A beloved coworker is returning from Canada soon and in his honor I have created the car horn prank box. Basically, when the red button is pressed, and an LED blinks indicating that there is thirty seconds in which to hide the box somewhere dark. Once the thirty seconds expire, the next time that the box sense light, the car horn honks very loudly. Some fun places to hide the car horn is includes drawers, underneath jackets, in closets, buried in a backpack, or simple left in a room in which the lights have been turned off. The amount of potential mischief that this device can produce makes it a perfect welcome back gift.
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Step 1: Materials
You will need:
(x1) Car horn
(x1) Enercell 12V/5Ah Sealed Lead-Acid battery (Radioshack #23-945)
(x1) Arduino Uno (Radioshack #276-128)
(x1) LED holder w/LED (Radioshack #276-068)
(x1) LED holder (Radioshack #276-080)
(x1) 7805 voltage regulator (Radioshack #276-1770)
(x1) TIP31 NPN transistor (Radioshack #276-2017)
(x2) 10K ohm resistor (Radioshack #271-1335)
(x1) 1K ohm resistor (Radioshack #271-1321)
(x1) 220 ohm resistor (Radioshack #271-1313)
(x1) 10uF capacitor (Radioshack #272-1025)
(x1) 1uF capacitor (Radioshack #55047773)
(x1) 1N4004 diode (Radioshack #276-1103)
(x1) Small photocell (Radioshack #276-1657)
(x1) Red pushbutton switch (Radioshack #275-646)
(x1) Aluminum heatsink (Radioshack #276-1368)
(x1) Thermal compound(Radioshack #28-1098)
(x1) 12V automotive relay (Radioshack #275-001)
(x1) Dual PC Board (Radioshack #276-159)
(x2) Pack red and blue quick connects (Radioshack #64-3088)
Step 2: Mark and Drill
Measure in 2" from one of the 6" edges of the enclosure's lid and make a centered mark.
Drill through this mark with a 1/4" drill bit.
Step 3: Insert
Insert epoxy into the base of the LED holder and then insert a small photocell.
Clean the epoxy of the photocell leads if necessary.
Let the assembly dry.
Step 4: Attach
Crimp on 4 red quick connects to 6" red wires, and 4 blue quick connects to 6" black wires.
Step 5: Drill
In one of the opposite corners from the last hole drilled in the lid, drill a 1/2" hole, and two 1/4" holes, which are all spaced about 1/4" apart.
Step 6: Insert
Insert the pushbutton switch into the 1/2" hole and fasten it in place with its mounting nut.
In the adjacent 1/4" hole, install the red LED (in holder).
Finally, next to this install the photocell assembly.
Step 7: Break
Take the dual pc board and snap it in half such that it is divided into two seperate boards.
Step 8: Heat Sink
Apply thermal paste to a heatsink and then mount a 7805 voltage regulator using a 1/4" x 4-40 bolt.
Clean away any excess thermal paste from the sides of the voltage regulator.
Step 9: Cut and Glue
Download the attached template and use it to cut both shapes out of 1/4" plywood.
Glue the plywood shapes together to form an L-assembly.
Step 10: Attach the Arduino
Mount the Arduino to the L-assembly using 4-40 x 1/2" nuts and bolts.
Step 11: Start the Circuit
Assemble the circuit board by including the voltage regulator components (these are the 7805 regulator, 10uF capacitor, and 0.1uF capacitor), and also the resistors for the pushbutton switch (these are the two 10K resistors and the 220 ohm).
Step 12: Twist and Attach
Cut a 6" red wire and strip a little insulation off the end.
Using a wire nut, twist this together with the ends of three of the red wires with quick connects attached.
Solder the wire without the quick connect attached to the voltage input pin of the 7805 regulator.
Step 13: Twist and Attach Again
Cut a 6" black wire and strip a little bit of insulation of the end.
Using a wire nut, twist this together with two of the black quick connect wires.
Solder the other end of the 6" black wire to ground on the circuit board.
Step 14: Transistor
Solder the TIP31 transistor to the circuit board connect a 1K resistor to the base pin (the leftmost pin).
Connect the emitter to ground on the circuit board (the rightmost pin).
Take a black wire with the quick connect attached, and solder it to the transistor's collector (the center pin).
Step 15: Wires
Attach a 6" red wire to one of the terminals on the pushbutton switch, and a 6" green wire to the other.
Attach a 6" red wire to the LED's anode (longer lead) and a 6" green wire to the cathode (the other shorter lead).
Attach a 6" red wire to one of the leads on the photocell and a 6" green wire to the other lead.
Step 16: Insulate
Insert a drop of hot glue between the leads of the photocell and LED to insulate them and keep them from bending into contact with one another.
Step 17: Diode
Solder a 1N4004 diode between the coil pins (pins 85 and 86) of the relay as close to the base of the relay as possible.
Step 18: Attach the Circuit Board
Attach the switch, LED and photocell to the circuit board as appropriate based on the circuit diagram.
Additionally, solder a 6" red wire to +5V power on the voltage regulator, and 6" black wire to ground.
Extend the output for the photocell, LED, pushbutton switch, and transistor base with 6" green wires.
Step 19: Mount the Relay
Fasten the relay to the singular hole on the base of the wooden L-bracket using a 1" x 6-32 but and bolt.
Step 20: Mount the Circuit Board
Fasten the circuit board to the base of the wooden L-bracket with 1/2" x 6-32 nuts and bolts.
Step 21: Plug In
Connect the red wire from the circuit board to the 5V pin on the Arduino.
Connect the black wire to one of the ground pins on the Arduino.
Attach the wire from the photocell to pin A0.
Attach the wire from the 1K resistor (connected to the base of the transistor) to pin D2.
Attach the wire from the LED wire to pin D3.
Attach the wire from the pushbutton switch to pin D4.
Step 22: Connect the Relay
Connect a 12V quick connect wire to the cathode side of the relay coil.
Connect the black wire from the TIP31 collector pin to the anode side of the relay coil.
Connect a 12V quick connect wire to one of the sides of the relay switch.
Connect a black quick connect wire (that is not connected to ground) to the other side of the relay switch.
Step 23: Another Connector
Attach another quick connect to the other end of the lone black wire attached to the relay.
Step 24: Program
Step 25: Insert
Put the wooden L-bracket into the enclosure.
Step 26: Battery
Place the battery in the cutout adjacent to the relay and the circuit board.
Connect a black wire from the circuit board's ground to the ground terminal on the battery.
Connect a red wire from the circuit board's 12V connection to the power terminal on the battery.
The circuit is now activated.
Step 27: Horn
The last thing that needs to be wired into the circuit is the car horn. It is highly recommended that you wear hearing protection when wiring the horn into the circuit, and debugging the circuit while the horn is connected (it is VERY LOUD!).
Connect the remaining black wire from the relay to one terminal on the horn and a black ground wire from the circuit board to the other terminal on the horn.
Step 28: Connect
Mount the horn to the inside of the lid using the 1/4" hole drilled earlier.
Step 29: Close
Close the lid and fasten it shut.
Step 30: To Use
To arm the Car Horn Prank Box, press the red button.
The LED will flash on and off for 30 seconds. During this time, you should hide the prank box somewhere very dark.
After 30 seconds, the next time the prank box sense light, it will honk.
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