Sep 202018

Car Burglar Alarm using two 555 timers

This is a simple Car Burglar Alarm circuit that uses a dual contact relay and two 555 timers, each of them working as a monostable multivibrator. The alarm will activate the car horn, approximately 5 seconds after either door is opened and has not been properly closed.

The delay time is used in order for people to have time to:

  • Activate the alarm when they leave the car, and so it remains protected …
  • Enter the car and deactivate the alarm before the horn starts to sound.

Car Burglar Alarm using two 555 timers

Car Burglar Alarm using two 555 timers operation

Only the owner of the car knows the location of the switches that activate and deactivate the alarm. To power the circuit, switch SW1 is closed.

Switch SW2 allows canceling the operation of the alarm momentarily in case it takes more than 5 seconds to enter or exit the vehicle. When SW2 is closed, the output of the first 555 goes to 12 volts. Then the switch must be returned to its original position (normally open)

Note that the two activation terminals of the relay are connected to the outputs of the two 555. (On the second 555, through transistor Q1).

When the alarm is on the standby state and no door opening has been detected, the output of the first 555 is 0 volts, the output of the second 555 is 12V, Q1 does not conduct and between the relay terminals there is 0 volts (Relay is disabled).

When one of the doors is opened, capacitor C2 is charged through resistor R3. After about 5 seconds, the output of the second 555 goes to zero volts, saturating Q1 and activating the relay.

Remember that the first 555 will only have 12 volts at its output when SW2 is closed. (This 555 output is normally 0 volts, see SW2 operation in a previous paragraph)

The relay when activated connects the car horn to the normally open (NO) contact and locks the door switches with the normally closed (NC) contact, which keeps the car’s horn active despite the doors being closed.

SW switches are placed on all doors of the car, including engine cover and trunk. These are connected in parallel. (See diagram)

Component List of the Car Burglar Alarm circuit

  • 2 555 timer (IC1, IC2)
  • 1 2N4403 or similar PNP bipolar transistor (Q1)
  • 1 1N4001 semiconductor diode (D1)
  • 1 270 ohm resistor (R1)
  • 1 resistor (R2)
  • 1 2M resistor (R3)
  • 1 10uF electrolytic capacitor (C1)
  • 1 1uF electrolytic capacitor (C2)
  • 1 double pole double throw 12 volt relay (DPDT) (RL1)
  • 2 switches (SW1, SW2)
  • ? normally closed contacts for car doors, trunk, etc. (SW)

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