See how to make an LED light up in the dark - by just using an LDR and a resistor !!
NO IC chips, no arduino, no NPN or PNP transistors!

Simplest and best (free) video & circuit!

Step 1: Parts & Arrangement:

Parts:

Just four parts: An LED, An LDR (Light Dependent Resistor), a resistor 100 kilo ohm and a DC battery (Any -- 3 volt, 6 volt, 9 volt or a 12 volt one)

Arrangement:

1.Just connect any leg of LDR to any leg of LED. You can increase the light sensitivity by connecting two or more LDRs "in parallel" (like i have).

2.a) For dark detecting sensor (lights up when dark):

Choose a resistor, from 10 kilo ohms to 150 kilo ohms, in decreasing order of sensitivity to light.
Connect the resistor to the positive leg of the LED and the DC battery.
Any voltage DC battery will do.

2.b) For light detecting sensor:

Connect the resistor to the negative of the LED and the DC battery.

Done !!

http://www.youtube.com/watch?v=dh01O22A0Xg&feature=youtu.be

Advantages:

1. Simple sensor circuit - actually no circuit at all!
2. No soldering required - relief for those allergic to it ! (like me!)
3. I have just pressed the circuit between a 9 volt battery and its cap. Even the battery cap wires were not used!!

Genius! (Idea, of course!) ;-)

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