Cellphone Triggered Car Amplifier Auto Mute Circuit

This post shows you how to build a circuit that automatically mutes your car stereo when your phone rings.

This way, you can answer your call without scrambling to turn down the music.

What is a Cellphone Triggered Car Amplifier Auto Mute Circuit:

The Cellphone Triggered automobile Amplifier Auto Mute Circuit is an electrical circuit that, upon detection of a cellphone call inside the automobile, automatically mutes the music being played via the amplifier.

The purpose of this circuit is to offer a practical remedy in scenarios where loud music might impede the intelligibility of a phone conversation.

The user experience is enhanced by the automated muting of the car speaker during a phone conversation, which removes the need for manual intervention.

Circuit Working:

Parts List:

Let us explore how this functionality can be achieved using the circuit explained above.

It is known that cellphones emit a significant amount of radio frequency RF when activated during incoming or outgoing calls.

Although the emitted RF levels may vary among different cellphones some degree of RF is always present around active cellphones regardless of restrictions.

The emitted RF from a cellphone makes it easy to detect its operational status and can be effectively utilized for relevant toggling functions through connected circuitry.

The RF Sniffer: The circuit above illustrates a basic RF sniffer or detector circuit that can be incorporated for muting the vehicle amplifier when a cellphone call is received or dialed within the designated area.

Referring to the diagram above, the design comprises two stages: the RF sensor A1 and A2 and a relay driver stage featuring the subsequent BC547 driver stage.

A1 and A2 are configured as high gain amplifiers connected in series to achieve maximum sensitivity.

The feedback resistor 2M IC2 determines the gain or sensitivity level of the op amps.

Increasing this resistor increases sensitivity and vice versa.

These op amps are ideally suited for detecting various RF signals in the vicinity.

The sensitivity can be set by adjusting the 2.2M preset based on the available RF level in the surroundings.

In a car, various disturbances may exist apart from cellphone RF, so the sensitivity must be optimized to ensure the sensor picks up only cellphone RF and not interference from the vehicles ignition system.

Multiple units can be placed at different corners of the car interior with their outputs integrated into the main relay driver.

This setup allows the receiver to detect RF from all areas inside the car including cellphones in the rear seats.

This approach ensures that individual muting circuits have minimal sensitivity detecting only cellphone RF and disregarding other spurious disturbances.

Returning to the proposed car amplifier mute circuit, when a cellphone call is initiated the circuits antenna promptly detects the RF converting it into an amplified DC signal that fluctuates based on the cellphones emission levels.

The amplified output across A2 is appropriately filtered by the associated diode and capacitor network driving the relay stage.

The relay clicks activating the mute terminals of the car amplifier and temporarily silencing the music until the call is concluded by the user.

Formula:

The connection between input current (Iin), output voltage Vout, and resistance R3 in a basic inverting amplifier circuit is expressed by the formula:

V_out = -I_in * R3

where,

• V_out: This is an indication of the voltage produced at the amplifier circuits output.
• The notation Vout in this instance denotes an inverted output, which means that the input current and output voltage are of different polarities.
• Iin: The input current that is entering the amplifier circuit is represented by this.
• R3: This is an illustration of the resistance of a particular resistor R3 in the circuit of the amplifier.

How the formula functions:

The input current Iin is amplified by the amplifier circuit, which then outputs the amplified voltage Vout.

The output voltage is inverted when it has a negative sign (-).

Put more simply, a positive input current will result in a negative output voltage and vice versa.

How to Build:

Building the proposed circuit for automatic car amplifier muting when a cellphone call is detected involves assembling the components and following the schematic diagram provided.

Gather Components:

• Collect all the necessary components mentioned in the diagram.

Circuit Connections:

• Refer to the schematic diagram provided and connect the components on a PCB.
• Be careful with the orientation of diodes, transistors and the IC.

RF Sensor Stage A1 and A2:

• Connect the Op Amps LM358 in series as A1 and A2 for the RF sensor stage.
• Connect the feedback resistor 2.2M preset to determine sensitivity.

Relay Driver Stage BC547:

• Set up the relay driver stage using a transistor BC547.
• Connect the relay to the transistors collector, and connect the relays common contact to the car amplifiers mute terminals.

Adjusting Sensitivity:

• Adjust the 2.2M preset to set the sensitivity level.
• This helps in filtering out unwanted RF signals.

Additional RF Sniffers:

• If needed, replicate the RF sensor stage at different corners of the car interior using multiple units.
• Integrate their outputs into the main relay driver.

Power Supply:

• Connect the circuit to a suitable power supply.
• Ensure it is within the specified voltage range for the components.

Test and Optimize:

• Power up the circuit and test it with a cellphone call inside the car.
• Adjust the sensitivity and preset settings to optimize the circuits performance ensuring it detects cellphone RF signals accurately.

Final Check:

• Double check all connections, and ensure there are no loose wires.
• Verify that the circuit reliably mutes the car amplifier when a cellphone call is initiated.

Important Notes:

• Be cautious with polarity when connecting diodes and electrolytic capacitors.
• Double check the pin configurations of components especially transistors and ICs.
• It is recommended to use a PCB for testing before moving to a permanent circuit.

Conclusion:

The Cellphone Triggered Car Amplifier Auto Mute Circuit provides a hands free solution for managing audio playback in a vehicle ensuring that phone calls take precedence over loud music.

It enhances user convenience and safety by preventing distractions caused by conflicting audio outputs.

References:

Datasheet IC LM358