Extreme Noob needs help.

dvoros · Semi Newbie Joined: 05 Dec 2008 Posts: 8

Ok, I am trying to teach myself a little electronics, all I want is a led that
senses darkness and turns on. Here is the schematic I was trying to follow

Here is a crappy pic of what I thought was a correct way to set it up on a breadboard:

As it turns out, when i apply the 9 volts, the LED turns ON in a lit room and off in a dark room, opposite of what I want. This is my first try ever so Im sure its somthing easy but I need some help.

These are the parts I am using:

9 Volt Battery
Photo-Cell
Transistor (PN2 907A 339) At least that is what is written on it.
White LED
100K ohms Resistor
470 ohms Resistor
9 Volt battery Snap

One note: On the schematic I was using it calls for a different number
of transistor. I used the only one I had.

Any help would be appreciated folks Smile

Dave

Turd · Posted: Fri Dec 05, 2008 5:56 pm Post subject:

This is what it should look like Very Happy

eric

dvoros · Semi Newbie Joined: 05 Dec 2008 Posts: 8

Turd Thanks for the quick reply, I made mine the way yours looks and it is still acting bass ackwards lol.

could i have the wrong resistors hooked up? Im not sure why my setup would act like this.

Turd · Posted: Fri Dec 05, 2008 6:47 pm Post subject:

The 100K resistor should be: brown, black, yellow and then gold. The 470 ohm resistor should be: yellow, purple, brown and gold.
And note the bare wire jumper from the emitter to the ground rail ( - ).

eric

dvoros · Semi Newbie Joined: 05 Dec 2008 Posts: 8

Never mind I got it working now Thanks alot Turd Smile

Dave

Turd · Posted: Fri Dec 05, 2008 6:54 pm Post subject:

No problem. Glad you got it to works Very Happy

eric

dvoros · Semi Newbie Joined: 05 Dec 2008 Posts: 8

Thanks man Smile

No one more question if you dont mind.

What would be an easy or best way to make that simple circuit more sensitive to light, to where it would only activate in a very dark place?

Dave

Turd · Posted: Fri Dec 05, 2008 7:10 pm Post subject:

You could change the 100K resistor to a 200K resistor (red, black, yellow, gold).
If you don't have a 200K resistor use two 100K resistors in series.

eric

dvoros · Semi Newbie Joined: 05 Dec 2008 Posts: 8

that worked perfectly Smile

thanks

Dave

dvoros · Semi Newbie Joined: 05 Dec 2008 Posts: 8

Hey there Turd, my simple circuit is working great. I wanted to ask you , if i wanted to switch my power supply to 3v (coin battery), how do i figure out the changes i would need to do with the resistors?

Turd · Posted: Mon Dec 08, 2008 4:18 pm Post subject:

What colour is the LED?

eric

dvoros · Semi Newbie Joined: 05 Dec 2008 Posts: 8

it is a 5mm red led , 1.7volt , 20mA, 3000mcd

Dave

Turd · Posted: Mon Dec 08, 2008 4:34 pm Post subject:

You could replace the 470 ohm resistor with a solid wire but then make sure you don't use the 9V battery Very Happy

eric

dvoros · Semi Newbie Joined: 05 Dec 2008 Posts: 8

ok than k you very much, think i need to get an Electronics for Dummies or somthing like that hehe

Smile

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