Auto Dark Detection Lamp

Hello, for the first "instructable" I wanna build a LED lamp or flashlight which will turn on at some points of darkness.

Here is the list of what you need!

1- Some Smile. :-)

2- BC547 Transistors(x2)

3- 1N40001 or 1N40007 Diod (x1)

4- LDR (Light Dependent Resistor) (x1)

5- Some Resistors (1k and 0.5k) or 10k Potentiometer(The More, The Better) for making it more accurate.

6- 5 Volts Relay(x1)

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Step 1: The Circuit

Here is the circuit. Try wiring it first on Bread Board, then solder it on another boards to sure you debug your stuff. For checking it just put your finger on LDR, your relay must connect or disconnect with a click sound. if you found anything OK but no click from relay, change the 330 ohms to a bigger resistor, I used 2k and was good.

Step 2: Share Yours

The last part is the result. Pictures showing mine. the last one is in darkness. These LEDs found from an old flashlight which my school gave it to me as a prize six years ago, but anyway, that's it!

A Persian article is available on my website.

Happy Making!

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    7 Discussions


    4 years ago on Introduction


    Why not run it off just the 12V battery? A second battery is just an annoyance.

    Making R1=47k and connecting the LDR from Q2 base to emitter (replacing Q1), you can then leave out B1, Q1 and R2 and feed R1, D1-cathode and the relay from +12V (drawing only around 250µA in standby).

    2 replies

    Reply 4 years ago on Introduction

    Thanks Omnivent, you're right.

    I think this is what you're talking about.

    Thanks for your comment.

    ldr omnivent.png

    Reply 4 years ago on Introduction

    Yes exactly :)

    Remember to use a 12V relay if you use this. Depending on your LDR, it may help to use a resistor of, say 22kOhm, in series with a trimmer potentiometer of 50..100kOhm, if you want to be able to adjust the turn-on point.

    Have a nice day.