Bioquizzical: $5 Flashing LED Light Alarm Clock

Don't mind the crying kid in the background

I had two basic problems that led me to this project:

1. I needed something to wake me up without waking up my wife. (Headphones fall out of my ears overnight. I thought of that.)

2. I hate the sound of alarm clocks.

My answer: the LED light alarm clock! LEDs are problematic for normal lighting because the light is only bright within a narrow range. For us, this is perfect as it lets us focus the light on just one person, letting the co-sleeper snooze on.

Parts and Cost:

1 Cheap Alarm Clock ($1.99, eBay)

5 White LEDs ($0.35, 50 for $3.29, eBay)

1 2N3904 (or other NPN) Transistor ($0.20, 15 for $2.99, RadioShack)

1 8 ohm Resistor ($0.01, 1000 for $7.50, eBay)

1 2 x AA Battery Holder ($1.99, RadioShack. abt. $0.50 on eBay)

2 AA Batteries (You probably have some around)

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Total = About 4 or 5 bucks

Warning: Some of this goes against good electronics practice. Just so you know (see step 3).

The basic concept here is that somewhere in your average alarm clock is something that makes sound. For it to make sound, it needs pulses of current. Using a transistor, you can use that current to switch on current for whatever you want!

The left circuit is the part built into the clock. The right circuit is the new part.
The question marks stand for whatever is going on inside the clock. We don't need
to know how the left part works, just that when the alarm goes off we get current.

Step-By-Step:

Step 1: Open your alarm clock, find something that looks it makes sound. Use your multimeter to figure out which side is giving it positive current and which side is negative. Cut out the sound part

Step 2: Connect the positive side to the base of the transistor. Connect the negative side to the emitter.

Step 3: Connect the LED circuit as shown in the diagram. The positive end goes to the collector, and the negative to the emitter. I use old CDs to set the LEDs because they're rigid, easy to put holes in, and I have a stack of 50 CD-Rs that I'll probably never use in the thumb drive/MP3 age.

(Note: Using just one current-limiting resistor for the whole LED array isn’t really a great idea. Here, the 8 ohm resistor limits the current to (3 V – 2.2 V) / 8 ohms = 100 milliamps. We just assume that the current will divide evenly giving us 20 milliamps per LED. With variation among LEDs, it might not be evenly split. Also, a bigger problem is that if one of the LEDs goes out we still have 100 milliamps, but only divided 4 ways. This gives us 25 milliamps each, which is more than the LEDs are rated for. It would be better to use a resistor in series with every single LED. I just didn’t feel like it at the time.)

Step 4: Enjoy!