Fire Alarm

Dummy Alarm Circuit

Have you ever heard about dummy alarm? This is similar to the alarm but this flashes high power Red LEDs for every 5 seconds. This article describes the circuit of dummy alarm.


  • Dummy Alarm Circuit Principle:
  • Dummy Alarm Circuit Diagram:
    • Dummy Circuit Design:
    • Dummy Circuit Simulation Video:
    • Dummy Circuit Working:
    • Dummy Project Output Video:
    • Dummy Circuit Applications:
      • Limitations of the Circuit:

Dummy Alarm Circuit Principle:

The core concept of this circuit revolves around intermittently illuminating an LED at a 5-second interval. At the heart of the circuit lies the 7555 timer IC, belonging to the low-power 555 series. The 7555 timer operates in an astable mode, consistently generating a recurring waveform. This waveform is then directed to the high-intensity LED, causing it to flash once every 5 seconds.

Dummy Alarm Circuit Diagram:

Dummy Alarm

Circuit Components:

  • 555 Timer IC.
  • Resistors R1, R2, R3.
  • Capacitor C1.
  • High power Red LED.
  • Battery.
  • ON/OFF switch

Dummy Alarm Circuit Design:

In this circuit, a 7555 timer IC is employed, equipped with a total of eight pins. The initial pin, the ground pin, is connected to the first pin. Pins 2 and 6 are joined together and linked to the positive terminal of a capacitor. The negative terminal of the capacitor is connected to the ground. A 10K resistor is attached to the positive terminal of the capacitor, with a 680-ohm resistor connected in series to the 10K resistor. The seventh pin establishes a connection between the two resistors in series. The 4.5V battery is connected to the opposite end of the resistor arrangement. The battery is further connected to the fourth and eighth pins, which are interconnected (the eighth pin may not be visible in the circuit diagram). Finally, the LED is linked to the timer’s output pin, specifically the third pin, with a 330-ohm resistor.

Dummy Alarm Circuit Simulation Video:

Dummy Alarm Circuit Working:

  1. Turn on the circuit for the first time.
  2. Now push the ON/OFF button to turn the device on or off.
  3. When the button is pressed, the LED glows constantly with a five-second delay. Because voltage is provided to the capacitor through two series resistors when the circuit is energised, this is the case.
  4. At the same moment, the 555 timer IC receives power.
  5. As a result, the capacitor begins to charge up to a voltage of 2/3 VCC.
  6. The 6th pin detects when the charging voltage reaches 2/3 of VCC and connects the 7th pin to the ground.
  7. As a result, the capacitor begins to discharge through the 10K resistor.
  8. The capacitor begins to charge again when the voltage in the capacitor reaches 1/3rd of the VCC.
  9. This procedure continues, resulting in a wave with a 5 second delay.
  10. The timer IC’s RC circuit determines this delay.
  11. T = 0.7(R1+2R2)C1 T = 0.7(R1+2R2)C1 T = 0.7(R1+2R2)C1 T = 0.7*(R1+2R2)
  12. The values of resistance R1, R2, and C1 can be calculated using the formula above.
  13. This RC circuit can thus be used to create precise delays.
  14. When a low pulse is detected at the output, the LED, which is attached to it, begins to glow after a 5-second delay.
  15. When you let go of the button, the LED will turn off.

Dummy Alarm Project Output Video:

Dummy Alarm Circuit Applications:

  • As it produces exact delay of 5 sec, it can be used in timing applications.
  • It can be used in cars for security that is, when any theft is detected in the cars it starts flashing for every 5 seconds.

Limitations of the Circuit:

  • This is dummy alarm and it does not produce any sound, an LED is simply flashed for the given delay.
  • Improper values of R1, R2 and C may cause improper time delays.

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