A simple sound activated alarm circuit is a device which is used for triggering an alarm on detection of a sound vibration. The sensitivity of the unit is set externally according to the requirement of the user.
The circuit discussed in this article can be implemented for the above purpose or simply as a security device for detecting an intrusion. For example it can be fitted in a car for detecting a possible intrusion or a break-in.
Looking at the circuit diagram we see that the circuit uses only transistors and therefore becomes very easy even for a new hobbyist to understand and make the system at home.
How the Circuit Works
Basically the whole circuit is made up of two small signal amplifiers which are connected in series for doubling the sensing power.
T1, T2 along with the associated resistors becomes the first small signal amplifier stage.
The introduction of the 100K resistor across the emitter of T2 and the base of T1 plays an important role in making the amplifier stage very stable due to the feedback loop connected from the output to the input of the stage.
The input of T2 is connected to a piezo transducer element, which is used as a sensor here.
Sound signals hitting the piezo transducer surface is effectively converted to tiny electrical pulses which are amplified by the amplifiers made from T1 and T2 to a certain higher level.
This amplified signal which becomes available at the collector of T2, is fed to the base of a high gain PNP transistor T3 via the 47uF coupling capacitor.
T3 further amplifiers the signals to yet higher levels.
However, the signals are still not strong enough and won't detect the minute sound vibrations, probably which might be emitted by human physical contacts over a particular body.
The next stage which is a replica of the first stage, consists of the transistor T4 and T5.
The amplified signals generated at the collector of T3 is further coupled to the above stage for the final processing.
T4 and T5 makes sure that the signals are amplified to the required limits as per the units expectations.
If the piezo is attached to, say for example a door, even a slight knock over the door will be easily sensed and the alarm connected to T5 will become active.
The 10uF capacitor across the 10K preset keeps the alarm activated for a few seconds of time, its value may be increased for increasing the above delay of the alarm sound.
The discussed sound activated alarm circuit will work with any supply in between 6 and 12, however if the alarm is a powerful one, the current might have to be selected accordingly.
The preset may be used for setting the sensitivity of the circuit.
For the sensor, a 27mm piezo transducer will work the best, the following figure shows the image of this device: