Cellphone Triggered Car Amplifier Auto Mute Circuit

This article for Cellphone Triggered Car Amplifier Auto Mute Circuit show how to make one simple circuit.

When phone ring it turn OFF the car music by itself.

So no need to rush or search button to stop music when call is coming.

What is a Cellphone Triggered Car Amplifier Auto Mute Circuit:

The Cellphone Triggered Car Amplifier Auto Mute Circuit is one small electric setup.

It stops car music when phone ring inside the car.

Main idea of this circuit is to help when loud music make hard to hear or answer phone call.

It work by itself with no need to press any button.

When phone call come it mute car speaker and this makes things easy and better for user.

Circuit Working:

Parts List:

Let us see how this circuit help to do this job.

We know when mobile phone is ON and making or getting call it make some strong radio signal and we call it RF (radio frequency).

All phones give out this RF when calling even if signal strength is not same for all.

There is always some RF around phone even with rules in place.

This RF from phone can help us know if phone is working or not and we can use this signal to do switching using circuit.

RF Sniffer Part:

When someone make or get phone call inside car this circuit which is called RF sniffer can catch the signal and then it uses this to mute the music from car amplifier.

This circuit have two parts:

• One relay driver with BC547 transistor
• And RF sensor using two amplifiers A1 and A2

To catch even weak signal A1 and A2 are made strong and they are high gain amplifiers which are connected one after another.

There is one feedback resistor 2M on IC2 that control how strong the op-amp is.

If we increase this resistance and its sensitivity goes up and if we reduce it the sensitivity goes down.

These op-amps are very good at catching different RF signals in air.

We can adjust one 2.2M preset to set how sensitive the circuit is and depending how much RF is in the place.

It is important to set the sensitivity correct so it can only catch RF from phone and does not get confused by noise from car engine or other things.

We can also put many of these sensor units in different places in car and join their outputs to the main relay driver.

So even if phone is in backseat the signal is still caught.

With this way we keep each sensor not too sensitive but just enough to catch mobile RF and not the other noise.

How It Works:

So when phone call happen antenna in circuit catches the RF signal.

Then it turn that into DC signal with more power which is depending how strong phone signal is.

This strong signal goes to relay part which have some diodes and capacitors to clean the signal.

When the relay turns ON it send signal to mute pin of car amplifier and this make music stop for some time until the phone call is finished.

Formula:

Some formulas used for the Cellphone Triggered Car Amplifier Auto Mute Circuit:

We use this formula for inverting amplifier circuit:

Vout = -Iin × R3

where:

• Vout is output voltage from the amplifier
• Iin is input current going into amplifier
• R3 is resistor in the circuit with certain resistance

How this formula works:

This circuit take small current (Iin) and make it bigger and give it out as voltage (Vout).

The minus sign (-) in formula mean output voltage is reversed.

So if input current is positive then output voltage become negative.

This is how inverting amplifier behave.

How to Build:

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

Assemble the Parts:

• First collect all the parts as we can see in the circuit diagram above.

Circuit Connections:

• Put all parts on PCB (printed circuit board) using the diagram as our guide.
• Be careful while fixing IC, diode and transistor and place them correct way.

RF Sensor Stages A1 and A2:

• For RF sensor connect two op-amps LM358 one after another which is called A1 and A2.
• Now connect 2.2M preset resistor as feedback to control sensitivity.

Relay Driver with BC547:

• Use BC547 transistor to make relay driver stage.
• Connect the relays common pin to the car amplifiers mute terminal and connect relay coil to collector of transistor.

Adjusting Sensitivity:

• To change sensitivity turn the 2.2M preset.
• This helps remove unwanted RF signals and it only let phone signal trigger the mute.

Adding More RF Sniffers:

• If needed we can add more RF sensor units in different places in the car.
• Connect their output wires to the main relay circuit.

Power Supply:

• Give power to the circuit using correct power source.
• Be sure the voltage is safe for all parts.

Test and Adjust:

• Put circuit inside car and turn it ON and now try calling from a phone.
• Adjust the sensitivity to ensure circuit catches RF from phone and not from engine or other things.

Final Check:

• Before finishing check all wires again.
• Be sure there is no loose wire and the mute works every time call happens.

Important Tips:

• Diodes and electrolytic capacitors must be connected in correct direction.
• Check pins of ICs and transistors that they do not mix up.
• Try the circuit first on testing PCB before fixing permanent.

Conclusion:

This Cellphone Triggered Car Amplifier Auto Mute Circuit help to stop loud music in car when phone call comes.

It work without touching anything so it makes driving safe and easy.

References:

Datasheet IC LM358