Smart Door Bell


Microcontroller-based doorbell system which is being used in an industrial environment. This is the basic function: When you push the bell-button, a horn will sound for about 10 seconds. At the same time a light, which keeps burning until someone pushes a reset-button, will be activated.


The system has a lot of features:
  • There are 2 different lamp outputs: these can be used to control a LED lamp with two different decks of LEDs.
  • Possibility to enter a ‘secret’ code: push the bell button 3 times to activate a secret code (the light will now flash).
  • Possibility to enter an ’emergency’ code: push the bell button 10 times to activate a secret code (the light will now flash very rapid and the horn will sound for 1 minute).
  • Possibility to enable a temporary mute-mode by pressing the reset-button for 5 seconds: now the horn won’t be activated unless someone pushes the secret/emergency code.
  • The temporary mute-mode is automatically turned off after 45 minutes. It can also be disabled manually: push the reset button again for 5 seconds.
  • During the mute-mode the light flashes once per minute to indicate the mute-mode is active.
  • Jumper selection: choose the horn-sounding time (5 seconds/10 seconds).
  • The light is turned off after 15 minutes as some sort of overheat protection.
  • The PIC is placed into a low-power sleep mode after 15 minutes.

This circuit uses a PIC16F84A microprocessor whose frequency is regulated by a 14 MHz crystal. The circuit itself should be powered by a 12VDC voltage. Attention: I didn’t add a protection against reversing the polarity of the inputs.
The outputs are controlled by an UNL2003A IC. This is a driver chip that can deliver currents up to 500 mA. According to the datasheet you can put drivers in parallel to increase the output current.
  • By setting jumper SJ1 you can make the pin RB1 of the microprocessor HIGH: this will shorten the duration of the horn sounding time from 10 seconds to 5 seconds.
  • The push-buttons are connected to the ground: pushing them will make the inputs of the microcontroller low. I did not foresee any circuitry to avoid contact bounce: this is done in the software of the microcontroller.
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