Cellphone Triggered Car Amplifier Auto Mute Circuit

This article explains how to construct a circuit that will automatically turn off the cars audio when the phone rings.

In this manner one wont have to fumble to switch off the music while taking calls.

What is a Cellphone Triggered Car Amplifier Auto Mute Circuit:

The Cellphone Triggered Car Amplifier Auto Mute Circuit is an electrical circuit that automatically mutes the music being played through the amplifier when it senses a telephone call inside the car.

This circuits main objective is to provide a workable solution in situations when playing loud music might make a phone call difficult to pick up.

By avoiding the need for a human touch an automatic muting of the automobile speaker during a phone conversation enhances the user experience

Circuit Working:

Parts List:

Let us see how the circuit described above are used to provide these feature.

It is well known that when a smartphone is turned on for an incoming or outgoing call it generates a significant amount of radio frequency (RF).

Even though the amounts of RF produced by different cellphones might differ but there is always some radiofrequency (RF) present around them despite of any regulations.

In addition of being used for essential switching operations through connected circuits a cellphones radiated radio frequency (RF) makes it simple to figure out its working condition.

The RF Sniffer: When a mobile phone call is received or made inside the specified region the circuit above shows a simple RF sniffer or detector circuit that can be added to mute the car amplifier.

The design consists of two stages as shown in the following diagram: a relay driver stage with the thriving BC547 driver stage and the RF sensor A1 and A2.

To get maximum sensitivity A1 and A2 are set up as high gain amplifiers connected in series.

The op amps strength or sensitivity level is controlled by the feedback resistor 2M IC2.

Sensitivity rises when this resistance is increased and vice versa.

These operational amplifiers are perfect for picking up different radio frequency signals in the area.

Depending on the available RF strength in the environment the 2.2M preset is adjusted to set the sensitivity.

The sensitivity has to be adjusted to make sure the sensor detects just cellular radio frequency (RF) and not interference from the vehicles ignition system as there may be other disturbances in the car than RF.

With their outputs connected to the main relay driver many units can be installed in various interior automobile corners.

With this configuration the receiver can pick up radio frequencies from wherever in the vehicle including cell phones in the backseat.

With this method individual muting circuits are guaranteed to have low sensitivity detecting just radio frequency from cellphones and ignoring other misleading noises.

Going back to the described automobile amplifier mute circuit the antenna quickly detects the RF when a phone call is made and transforms it into an amplified DC signal that changes according to the emission levels of the phone.

The relay stage is driven by a network of diodes and capacitors that correctly filter the amplified output across A2.

When the relay clicks the automobile amplifiers mute terminals are activated temporarily turning off the music until the user ends the conversation.

Formula:

Below formulas are used for Cellphone Triggered Car Amplifier Auto Mute Circuit:

The following formula describes the relationship between input current (Iin), output voltage Vout and resistance R3 in a simple inverting amplifier circuit:

V_out = -I_in * R3

where,

• The voltage generated at the output of the amplifier circuit is shown by Vout.
• Iin represents the input current entering the amplifier circuit.
• R3 is an example of the resistance of a specific resistor in the amplifiers circuit.

How the formula works:

The amplifier circuit increases the input current Iin and outputs the amplified voltage Vout.

When the output voltage has a negative sign (-) it gets reversed.

In other words if the input current is positive the output voltage will be negative and vice versa.

How to Build:

To build a Cellphone Triggered Car Amplifier Auto Mute Circuit following are the connections steps:

Assemble the parts:

• Gather every component that is required as shown by the above circuit diagram.

Connections in Circuits:

• Connect the components on a PCB using the included schematic diagram as a guide.
• Take care when placing the IC, diodes and transistors.

Stages A1 and A2 of the RF Sensor:

• For the RF sensor stage connect the Op Amps LM358 in series as A1 and A2.
• To find the sensitivity connect the 2.2M preset feedback resistor.

BC547 Relay Driver Stage:

• Use a BC547 transistor to set up the relay driver stage.
• Connect the relays common contact to the automobile amplifiers mute terminals and the relay to the transistors collector.

Changing the Sensitivity:

• The sensitivity level is controlled by adjusting the 2.2M setting.
• It helps in removing unwanted radio frequency transmissions.

Further RF Sniffers:

• If necessary use numerous devices to replicate the RF sensor stage at various interior automobile corners.
• Connect their outputs to the relay drivers main circuit.

Source of Power:

• Connect the circuit to a proper power source.
• Make sure it falls within the components specified voltage range.

Examine and Improve:

• Inside the automobile turn on the circuit and test it by making a phone call.
• To improve the circuits functionality and ensure that it correctly detects smartphone RF signals adjust the sensitivity and default setting.

Last Verification:

• Verify every connection one more time to be sure no wires are loose.
• Check to see if the circuit regularly mutes the vehicle amplifier when a phone call is made.

Crucial Information:

• When connecting diodes and electrolytic capacitors be careful when it comes to polarity.
• Verify component pin configurations particularly for ICs and transistors.
• It is advised to test on a PCB before switching to a permanent circuit.

Conclusion:

In order to control audio playback in a car without using your hands the Cellphone Triggered Car Amplifier Auto Mute Circuit makes sure that phone calls take advantage over loud music.

It improves user safety and convenience by avoiding distractions from clashing audio sources.

References:

Datasheet IC LM358