Simple Siren circuits are an important part of electronic security devices nowadays. In this project, we will focus on a simple & low-cost transistor-based circuit with a high-power loudspeaker, which can be used for home security & as a warning system against intruders. The electronic siren circuit given here is based around a transistor pair Q1 & Q2 (2N2222 & 2N2907 respectively) connected as an astable multivibrator, which drives the speaker directly.
The main advantage of a simple siren circuit is that it provides round-the-clock constant protection against burglars & break-ins. Also, due to its compact design, its location can be changed easily & frequently.
Hardware Components
The following components are required to make the Simple Siren Circuit
| S.No | Component | Value | Qty |
|---|---|---|---|
| 1. | Breadboard | – | 1 |
| 2. | Battery | 9v | 1 |
| 3. | Connecting wires | – | 1 |
| 4. | Speaker | 8 ohm | 1 |
| 5. | Transistor (NPN, PNP) | 2N2222, 2N2907 | 1 |
| 6. | Resistors | 22k, 47k, 39k | 1,1,1 |
| 7. | Polar Capacitor | 22uF, 0.47uF | 1,1 |
2N2222 Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of 2N2222
2N29047 Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of 2N2907
Simple Siren Circuit
Working Explanation
This circuit produces a sound very similar to a siren. This is due to the generator part of the circuit, which is a combination of PNP (2N2907) and NPN (2N2222) transistors. Combined, the two transistors represent a self-triggering multivibrator.
When the circuit is switched on, the capacitor C1 charges slowly till it reaches its maximum voltage level. This increasing voltage results in a decreasing time constant at the R2/C2 junction, Which also results in an increased frequency of the multivibrator. On disconnecting the circuit from the power, the capacitor C1 discharges slowly resulting in a decreasing frequency cycle. The tone signal from the speaker will be an increasing or decreasing tone depending on whether the circuit is switched on or off.
Applications
- Commonly used in home security setups & monitoring systems.
- Industrial uses such as fire & substance danger alarms.
- Civil defense uses such as alarms for early warning systems.
- Usage in Emergency services such as police & ambulance sirens.
- Usually found in devices such as screamers, PA Systems & loudspeakers.
