Have you ever heard about a 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 a dummy alarm.

Dummy Alarm Circuit Principle:

The main principle of the circuit is to flash an LED every 5 seconds. The circuit consists of a 7555 timer IC as the main component. This is a low power IC of 555 series. This 7555 timer is operated in an astable mode to continuously and produces the waveform. This output is applied to the high power LED, which flashes for every 5 seconds of delay.

Dummy Alarm Circuit Diagram:

Dummy Alarm Project Circuit Diagram

Dummy Alarm Project Circuit Diagram

Circuit Components:

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

Dummy Alarm Circuit Design:

This circuit consists of a 7555 timer IC. It has 8 pins. The first pin is connected to ground. Pin2 and Pin6 are shorted and connected to the positive terminal of the capacitor. The negative terminal is connected to the ground. A resistor of 10K is connected to the positive terminal of the capacitor. Another resistor of 680 ohms is connected in series to the 10K resistor. The 7th pin is connected between the two series resistors. Another end of the resistor is connected to the battery of 4.5v. 4th pin and 8th pin are shorted and connected to the battery. In the circuit diagram, the 8th pin is not shown. The LED with a resistor of 330 ohms is connected to the output pin from the timer i.e. 3rd pin.

Dummy Alarm Circuit Simulation Video:

Dummy Alarm Circuit Working:

  1. Initially, switch on the circuit.
  2. Now press the ON/OFF button.
  3. By closing the button, one can observe LED glowing continuously with a delay of five seconds. This is because when the circuit is powered voltage is applied to the capacitor through two series resistors.
  4. At the same time, the 555 timer IC also gets a supply voltage.
  5. Thus capacitor starts charging up to a voltage of 2/3 VCC.
  6. When the charging voltage reaches to 2/3 of VCC, it is detected by the 6th pin and it connects 7th pin to the ground.
  7. Thus the capacitor starts discharging through a 10K resistor.
  8. When the voltage in the capacitor reaches 1/3rd of the VCC, the capacitor again starts charging.
  9. This process continuous which generates a wave with a delay of 5 seconds.
  10. This delay is decided by the RC circuit of the timer IC.
  11. The time delay can be calculated using the formula: T = 0.7*(R1+2R2)*C1.
  12. Using the above formula, one can calculate the values of resistance R1, R2, and C1.
  13. Thus using this RC circuit, one can produce exact delays.
  14. As the output is connected to the LED, it starts glowing with a delay of  5 seconds when a low pulse is detected at the output.
  15. Now release the button, the LED stops glowing.

Dummy Alarm Project Output Video:

Dummy Alarm Circuit Applications:

  • As it produces an 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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