Published 7/17/09 4 months ago | Views 1,053 Grade C Electronics / Hacks, Mods & Circuitry

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Grade C Views 1,053
Last edited 2 months ago

You’re sleeping or your back is turned as a friend is trying to sneak up and scare you. Well not anymore!

Step 1

Go to your local RadioShack and purchase the following: 2 - Cds Cells (Photocells), 1 - 741 Op Amp, 1 - 1 kilo ohm resistor and LED, 1 - 9 volt battery breadboard, 1 - 10K potentiometer.

Step 2

Take the bread board and put the 741 Op Amp in the middle, shown in the picture below.

Step 3

The next steps can be seen in photo above.

Step 4

Nodes, if looking at the breadboard vertically, are horizontal and do not continue beyond the break in the middle. So there are two different nodes horizontally across each row of the breadboard.

Step 5

The 741 Op Amp has 8 pins, 1 through 8. Pin number 1 will be the one closest to the little circle in the corner and 2 through 8 will be counterclockwise from there.

Step 6

After placing the Op Amp on the breadboard, put one pin on a Cds cell in the node connected to pin two of the op amp and the other Cds pin in the seventh pin of the op amp. The other cds cell likewise connects to pins two and four of the op amp.

Step 7

Connect the middle pin of the potentiometer to pin three of the op amp, the right pin of the pot. to pin seven of the op amp and the left pin of the pot. to pin four of the op amp.

Step 8

Connect the negative of the battery to pin 4 of the op amp.

Step 9

Connect the negative (usually shorter) lead of the LED to pin six of the op amp. Connect the other pin of the LED the 1K resistor. The other side of the 1K resistor should be connected to pin seven of the op amp.

Step 10

Connect the positive battery wire to pin seven of the op amp.

Step 11

All that is left to do is connect the battery, and adjust the pot. as needed.

Tips

If the LED doesn’t turn on try twisting the potentiometer and if that doesn’t work the charge could be wrong and just go ahead and switch the LED around.
To have the highest sensitivity with the photocells turn the potentiometer so the LED just barely turns off.
If you are not excited with this circuit go ahead and replace the 1 killi ohm resistor and the LED with a buzzer. This can also be found at RadioShack. Although it may be a little annoying it is much more efficient if you are using it to be warned of your friends sneaking up on you it is perfect.
You can also replace the LED with a relay to switch whatever you want. (Note: this will also require an extra transistor)
Photocells work by changing their resistance as light enters the cell. I found that if you replace the top photocell with a 1K resistor, you can shine a beam of light into the other one, and the circuit trips when the beam is broken