Car Horn Prank Box

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.

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)

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.

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.

Crimp on 4 red quick connects to 6" red wires, and 4 blue quick connects to 6" black wires.

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.

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.

Take the dual pc board and snap it in half such that it is divided into two seperate boards.

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.

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.

Mount the Arduino to the L-assembly using 4-40 x 1/2" nuts and bolts.

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).

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.

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.

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).

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.

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.

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.

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.

Fasten the relay to the singular hole on the base of the wooden L-bracket using a 1" x 6-32 but and bolt.

Fasten the circuit board to the base of the wooden L-bracket with 1/2" x 6-32 nuts and bolts.

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.

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.

Attach another quick connect to the other end of the lone black wire attached to the relay.

Program the Arduino with the following code:

/*
Car Horn Prank Box
by Randy Sarafan - 2013
 
When the red button is pressed, an LED blinks for thirty seconds and then the prank is armed. During this time, the box should be hid somewhere very dark. The next time that light is sensed, the car horn honks for 2 seconds and disarms.
 
For more information visit:
https://www.instructables.com/id/Car-Horn-Prank-Box/

This code is in the Public Domain.

 */

// Set the name of pin 2. This pin controls the horn.
const int honk = 2;

// Set the name of pin 3. This pin controls the LED.
const int led = 3;

// Set the name of pin 4. This pin monitors buttons presses.
const int buttonPin = 4;     

// Set the name of analog pin 0. This pin monitors the photocell.
const int sensorPin = A0;   

// Set the initial armed state to 0. 
// Make sure it does not trigger when powered on.
int armed = 0;

// Variable for storing photocell data
int senselight;

// Variable for 
int buttonState = 0;


// This happens once.
void setup() {
  
  // Set the digital pins to either be inputs or outputs.
  pinMode(buttonPin, INPUT);     
  pinMode(honk, OUTPUT);   
  pinMode(led, OUTPUT);   

}


//This all happens over and over.
void loop(){
  
  // Read the state of the pushbutton value.
  buttonState = digitalRead(buttonPin);

  // If the button is pressed
  if (buttonState == HIGH) {     
    
  //Blink the LED for 30 seconds
    for(int i = 0; i < 15; i++){
      digitalWrite(led, HIGH);   
      delay(1000);           
      digitalWrite(led, LOW);    
      delay(1000);   
    }
    
    // Arm the prank box. The next time it senses light, it will honk.
    armed = 1;
  } 
  
  // Read the light sensor and store it as a variable.
  senselight = analogRead(sensorPin);    

  // If the light sensor senses a small amount of light, honk the horn for two seconds and then disarm.
  if(senselight > 300){
    if(armed == 1){
      digitalWrite(honk, HIGH);  
      delay(2000);              
      digitalWrite(honk, LOW);  
      delay(1000);
      armed = 0;
    }
  }
}

Put the wooden L-bracket into the enclosure.

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.

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.

Mount the horn to the inside of the lid using the 1/4" hole drilled earlier.

Close the lid and fasten it shut.

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.