In this article, we have designed a transistor-based security alarm circuit with a high power siren that can be used to provide security for homes or as a warning for intruders trespassing. Only the basic design is provided in the circuit. You can check for advanced security alarm circuits using Arduino and Micro-controllers in the links given below.
A complementary transistor pair (BC 557 & 337) is arranged as an oscillator, directly drives the speaker. Transistor Q1 (BC 557) is used to provide full charging of capacitor C2. When the push button switch P1 is pressed, capacitor C2 discharges through R8 and the circuit starts oscillating at a low frequency that increases slowly until a high-frequency steady tone is reached and it is kept. When P1 is released C2 starts discharging and output frequency slowly decreases. When C2 is charged fully the circuit stops oscillations. Please note that the pressing of the push button P1 denotes the cause of the siren alarm circuit to activate. This could be interpreted as the presence of a burglar or someone unwanted.
Adjust the values of C2 & R6 to vary the tune. To get a better result for the security alarm circuit, it is highly recommended that you wire the circuit on a good quality PCB. A good quality common board may also do the trick.
We have a big collection of security and safety circuits on our website. If you like to build other circuits, try our sensitive intruder alarm circuit and also burglar alarm using an 8051 microcontroller. You can also try the burglar alarm circuit using Arduino.
Parts List – High Power Siren Alarm Circuit
|Resistors, R1 & R3||1/4W|
|Resistors, R2 & R5||1/4W|
|Electrolytic Capacitors, C1 & C2||25V|
|Polyester or Ceramic Capacitor, C3||63V|
|Electrolytic Capacitor, C4||25V|
|Transistors, Q1, Q2||BC557|
|Toggle or Slide Switch, SW1||SPST|
|Pushbutton Switch, P1||SPST|
|Speaker, SPKR||8 Ohms|
Siren Circuit Diagram
Security and safety circuits are always fun to build. During my childhood days, I used to enjoy building burglar alarms and giving to my friends. You see, that little joy of amazing your friends with circuit building skills. They treat you like a hero! Definitely those were good old days.
I hope you understood the concepts behind this circuit. You may try building other safety circuits as well.