Sound Activated Relay Circuit

This article show how to make one Sound Activated Relay Circuit.

It uses sound for turning device ON and OFF.

Relays are special parts and they control big power things like lights and motors.

Here are some ways to use it:

• When with clap hand the light turn ON.
• It can also make sound effect machine.

This article tell how to build and use this type of circuit. of circuit.

What is a Sound Activated Relay Circuit:

Sound activated relay circuit is one type of electronic circuit which turn ON the relay when it hears sound or audio signal.

Relay is electromechanical switch and can control electric current when it gets signal from outside.

In this circuit the sound makes relay work.

This type of circuit are useful for things like sound switch, voice control device or any project where sound make something happen with electricity.

Circuit Working:

Parts List:

Capacitor C2 makes relay turn ON for short time when power come ON.

We can change how long relay stay ON by changing C2 microfarad value.

Bigger value will give more time but not more than 47μF.

The circuits sensitivity and how MIC hear sound is set by resistor R1.

Big R1 value make less sensitivity but small R1 makes more sensitive.

How good circuit hear sound depend on R1.

To get best R1 value we need to try different ones and see what work best.

Most electret microphones have small FET inside and this FET need bias voltage.

To give this bias voltage R1 resistor is very important.

Formulas and Calculations:

How to do calculation for R2 and C2:

Formula is:

ton = R2 × C2

where:

• ton is time in seconds.
• R2 is resistor in ohms Ω.
• C2 is capacitor in farads F.

Time constant (ton) means how long it take for capacitor C2 to charge around 63.2% of supply voltage.

This happen in RC resistor-capacitor circuit when power is ON.

Example values:

R2 = 47kΩ → this is 47,000Ω

C2 = 1uF → this is 1 x 10⁻⁶ F

Now calculation:

ton = 47,000 × 1 x 10⁻⁶ = 47 x 10⁻³ seconds

So ton = 47 milliseconds (ms)

Important:

Always use same units like ohms, farads and seconds when we do this calculation.

This formula is good for simple relay circuits but with some extra resistance or wires may also affect the result a bit.

How to Build:

To build a Sound Activated Relay Circuit connections details are mentioned below:

PCB Design and Making:

• First design the circuit layout for PCB (printed circuit board).
• Use proper method like toner transfer or UV method to print design on copper board.
• Etch the board to remove extra copper and clean it.

Placing and Soldering Parts:

• Put all components on PCB as per the circuit diagram.
• Place resistor R1 and R2 in right place and solder them.
• Use shielded wire to connect the electret microphone (MIC) to PCB and helps reduce the noise.
• Place capacitor C1 on board and solder it.
• Connect base of transistor Q1 to the point where R1 and C1 meet.

Wiring the Transistor and Relay:

• Connect collector of Q1 to positive supply through relay coil.
• Connect emitter of Q1 to negative supply of ground.
• Put diode D1 across relay coil and be careful with diode direction means polarity.
• Add capacitor C2 between collector of Q1 and negative supply.

Load and Power Connections:

• Connect AC or DC load to the relay output terminals.
• We can also add LED and current limit resistor across relay coil to show its working.
• Check the relay coil voltage should be same as power supply voltage.
• Always connect power supply with right polarity.

Safety:

• Be careful when using AC power and use insulation and avoid touching live wires.
• Follow basic electronic safety rules.

Testing the Circuit:

• Power ON the circuit and we will see relay click ON for short time and this happen because of capacitor C2.
• Try different values of R1 like Lower R1 → More sensitive to sound Higher R1 → Less sensitive
• Connect the load like lamp, fan, etc. to the relay.
• It will turn ON and OFF when circuit hears the sound.
• Try changing C2 value to adjust how long relay stays ON.
• Bigger C2 = longer ON time
• But do not use more than 47μF or circuit may behave bad.

Conclusion:

We can build a easy and Sound Activated Relay Circuit if we follow all above steps carefully.

Be sure all wires are connected right and parts are put in right place and polarity is correct.

To get good sound sensitivity and right relay ON time try changing resistor and capacitor values.

When working with circuits especially mains power always be careful and follow safety rules.

References:

DESIGN AND CONTRUCTION OF A SOUND ACTIVATED SWITCH

Sound operated relay