how does light sensors in lamps work? like those being used in street lamps?

Question

it turns the lamps on automatically when it is dark.. how does its mechanism work?

Answer 1

Some street lights do not have photocells, but rather are turned on at a certain time at night, controlled from a center. It may take them 5 minutes to turn on. If all street lights seem to turn on within one minute, suspect that they are controlled centrally rather than by light detection at the lamp.

Some street lights seem to have a light sensor (photocell). Close to my university there is one street that had an always-on street light for a while. It was probably due to defective light sensor.

Wikipedia says modern light "may" have a sensor:

http://en.wikipedia.org/wiki/Street_light

Street lights are High-Intensity Discharge (HID) lamps:
http://en.wikipedia.org/wiki/High-intensity_discharge_lamp

As I mentioned, they may take 5 minutes to turn on. But once they are on, they are the most efficient type of light out there. Quite bright, and they often have a nice white daylight color (rather than yellowish/reddish incandescent light bulb color).

(HID lights are also used in bluish colors for those bright car headlights)

Look at some security lights for buildings. If they turn on at night, and look quite bright/white compared to incandescent (standard) home light bulbs, they are probably the same type as street lights (HID), and they definitely are using a photocell (also called photoresistor, commonly a CdS cell = Cadmium Sulfide cell).

Photocells can be quite small. I have built many simple night-light circuits with them. They look like this (Wikipedia shows an old model for some reason):

http://en.wikipedia.org/wiki/Cds_cell
http://edge.cs.drexel.edu/assemblies/tests/assemblies-200209/cds-cell-sensor.html

Essentially, when there is light, they show less resistance to electricity. When it is dark, they resist more to electricity. In a circuit, it's not hard to say "if CdS passes electricity, supply no electricity to lamp".

Photocells can be quite small:

http://images.google.com/imgres?imgurl=http://www.solarbotics.com/assets/images/cds/cds_pl.jpg&imgrefurl=http://www.solarbotics.com/products/cds/&h=450&w=600&sz=22&hl=en&start=8&um=1&tbnid=jVxWo5bFo1vvYM:&tbnh=101&tbnw=135&prev=


Wikipedia page for "photoresistor"s also has a close-up of "the internal components of photoelectric control of a typical American streetlight":

http://en.wikipedia.org/wiki/Image:Streetlight_control.jpg

It looks like the old buzzer doorbell inside. This is quite probably a "relay" that uses a wire wrapped around some metal, which would get magnetized when electricity (current) passes, and will pull a middle metal piece to close or open a circuit:

http://home.howstuffworks.com/doorbell2.htm
http://home.howstuffworks.com/relay.htm

In dark, photocell won't allow electricity to pass through, magnet will not hold anymore, and a spring will pull the relay switch away from the magnet, which will then connect two metal pieces that make the light circuit complete, and the street light will (slowly) come on.

Answer 2

Street Light Sensors

Answer 3

Photocells are semiconductors just like solar cells. They are used as part of a switch but its a little bit more than that. When light increases on the cell, the resistance changes. Its like a switch in your house. When you move it up (on) the resistance goes to zero and completes the circuit. Down (off) the resistance goes to towards infinity and opens the circuit. Get a meter that measures resistance (Ohms) and hook it up to a photo cell. You will see something cool. Its not used for the direct switch for the fairly high voltage of street lights or even the security light on your house. A relay or chip monitors the resistance and switches a larger contactor for the lights.
Get some photo cells from Radio Shack or remove one from an old secuity light. Have fun, work safe.

Answer 4

A sensor which turns the lamp on as it gets to a certain light fade - that's why streetlamps under trees come on earlier. And it costs circa 5p to run a street lamp per night.

Answer 5

I not sure I can explain this.
It use a Photoelectric cell. When light hits it then it produce electricity to act as a switch and turn off the light. This why in the day, when there a thunder storm or heavy cloud cover they will still come on as there not enough light to switch it off.