Description of Sensor Alarm Using Thyristor

The thefts in the public places like shops, banks, treasuries etc are rising day by day. All these public places occupy a very large area. To alert the concerned authorities in these areas we require an which can function and hold up large power. To overcome this problem, here is a solution namely Sensor alarm system using Thyristor. It is a device used to perform this task. This article discusses the operation, advantages, disadvantages and the future scope of this project.

Sensor Alarm System using Thyristor

The circuit of sensor alarm using thyristor can be built with capacitors, diodes, resistors for biasing the thyristor and generating the alarm. This project is designed to detect interruption, illegal entry into any area or building. The sensor alarm can be used in commercial, residential, industrial, and military properties where the reporting area for alarm is very big for guard against robbery and as an instrument for individual protection against intruders. The basic thought was to build a sensor alarm which can hold up and work on huge power and give out a purpose of robbery protection and interruption protection.

What is a Thyristor?

A thyristor is a 2 to 4 lead solid-state semiconductor device with four layers of alternating N-type and P-type material. They perform totally as bistable switches, conducting when their gate terminal gets a current trigger, and maintain to conduct while they are forward biased. A 3-lead thyristor is intended to control the larger current of its 2-leads by joining that current with the lesser current or voltage of its additional lead that is known as its control lead. On the other hand, a 2-lead thyristor is intended to ‘turn on’ if the potential difference between its leads is suitably large – a value symbolizing its breakdown voltage. Please refer the link to know more about Thyristor or Silicon Controlled Rectifier Tutorial basics and Characteristics


Sensor Alarm System using Thyristor Circuit

The required components of this sensor alarm system using Thyristors include voltage supply, switches, Thyristors, resistors, LEDs, Capacitors, and a variable resistor (potentiometer).

The D1 led shown in the above circuit diagram specifies the state of input supply voltage, when D1 led is on it shows that supply voltage is on. The R2 resistor and C1 capacitor form the R-C charging system. As R5 resistor is used to regulate the firing angle of SCR. The D3 led is the actual o/p, it blinks only when SCR gets activated.

Sensor Alarm using Thyristor
Sensor Alarm using Thyristor

At the start suppose supply voltage is OFF so both the LEDs will get off. Now, the supply voltage is switched ON so simultaneously D1 led glows but D3 led will off. Now if the switch is pushed by the user after adjusting resistor for certain value, without delay D3 LED glows as soon as the voltage across capacitor equals activating voltage of SCR. Due to this SCR goes in forward conducting region and D3 led glows. Now, when the switch is free C2 capacitor releases through a resistor and after some time SCR gets switched OFF and D3 led to turns off.

Thus the firing angle of SCR and in rotating the time for which D3 led glows can be controlled by the user by correcting the value of the resistor.

Advantages and Disadvantages

The advantages and disadvantages of sensor alarm system using thyristor include the following.

  • This circuit is dense and very easy to design
  • Thyristors can provide DC from an AC source (controlled rectification) with a broad range of voltage or current control.
  • Firing angle can be familiar from 0-180 degrees with extremely less difference of resistor
  • This Circuit is application specific and for using similar circuit for associated applications some changes are require
  • Thyristors “latch” in the on state formerly they start to perform and stay on after the gate pulse is detached until they are reverse biased.


The applications of sensor alarm system include the following

  • Signaling for facilitating through SOS
  • In communication between ships
  • During marine operations
  • During circumstances regarding safety and easy transmission of significant messages

From the above information, finally, we can conclude that this sensor alarm system can resist large power and can be used for a variety of defense applications. A buzzer can be included for better results. For multilevel disasters, a firing circuit can be used using which the firing angle of the thyristor can be varied. For any queries regarding this concept or to implement any Thyristor based projects please give your feedback by commenting in the comment section below. Here is a question for you, what is the function of a Thyristor?

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