Consider a situation in a bank or any organization where a sudden invasion by the burglars has taken place. Now all the staff has been handcuffed by the robbers and the main locker room full of valuable resources is at the mercy of the robbers. So is there any way to prevent this theft?? Yes, there is, if any buzzer is connected which can start ringing, alarming the local police or a camera fitted along with a GSM modem that can send the video to the police stations. But still the problem lies in the way to switching on these devices.
Consider another situation, when a person is at his room (in a hostel or a hotel) and a thief tries to enter the room at night when the room is dark and the owner is fast asleep. There needs to be a system for automatic switching on of the light and ringing of the buzzer alarm.

In both these situations, the solution lies in devising a way for automatic operation of a switch and to achieve this, one of the most efficient ways is a Sound Operating switch.

### Two ways to design your own sound operated switch

• Using a audio Amplifier and a timer
A basic sound operating switch can be constructed using an audio amplifier IC, a comparator, a 555 timer operating in monostable mode, a relay and a load. Here the basic idea is to change the output of the timer with input from the microphone, in order to switch on the load. The load can be a incandescent lamp or a LED lamp or a motor.

The audio signal is received by the microphone, which converts the audio signal to electrical signals. The signal is given to the pin2 of comparator IC 741. The other input pin 3 of the comparator is given by the reference voltage set by the potentiometer arrangement.
Simple Sound Operated Switch circuit

In absence of any audio signal, pin 2 is at logic high and the output of comparator is at logic low, giving a low signal to the trigger pin of the 555 timer. The output of the timer is thus at logic high, keeping the relay at off condition. When a sound is heard the microphone detects it and converts it to electrical signal and the signal is applied to pin2 of the comparator and since this pin is now at logic low, the comparator output is at logic high, triggering a logic high signal to the trigger pin of the timer. The timer output is thus at logic low, driving on the relay, which in turn switches on the load( A bulb) for a time determined by the RC combination.
• A Sound Operated Switch using Counter IC
You can use this circuit to operate a Relay through the sound of claps. It is highly sensitive and can detect the sound of the clap from a distance of 1-2 meters. AC loads such as lamps, fans etc can be connected through the relay. This circuit has three sections. A sensitive MIC amplifier, A Toggle switch based on IC CD4017 and a Relay driver. IC CD4017 is a decade counter where the output count number is shown by the corresponding output number pin going high.

Condenser Microphone picks up the sound vibrations and generates minute voltage across its terminals. These feeble signals are amplified by IC1. Resistor R1, R3 and variable resistor VR1 adjust the sensitivity of the amplifier. Resistor R1 set the sensitivity of Mic. The amplified output pulses from IC1 passes to the input of IC2 (CD 4017). Resistor R4 keeps the input (pin14) of IC2 low so as to prevent false triggering. IC2 is a decade counter IC which is wired as a toggle switch. That its outputs 1 and 2 (pins 2 and 3) becomes high and low alternately when the input pin14 receives pulses from IC1.  Pin4 (output4) is connected to the reset pin15 so that further counting will be inhibited. The high output from IC2 passes through the current limiter R6 to the base of switching transistor T1. When T1 conducts, Green LED and the Relay turns on. In the next clap, output pin 2 becomes low to switch off the relay and the Green LED. Red LED indicates the OFF position of the load.

In other words, when we first make a clapping sound, the audio signal gets converted by the microphone to electrical signal and amplified by the operational amplifier, which gives the output pulse. When the counter receives the first pulse the pin 2 goes high and the relay driver is switched on to energize the relay and thus switch on the LED. When we again make a clapping sound, the amplifier IC generates another pulse. This time the pin 2 goes low and pin 3 goes high, causing the red LED to glow. The relay is now switched off and the LED is in off condition.
Note: It is better to connect the Microphone directly on the PCB using two pieces of the trimmed leads of resistor or capacitor. If Microphone is connected with wires, sensitivity may be reduced. Enclosing the Microphone in a tube will increase the sensitivity considerably. Adjust VR1 to get maximum sensitivity and range.

### 4 Sound Operated switches available in the market

It is a sound operated electronic switch kit by Electro Kits. It operates on a 9V battery and uses a electrets microphone. It switches on and off an LED mounted on the kit.
• Automatic Voice Activated Switch
Its switching time is up to 60seconds and supports loads upto 3A-115V or 5A-12V
It is a microprocessor controlled switch operating on 1 or 2 claps. It has a relay power rating of 24VDC/AC 3A. It works on a power supply of 12V dc.

1382313 Model number Sound Operated Switch

It is a 1382313 model number sound operated switch operating on a power of 250W max with a supply voltage of 120V. It works in two modes. In Home mode, it can operate one or two appliances. In the Away mode, it will operate on any noise to switch on the connected appliance.

Now you have got an idea about how the sound activated switch is used if any queries on this topic or on the electrical and electronic projects leave the comments below.