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Geek Night Light


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Got all the materials for it, tried it out in many different ways (the right way also) and still didnt get it. I used the pictures from instructables as a guide. When I plug in my battery, my led BARELY lights. It lights about

1/10th of its maximum power. It lights very little and it does so all the time when the battery is plugged in. The photoresistor doesnt seem to be functioning because light or no light, the led is still on.

So..... need ur help.


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There is no certain way for me to check my wiring because I am doing it according to the pictures only. However, if you understand schematics, you can go to


Just that its a schematic with a twist to it. It includes an ON/OFF/AUTO switch.

If you understand it, it would be nice if you made like a small drawing in Paint which clearly portrays "what goes where". Thanks

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Here's another schematic... it has on/off/auto but to be honest I didn't use the best way to do it, I have a switch cutting off the negative terminal of the battery which is bad practice but hey, it's low voltage DC so it's no problem, it'll work... thought I'd post it anyways :)

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Well I did that schematic in Crocodile Clips Technology and I could've sworn there used to be an option to view the components as they look in real life but I can't find it in the version I have, it's been a long while since I used the older version (which I can't actually find at the moment) but if I find it I'll post a real-world view of it :)

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The four color codes refer to the resistance. a Brown Black Black Orange means that the colored bands around the resistors would be in this order: brown black black orange, and then most likely a gold or silver band, indicating tolerance. the color codes are simply the bands on the resistors. Hope that helped a bit

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PoyBoy labled all the parts it's messy but it works the little zappy lines are resisters. take a look at it .

By looking at your drawing it could be the amount of sodder you have on your wireing did it work before you soddered it up?

You should allways allways see if a config works on a bread board or twisted befor soddering , yes I found this out the hard way.

I can't tell if your larger resitor is yellow blue brown gold , or yellow black brown gold it's to dark for me , I think it should be brown (1) black (0) yellow(4) = 100k 10 + 0000 , yeah any way the next resistor ; I cant make out what you have , but Yelllow(4) vilot (7) red (1), will give you 470 ohms.

Now I'm not perfect with my resistor codes , so I may end up being correcter but I hope this helps.

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Here are some common schematic symbols and what they mean:


As for the resistor color codes, the first color is the first digit, the second number is the second digit, and the third number is the multiplier (how many "0s"). Any bands after that are tolerance...usually there is only one that is silver (+ or - 10%) or gold (+ or - 5%)...but there are others, and sometimes 2 tolerance bands...for our application, the tolerance bands are not important.


100K ohms = Brown(1) Black(0) Yellow((4)add 4 zeros) = 100,000

470 ohms = Yellow(4) Violet(7) Brown((1)add 1 zero) = 470

Here's a little resistor color code calculator:


I can't make out exactly what is wrong based on your photo...maybe different angles, and more photos?

As for the circuit, I'll wire one up tonight and see what happens...http://' target="_blank">

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Ah, I see the problem now. You've got your resistors switched around. Desolder the lead of the physically larger resistor that is on the middle pin of the transistor, desolder the lead of the physically smaller resistor that is connected to the LED, then switch them...


Since you only had a 470 ohm resistor instead of the 100K ohm (HUGE difference) on the base of the transistor, you were driving it into hard saturation...Most transistors can easily handle anytihng below 9V, but there still may have been enough current to damage the transistor. If it does not work after you switch the resistors, you may want to try swapping out for a new transistor. The LED was dim due to the 100K resistor blocking most of the current, so at 9v (really the LED will drop 2 or more volts, but its a close enough approximation) and 100K ohms, you're looking at about 90 uA (micro amps), and LEDs are bright at around 20mA-50mA (milliamps)...I'm not sure about the current-handling capabilities of the photocell either, you may have to swap it out as well. The rest of your circuit looks right.

Let me know how it goes..

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If the LED does not light when the photocell is completely in the dark, you need to start switching components out. The first thing I would switch would be the NPN Transistor. The Instrucables website says to use a 2n4401, but any NPN small-signal transistor should work (2N3904 is the most common part #).

Switch that out, and try again, if still no success, try switching the photocell out.

Radioshack normally has all of the required parts...if a local store doesn't have them, they will order them, or if you can order online:

Transistor - http://www.radioshack.com/product/index.js...rentPage=family

Photocell -


I like to get my LEDs from www.besthongkong.com, but I usually buy large quantities.

Notice Radio Shack makes you buy more than 1 quantity of the parts. The best reason for this is exactly what is happening to you now. Parts often fry, and it becomes handy to have a bunch around. Also, it wouldn't be worth the shipping to sell quantities of 1. I normally would never recomend radio shack to buy parts from due to their outrageous prices, but for a few quick components, they are usually the way to go.

Your circuit looks correct, so that leaves the possibility of faulty parts. Get switchin 'em, and let me know what happens. Remember not to short anything.

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Damn!.!.!.! Thanks for telling me to check it in the complete dark. I just went to my parents room-dark. And suddenly it got very bright. TADA!!! it lit. Thank you everyone who has tried to help me. If you have any good ideas for putting the setup somewhere else other than a bawls bottle or a light bulb, let me know of your CooL ideas.


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My first suggestion would be to NEVER hook an LED (or any Diode) directly up to a battery...If you had it on there, and it didn't burn out, then that is amazing, but you can always try using a smaller value resistor on the LED in the nightlight circuit. I would suggest a 200 Ohm, but no lower...Also, you could use more than 1 LED, but if you use more than say 3 of them, the current may become too much for the transistor, in which case you can use more transistors...if you have any Ideas, I could help you out with the circuitry.

These formulas might help:

Ohm's law

Volts(V)=Current(I) times Resistance®


Power Law

Power(P)=Current(I) times Voltage(V)

Common Facts:

Super Bright white LEDs drop anywhere from 3-4V

20mA is the max rating

To figure out how to power your LED setup, use the formulas above.


so, we'll plug in 3.5V (typical) for the LED...with a 9V battery, this means that the Resistor will have the other 5.5V. Using Ohm's law, this gives us 5.5(voltage on the resistor), divided by .020 (20mA for the LED) gives us an ideal resistor value of 275 Ohms. On our setup, we have a 470 Ohm resistor, so there is a little room to play with lower values before burning it out.

The reason I said it was amazing that the 9V didn't burn your LED out, is that the LED drops 3.5V, but any diode acts like a short (zero ohms of resistance) once you get to that voltage. This means that the typical 300mA that a 9V battery provides was flowing through that diode, and shorting out the battery.not good.

That's the end of today's electricity 101...like I said lower resistor values, more LEDs, transistors, batteries, etc.

If you need help, let me know!

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