Simple Sound Activated Relay Circuit

This article explains how to create a circuit that uses sound to switch a gadget on and off.

Relays are specially designed parts that regulate power hungry devices like lights and motors.

The following are some ways that these can be useful:

• As soon as you clap a light turns on and
• Building a sound effects machine

This article will teach how to construct and run this kind of circuit.

What is a Sound Activated Relay Circuit:

A sound activated relay circuit is an electronic circuit that activates a relay in response to a sound or audio signals.

Relays are electromechanical switches that can control the flow of electrical current based on an external input.

In a sound activated relay circuit the relay is triggered by sound making it useful for applications such as sound activated switches, voice activated devices or other projects where you want an electrical response to sound.

Circuit Working:

Parts List:

The presence of capacitor C2 causes a short term relay activation when the circuit is powered on.

Modify the microfarad value of C2 to change the on time duration.

The period is extended by a higher value however it should not be higher than 47μF.

The sensitivity of the entire circuit and the microphone MIC is calculated by R1.

While larger R1 values reduce sensitivity lower ones increase it.

The ability to respond to audio signals is impacted by the biasing resistor R1.

The best R1 value for perfect audio signal response is just found through experimentation.

A bias voltage is necessary for the central FET found in most electret microphones.

When it comes to adjusting the microphones bias R1 is important.

Formulas and Calculations:

Below is how to calculate the formula working and calculations for resistor R2 and capacitor C2:

t_on = R₂ × C₂

here,

• ton represents the time constant in seconds.
• R2 represents the resistance in ohms Ω.
• C2 represents the capacitance in farads F.

According to the time constant (ton) it will take approximately 63.2% (1 – 1/e) of the power applied to the resistor capacitor RC circuit for the voltage across the capacitor C2 to achieve that level.

Values:

• R2 = 47kΩ (kilo-ohms) = 47,000Ω (converted to ohms)
• C2 = 1uF (microfarads) = 1 x ^10-6 F (converted to farads)

Calculation:

ton = 47,000Ω × 1 x ^10-6 F ton = 47 x ^10-3 seconds

Therefore, the time constant (ton) for the circuit with R2 = 47kΩ and C2 = 1uF is approximately 47 milliseconds (ms).

Note:

Be careful to use the same units of measurement throughout the calculations

This formula works with simple relay controlled circuits.

For example resistance or stray capacitance can cause the real time constant in complex circuits to vary slightly.

How to Build:

To build a simple sound activated relay circuit connections details are mentioned below:

• Design a PCB layout for the sound activated relay circuit.
• Transfer the layout onto the PCB using the right method.
• Place the components on the PCB according to the layout.
• Put resistors R1 and R2 in place and solder them.
• Use a shielded cable to connect the electret microphone to the PCB.
• Put capacitor C1 in place and solder it.
• Connect the base of the transistor Q1 to the junction of R1 and C1.
• Wire the positive supply to transistor Q1s collector pin.
• Connect the negative supply to transistor Q1s emitter pin.
• Keeping the correct polarity in mind connect diode D1 across the relay coil.
• Apply the required AC or DC load to the relay connections.
• Connect transistor Q1s collector pin to the relay coil.
• Join capacitor C2 between the negative supply and Q1s transistor collector pin.
• Modify the C2 microfarad value to regulate the supplies on time duration.
• For visual indication connect an LED if necessary in parallel with the relay coil and a current limiting resistor.
• Ensure that the power supply is connected to the circuit with the proper polarity.
• Verify that the relay coil voltage and the power supply voltage are same.

Safety Precautions

• Follow electronic protection measures especially when connecting mains AC powered loads.

Testing:

• While powering up the circuit observe the brief relay activation caused by capacitor C2.
• Experiment with different R1 values to calculate the maximum response from the microphone and the circuit.
• Lower R1 values enhance sensitivity while higher values reduce it.
• Connect the relay contacts to the desired AC or DC load.
• The relay will switch the load on and off in response to the activation and deactivation of the relay.
• Modify the microfarad value of C2 to adjust the on time period of the relay.
• Exercise caution not to exceed 47μF to prevent unwanted circuit behavior.

Conclusion:

One can effectively construct a basic sound activated relay circuit by following these steps.

Make sure that the connections, polarity and component placement are all accurate.

To get the correct sensitivity and relay activation duration experiment with the settings of the resistor and capacitor.

When working with electronic circuits especially when connecting to the mains always take safety precautions.

References:

DESIGN AND CONTRUCTION OF A SOUND ACTIVATED SWITCH

Sound operated relay