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I've got a small nightlight in my hallway that has a photocell on top so it only turns on when it's sufficiently dark out. Sometimes the light will be off and a shadow will cover the nightlight as someone walks by triggering the light to turn on. After the shadow passes by and is no longer on the nightlight, the nightlight will stay on but only under certain lighting conditions such as a dawn, dusk or a particularly cloudy day.

This isn't a high-tech nightlight so I'm assuming that there's a simple electrical circuit that's causing the behavior. Can someone explain what that circuit is and how it works?

(I think this is probably something common and simple but if not, I can open the thing up and look inside for more details)

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    \$\begingroup\$ Look up "hysteresis". \$\endgroup\$ – Olin Lathrop Mar 16 '12 at 21:03
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The "sometimes stays on" action is what is called "hysteresis". This is designed to stop the light flashing on and off when it is on the verge of night or day.
Once it is on it adjusts its trigger level point such that it needs to get somewhat lighter befre it will go off again. Once it is off it adjusts its trigger level point such that it needs to get somewhat darker lighter before it will go on again.

When you adjust the conditions to be "just right" you dro it into a "twiligh zone" (literally in this case" where it needs more dark or more light to cause action.

Here is a typical nightlight control circuit with hysteresis.
The ORP12 at R8 is a light variable resistance.
R7 is a sensitivity control.
As light level varies the voltage at the R8/R1 boundary varies. The opamp compares this voltage with the voltage from R2 / R3 divider and switches when the ORP8 controlled voltage transitions past that voltage i either direction.

Hysteresis is meant to be added by voltage from the output of the opamp, via R4, back to its input. The interesting this is that they have connected it to th wrong plac !!! :). R4 should connect to opamp pin 3 = non inverting input and not to pin 2 = inverting input. When it is connected to the + input, anything that happens at the output is fed back as positive feedback and enhances the action. This exact circuit diagram (same drawing without change) can be found in many locations on the internet with some mentioning hysteresis - which it has not got.

So - this is an example of the sort of cct that may be in your light - but they sound like they have connected it correctly in your case.

enter image description here


This one does it correctly but its action is slightly harder to follow.
Circuit from here with an OK discussion. He specifically notes R5 R6 & R7 as being involved in setting the amount of hysteresis, which they are. Bigger version of diagram here

enter image description here

Circuitry in box at top left varies the "attack & decay" rates of the LDR (ligh dependant resistor) effect - see his text for why.

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There must be some comparator with hysteresis, preventing the light from turning on and off when the light is at an intermediate level.

But, when the light is in this middle state and somebody covers it, it remains on instead of turning off. A solution could be to set it for turning on with a lower amount of light, so then a simple shadow or cloudy day won't turn it on.

There should also be a screw that allows you to regulate this threshold. Can you help us identifying the type of light\sensor?

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