There are several types: The main two, in layman’s terms, are a light dependant resistor – LDR  – the resistance of which increases with the amount/type of light falling on it; and conversely one the resistance of which decreases with the amount of light falling upon it.

One of the main uses for these devices is in switching on street lighting. Rumour has it that a timer mechanism was originally used for the purpose, before the LDR’s invention. That’s maybe a sound idea if the light happens to be situated at or near the Equator; where the seasonal variations between night and day are minimal. However when you start getting as far away from the Equator, even as “close” as Southern England – Where night can come as early as 3:50PM in the depths of winter; yet as late as 10:40PM in the heights of summer: The timer would require resetting at least weekly. I honestly can’t envisage electricians running round the country resetting street lamp timers every week. Having said that though; there were people who went around the major English cities in the evenings and mornings lighting and extinguishing gas street lamps in the Victorian era.

Thankfully technology has moved on a bit since then.

See the Wikipedia article hyperlinked above from the letters LDR for a full description of the component.

You may notice that, on the circuit below, I use a non-standard symbol for an LDR which is completely different from the standard symbol. The one that I use (A zig-zag line with two arrows pointing towards it.) is a shorthand circuit-diagrammatical representation. It’s the old symbol for a resistor with the two arrows indicating that its value is dependant upon the amount/type of light falling upon it. Similarly with the fixed-resistors in the circuit; a zig-zagged line without the arrows. The symbol used as standard for a resistor is a rectangular box with leads either end. i find this too much hassle and too time-consuming to draw; therefore I resort to my shorthand: I understand it even if not all other people do.

So how does the device work in-circuit? The circuit-diagram below shows a very basic circuit incorporating 2 fixed resistors, a transistor, and a light-emitting diode to display the output. The LDR is an inverse-effect type. That is to say its resistance decreases as more light is shone on its surface.

The LDR , along with R1, acts as a potential-divider, biasing the base of Q1. As the light shining upon the LDR gets brighter, so its resistance drops, and thus the voltage at Q1’s base drops via R3. (See “Ohms Law and the Potential Divider”. Also see “Base Voltage”) When the base voltage drops below 0.7 Volts the transistor switches off and the LED goes out.

The function of the circuit can be reversed by replacing R1 with the LDR and vice-versa: In that case as the light shining upon the LDR increases, so its resistance drops and the voltage at Q1’s base rises via R3. When the base voltage rises above 0.7 Volts the transistor switches on and the LED lights up.

This circuit does work; in fact I memorised it from my early self-tuition in electronics, as well as from college. (Where I gained C&G 300, 301: Analogue and digital electronics certificates. ( I also retook a Maths exam as I had a cold on the day of my original exam twenty-something years ago and didn’t do as well as I’d have liked. – I passed; but my grade wasn’t as good as I’d hoped for, and I knew that I could get a better result.))

You might like to experiment with different values of resistor for R1, although a kilohm is probably the lowest value you should use in this case. You could also try replacing R1 with a 1K resistor and a 10K linear potentiometer connected in series. I’ll leave you to experiment.

There are a few and there will be more posts on the subject of basic practical electronics in this blog. if you’re interested in the subject then do look further into the content. It’s not all listed at time of writing so Google is your friend; use it. (Or Windows Live Search, Ask, Yahoo… whatever: I’m not biased or sponsored by Google. I prefer to use Google myself as I find the GUI simple to use and the listings useful. Your opinions may vary.)

If you build the circuit and experiment with it please do tell me your results. I’ll be interested to know.