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how device detects that headphone is connected

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mdanh2002

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If I connect a headphone to my radio headphone's jack, the radio detects that a headphone is connected and mute the built-in speaker. Similarly if I connect a headphone to my laptop, Windows reports the same thing.

Just curious how the detection is being done? I am pretty sure it does not detect the resistance of the headphone alone because once when I am trying to connect a jack alone with open wires (no actual speaker), it is still detected. I open my radio, observe the headphone jack and do not notice any obvious mechanical switch that gets activated when something is connected to the jack.
 

audioguru

Well-Known Member
Most Helpful Member
A jack with a switch in it is common on radios.
a switched mono jack has 3 pins: headphones, switched contact for the speaker and common.
 

mdanh2002

Member
A jack with a switch in it is common on radios.
a switched mono jack has 3 pins: headphones, switched contact for the speaker and common.
By 3-pin mono jack are you referring to the following? Most of the cheap headphones purchased from department stores look similiar. So they are all mono? I use them to listen to music all the time and the quality is still good. Or is it because the switched contact has been removed to have enough pins for stereo sound?



Am I right to say that the following jack is stereo? It has 4 pins. The headphone that comes with my phone is of this type.



I even have a headphone whose jack is similiar to this. It has only 2 pins



So can I say that only 4-pins jack can be stereo? How about the 2-pin jack, how can it get the radio to detect its presence even though it has fewer than the number of pins required?

Sorry for the basic questions, just curious :)
 

Nigel Goodwin

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Most Helpful Member
These are all plugs, plugs don't have switches - it's the sockets that have the switches.

The first one (three pole) is stereo, the last one (two pole) is mono, and the middle one (four pole) isn't a standard connector - sometimes it's used for stereo plus a remote control signal, or for anything the designer wishes to use it for that requires four connections.
 

audioguru

Well-Known Member
Most Helpful Member
Most of your photos show plugs, not jacks.
The white plug with 3 contacts is for stereo. Its switched jack has 5 pins.
The black plug with 2 contacts is for mono. its switched jack has 3 pins.

Your headphones 4 contacts plug might be L, R, microphone and common. Or it might be two contacts for the left ear and two more contacts for the right ear.
An ohm-meter will make each ear click when connected to the proper terminals.
 

mdanh2002

Member
Thanks nigel and audioguru for the clear details :) I have been confusing the 2 terms, 'plug' and 'jack' :)

The last plug having 2 poles (mono) is used in my 5-meter long headphone purchased 6 months ago. The bilingual label (English & Chinese) on its packaging says "high quality stereo headphone". It indeed has 2 earpieces, but maybe connected to the same contacts. Now I know I have been cheated!
 
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