Instructables
Picture of Remote control school bell
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remote control doorbell.jpg
resist.gif
bc547.gif
What you'll need:
- a wireless doorbell
- 1 x transistor (like bc547)
- 1 x relay
- 1 x 1K resistor

Tools:
- some solder
- a soldering iron
- ....

 
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Step 1: How it works

Picture of How it works
Normal:
When you push the button
=> The remote sends a signal.
=> The doorbell receives  the signal and makes a pin high in an IC on the PCB.
=> That pin TRIGGERS the sound controller.
=> The sound controller sends a signal to a speaker.
=> The speaker makes an annoying sound.

Hacked:
When you push the button: 
=> The remote sends a signal.
=> The doorbell receives the signal and makes a pin high in an IC on the PCB.
=> That pin TRIGGERS our new circuit.
(the transistor lets a current flow so the relay can switch on).
=> The relay switches on and lets a current flow trough the school bell.
=> The school bell rings "tringggg!!!!!!!!!!!! ".
=> And you can leave the class :D


Step 2: How to find the trigger pin

Picture of How to find the trigger pin
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Use the flowchart

Step 3: Making the circuit

Picture of Making the circuit
resist.gif
bc547.gif
V+ is connected to the positive lead on the battery-pack of the doorbell.
GND is connected to the negative lead on the battery-pack of the doorbell.
Trigger
is connected to the triggering pin of the doorbell.

G5-8 = are normal battery's 
R1 = 1KOhm
D1 = A normal diode like 1n4007 or 1n4004
K1 = NO relay 5V
T1 = A standard transistor like BC547 

Step 4: Testing

Picture of Testing
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When the school bell is not connected you can hear the relay switching when you push the button.
And when everything is connected the bell will start ringing when you push the button till you lose the button.