Self Activating Door Bell

Self activating doorbells are of very much use in the modern age. This is also known as magic eye or electronic eye. This can be used as a guest indicator at the door. This can also be used to warn us of the intruders as a burglar alarm.

The circuit is easy to build and should be placed at the bottom of the door. The components are easily available in the local market.


  • CD4049 IC
  • LDR
  • 220k ohm resistor
  • LED
  • Buzzer
  • Switch
  • 9v battery
  • Multimeter
  • Connecting wires
  • Breadboard
  • Soldering gun
  • Flux

How to make it:

Connections are to be made as shown in the circuit diagram. Once the connections are done, check the circuit for continuity test and short circuit using a multimeter.

LDR (light dependent resistance) is placed at the bottom of the door in such a way that when somebody tries to enter, the shadow blocks the LDR. This activates the circuit and a sound is produced through the buzzer.

CD4049 IC has 6 independent NOT gates. In our project we are using only one NOT gate. Pin3 is the input pin and pin2 is the output pin. The NOT gate goes high (1) whenever the input signal at pin3 is lower than 1/3rd Of the supply voltage. On the contrary, the output goes low (0) when the input signal is below 1/3rd Of the supply voltage. The output has only two states; either high or low. This is powered by a 9v battery. The circuit is economic in power consumption. Pin1 is connected to the positive power supply and pin8 is grounded.

We have used LDR and a source of light (LED) to detect the presence of the object. LDR is a resistor whose resistance decreases with increasing light intensity. This is due to photoconductivity.

A potential divider circuit is made from LDR and a 220k ohm resistor which is connected in series. We know that voltage is directly proportional to conductance; hence more voltage is derived from divider when the LDR is getting light and low voltage when the LDR is in darkness.

As soon as the LDR gets dark, the voltage of the input NOT gate drops to 1/3rd of the supply voltage and pin2 gets high and buzzer or LED which is connected to the output gets activated.


About Pavan Shetty

Masters in Embedded systems, Intern Embedded Engineer.

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