Photoresistor

A photoresistor, also called an LDR, changes its resistance with light. In the dark it resists current more strongly; in brighter light its resistance falls. It senses light rather than making it.

It has two leads and no + or −: either direction works. The inward arrows in its symbol mean light arriving at the component. The circuit still needs a power source.

At dusk, a photoresistor helps the street-lamp control circuit notice the fading daylight. The control circuit switches on the lamps, so Sparkie can find the workshop.