In this instructable I will describe to you how I made a solar powered prank for April fools day.

The goal of the device was to be at least 50% recycled materials and to cost no more then $5 per unit to make. As a side effect these devices can be mass produced for a larger distribution of the prank or to intensify it in one area.

The simplest idea that I came up with to meet these goals was to use a trashed solar garden light attached to a 3v piezo buzzer. When in the sun the device will produce a rather annoying buzz.

The solar cell from the garden light was removed and the housing and circuitry was retained for future use.

Before buying any additional part I did some research on how much power I had to work with.

Using my multimeter in voltage mode I recorded an open circuit voltage of 2.8v in moderate light and 3.5v in full light.

Then I switched the multimeter over to current mode and measured the short circuit current to be 50ma in moderate sun and 75ma in bright sun.

Step 1: Purchasing Parts

Knowing that my solar cell will output 3.5V at 75mA in bright light and 2.8v at 50mA, I purchased a radioshack part #273-0053 3V Mini Buzzer. ~$3.75

The specifications met the power supplied of the solar cell.
Voltage rating 1.5-3V
Current Consumption 15mA MAX at 3V

These numbers indicate that I could run 3 of the buzzers in parallel simultaneously off of one cell.
1 buzzer consumes 15mA.
The solar cell provides 50mA in moderate sun.
50mA / 15mA = 3 and 1/3 buzzers.

To keep things simple I stuck to one buzzer per unit.

For mounting purposes, I found a piece of scrap wire on the street. You could substitute any sort of wire including coat hanger wire.

Overall the bill of materials include:
1 X Solar Cell salvaged from solar garden light
1 X Piezo Buzzer
1 X Scrap wire

Tools to build this project are
A soldering iron
A glue gun
possibly a multimeter

Step 2: How to Build It: the Solar Cell

If you salvaged your solar cell and it has the polarity marked (either printed on the board or by wire color, red = positive) then skip to the next step.

If you find that your solar cell does not have the polarity marked you will need to determine the polarity.

In order to accomplish this you will need to use either a multimeter that can determine reverse voltage or a couple of leds with a known polarity.

The multimeter technique:
1) Hook the leads of the multimeter to the different pads or contact points on the board.
2) Turn the multimeter to the voltage position
3) position the solar cell so it can receive sunlight or light from a bright non-cfl light bulb
4) if the voltage on the meter shows a "-" in front of it then the polarity is reversed from the color of the leads. If no "-" appears then the polarity is correct.
5) mark on the board with some sort of marker the correct polarity next to the pads.

The led technique:
1) wire two leds together in series
2) solder or wrap the positive lead of the led to one of the pads
3) solder or wrap the negative lead of the led to the other pad
4) hold the solar cell up to a bright non-cfl light or the sun
5) if the leds light up then you guessed the polarity correct. if not then switch the leads and try it again.
6) mark on the board with some sort of marker the correct polarity next to the pads.
notes: if neither direction seems to work try using one led at your own risk or use the multimeter technique

Step 3: How to Build It: Wireing the Buzzer

The buzzer should have two leads coming off of it. One red and one black.

Solder the red lead to the positive pad of the cell.
Solder the black lead to the negative pad of the cell.

Before continuing to mount the buzzer take the time to test the connection. Put the cell next to a bright non-cfl light or the sun. If it buzzes then you got the soldering correct. If not switch the leads and try again.

Once properly wired, secure the buzzer to the back of the cell with some hot glue.

Step 4: How to Build It: Making a Mount

In order to secure the device to its resting place I used a piece of scrap wire.

first off I made a 3/4 oval shape in the middle of the wire. The extra bit of wire was then bent ~90 degrees away from the oval. The oval part was secured to the back of solar cell with some hot glue.

Once an appropriate position is found for the device the wire will be used to wrap around or hang from some other structure

Step 5: Deploying the Prank

I found the best way to deploy the prank was to place the device high up in a tree where it can still receive sun. This way a passerby would stop and look at the buzzing tree.

It would be best to deploy the device in a public area where it is busy but still able to receive light. Also to avoid complications make sure the buzzer is out of reach or better yet not visible. Take the time to secure it well or people will figure it out quickly.

Another way to deploy the prank would be to hide it in someones cluttered car.

Step 6: Variations and Upgrades

Keeping with the original goals a lot of features were not included in the final project. Several other additions could be made simply.

One idea was to use a strobing led flasher to pulse the buzzer on and off. One could disassemble the flasher, remove the led, and solder the buzzer in its place. If a strobe is not at your dispose, a 555 timer could suffice.

With a resistor, one could hook the solar cell up to a disposable camera modified to strobe. In this fashion instead of buzzing continuously there would be random flashes.

Some sort of low current proximity sensor could be used to turn on the buzzer whenever someone is close.

One could position the buzzer on a branch likely to sway in a breeze. This way the solar cell will produce voltage and drop off at random intervals making the buzzer that much more annoying.

A sign could be placed over the buzzer that says "April Fools".

An array of the buzzers spread throughout a park could be especially annoying to those who pass through the park.

Practically anything that uses a low voltage and low current can be used in conjunction with the solar cells. How you make it annoying or into a prank is up to you!