Buzz
In the Flintstones, a little bird inside the streetlight turns it on every evening before going to bed. In modern streetlights, a small circuit takes over, turning the light on when the ambient light drops below a certain level.
One common light-sensitive component is the cadmium sulfide photo-resistor, or CdS cell. This component alters its resistance based on the amount of light that strikes it. When it is exposed to bright light, it has minimal resistance, allowing electricity to pass easily. In darkness, its resistance increases, making it harder for electricity to flow. In a basic circuit, a CdS cell can be connected to a relay (refer to How Relays Work), where ample light will activate the electromagnet, and dim light will not. However, the CdS cell often doesn't generate enough current on its own to trigger the relay, so a transistor is typically added to amplify the current flowing through the CdS cell. A typical setup would look like this:
- Base
- Emitter
- Collector
If a small current flows from the emitter to the base, a larger current can then flow from the emitter to the collector. Essentially, grounding the base activates the 'switch' (the path from emitter to collector) in the diagram. So, when light hits the photocell, it activates the transistor, which powers the relay's electromagnet, turning the light off. In darkness, the photocell has high resistance, preventing current from reaching the base, keeping the relay inactive, and allowing the light to remain on.
In an actual streetlight, the circuit might be slightly more complex, but not by much. It still uses the CdS cell, the transistor, and the relay, though multiple transistors may be needed depending on the relay size. It's essentially a very straightforward circuit!
Here are three helpful links:
- CdS Photo Cell
- Tally light indicator
