Simple Sound Activated Relay Circuit

This article shows you how to build a circuit that can turn things on and off with sound.

It uses a special part called a relay to control things that use more power, like lights or motors.

This could be useful for things like:

• Turning on a light when you clap
• Making a sound effects machine

We will show you exactly how to build it and how it works.

What is a Sound Activated Relay Circuit:

A sound activated relay circuit is an electronic circuit that activates a relay in response to 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:

Circuit working process is mentioned below:

Upon power up the circuit experiences a brief relay activation due to the presence of capacitor C2.

Adjust the on time period by modifying the microfarad value of C2.

A larger value extends the period, but it should not exceed 47μF.

R1 determines the sensitivity of the microphone MIC and the overall circuit.

Lower R1 values increase sensitivity while higher values decrease it.

The biasing resistor R1 influences the responsiveness to audio signals.

Experimentation is key to finding the optimal R1 value for effective audio signal response.

Electret microphones typically contain a central FET requiring a bias voltage.

R1 plays a crucial role in setting the appropriate bias for the microphone.

Formulas and Calculations:

Here 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.

The duration required for the voltage across the capacitor C2 to approach around 63.2% (1 – 1/e) of the voltage supplied to the resistor capacitor RC circuit is shown by the time constant (ton).

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 (rounded to two decimal places)

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

Note:

Make careful you utilize the same units of measurement throughout the computation (for example, farads for capacitors and ohms for resistors).

This formula works with simple relay controlled circuits.

In complicated circuits the actual time constant may vary somewhat because of resistance or stray capacitance for example.

How to Build:

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

• Design or obtain a PCB layout for the sound activated relay circuit.
• Transfer the layout onto the PCB using the appropriate method.
• Etch the PCB and drill holes for component placement.
• Place the components on the PCB according to the layout.
• Insert and solder resistors R1 and R2.
• Connect the electret microphone to the PCB using a shielded cable.
• Place and solder capacitor C1.
• Connect the base of the transistor Q1 to the junction of R1 and C1.
• Connect the collector of Q1 to the positive supply.
• Connect the emitter of Q1 to the negative supply.
• Connect diode D1 across the relay coil observing proper polarity.
• Connect the relay contacts to the desired AC or DC load.
• Connect the relay coil to the collector of Q1.
• Connect capacitor C2 between the collector of Q1 and the negative supply.
• Adjust the microfarad value of C2 to control the on time period of the supply.
• Connect an LED if required with a current limiting resistor in parallel with the relay coil for visual indication.
• Connect the power supply to the circuit ensuring the correct polarity.
• Ensure that the power supply voltage matches the relay coil voltage.

Safety Precautions

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

Testing:

• Upon powering up the circuit observe the brief relay activation caused by capacitor C2.
• Experiment with different R1 values to determine the optimal sensitivity for 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:

By following these instructions, you can successfully build a simple sound activated relay circuit.

Ensure accuracy in component placement, polarity and connections.

Experiment with resistor and capacitor values to achieve the desired sensitivity and relay activation duration.

Always prioritize safety precautions when dealing with electronic circuits especially when connecting to mains power.

References

DESIGN AND CONTRUCTION OF A SOUND ACTIVATED SWITCH

Sound operated relay