# Add line in to electret microphone and mute the microphone when plug is connected

I have a megaphone with an electret microphone. I want to modify the megaphone so that I can optionally conntect a notebook's headphone out to the megaphone. So I want to add a line in connector to the megaphone, but at the same time keep the electret microphone.

The electret microphone operates at 3.1V. I found a 20 dB PAD for line to electret microphone input circuit which is working well if I completely remove the microphone.

I have a 3.5mm audio jack connector (mono) with a switch. The switch closes when an audio jack is inserted into the connector. I want to use that switch to "activate" the line in and mute the electret microphone.

What I tried first is to short-circuit the microphone with a transistor when the switch is closed:

C1 = 10 uF
R1 = 10 kOhm
R2 = 1 kOhm
T1: NPN transistor
M1: Electret microphone


This in almost working as I want it to. But there are two problems:

1. The microphone is not completely muted, it's just turned very quiet. I want it to be completely muted.
2. I'm not sure if short-circuiting the microphone without any load will drain the battery or worse will damage the microphone amplifier. The microphone has a resistance of about 1 kOhm.

My next idea was to use a transistor and an inverter to cut off the microphone when the switch is closed:

However I don't know which part or circuit to use as an inverter.

How could I solve this problem? Is there an inverter I could use? Or another circuit design? Thank you!