if it is just a one-off etc.

  • oyubir
    Lv 6
    1 month ago

    If I understand correctly, you want a light to automatically switch on when it is dark, and switch off when it is no longer dark. And you are wondering how to prevent the light to switch off as soon as it has switched on, since when the light is on, it is no longer dark anymore.


    1. Either you do what is takes so that your sensor does not receive the light from your lamp. This is what is done in street lamps, whose sensor are above the light (when they are not just triggered by a clock).

    2. You use a Schmitt trigger.

    So that when light intensity (I) is under a threshold (I₁), the light switch on. And then when intensity is over another threshold (I₂) it switch off. The whole point of Schmitt trigger, is that I₁≠I₂ (I₁<I₂).

    That would also work for things like street lamps, because the artificial light is far dimmer than the natural one. So even with light on, intensity is way lower than when the sun was up.

    So you have a margin to have I₂ bigger that I₁, bigger than intensity of your lamp (I₀), in fact bigger than I₁+intensity of lamp, but still lower than sunlight.

    Then, when sunlight is <I₁, lamp is switched on. Intensity is then just above I₁ + I₀ (sun was just above I₁. And you add your lamp light I₀). Which is not enough to trigger lights off, since I₁+I₀<I₂.

    Then the sun continues to set, until intensity is just I₀ (the one from your lamp), then the sun rises again, until Iₛ+I₀ reach I₂, and your light switch off. (And then intensity is just Iₛ, since the lamps are now off. But that is still more than I₁, so the light do not switch off again).

    3. If you have variable light, you don't really use your sensor to switch on or off the light.

    You use it in a PID control loop to control the intensity. If intensity is bigger than the target, you slightly lower the light. If it is above your target, you slightly increase it. Using a PID.

    You won't get "money in shower" effect (a monkey in a shower would burn itself, then decrease water temp and freeze itself, then increase water temp and burn itself again, ...) because PID are meant to avoid that.

  • ?
    Lv 7
    1 month ago

    get a mechanic to fix it

  • 1 month ago

    not sure what you want, perhaps a latching relay, or an electronic latch?

    If this is bout a commercial unit, you would have to dig into the guts and modify it, probably not easily possible.

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