autosensing input and output?

[ahgu]

autosensing voltage devider circuit

How to design a circuit with a connector, you can either connect a input signal or an output speaker to the SAME connector.

So when a user plugs in either a input signal, or a speaker to the same connector, I can autosense which, and do either input or output?

One idea I have is to use ADC, any other easier ways?

thanks
ahgu

 

[jourval]

It seems not too difficult.
Optocoupler with 2 antiparallel photodiodes or even simplest
relay coil can detect low impedance of speaker and make
all commutation needed. With high impedance input signal
it will be inactive - "default state". You'll have a bit nonlinear
distortion and power loss, but not too much, if speaker have not more than 4-8 Ohm

 

[ahgu]

still unclear

still unclear, how you can distinguish low impedance and high impedance using photo-relay, you mean low-impedance, LED will turn on and it will give my uController a signal?

 

[Aoxomox]

Hi all,

relay is in my opinion not a good idea. in any case you have a voltagedifference between your ground and signal pin of some volts. when connecting a low impedance speaker to it the current will flow through the moving coil. on the one hand a relay (or diode) will be enabled by the current, on the other the membran of the speaker will move (producing more distortion), if you connect or disconnect the speaker you will hear a plopp sound and if the supply of your circuit is derived from AC on not stable enough you will hear the 50/60Hz sound.


I would suggest taking a connector with a switch so that your micro can detect wether a plug is in or not. If no plug is in you can put about 1V with a relative high impedance to the life pin of your input jack. With a simple comparator and an input pin you monitor the state of the comparator. If a high impedance or an ac coupled impedance is connected the voltage of the input will not drop under a certain level.

If your speaker is connected it forms a voltage divider with the high impedance so this is almost shortcircuit.

This method has to disadvantages. I will not work with speakers with AC coupling and high Z input signals with DC Offset.

 

[jourval]

What about this straight-forward solution?

 

[ahgu]

comparator good idea

What is the chararcteristics of the the RCA jack recorder? when you switch between recoder and microphone input? I know mic output has low output impedance. Don't know what the RCA recorder is made of. probably high input impedance?

How is the possible in this case?

 

[jourval]

It will be better if describe your project more in deep.
1. Mic. is not out it is IN
2. RCA jack - is audio connector type, looks like stick with 2 0r 3 contacts
3. RCA jack recorder - I don't know what is it
4. Input impedance of mic. input vary for different microphone types,
from 15k to more then 100Meg , what mic you intend to use?
5. Speaker output impedance may be from 10 milliOhm to 10 Ohm
what is your output power and what is power amplifier VCC?

E.T.C

 

[ahgu]

details

Input: mic input, 700ohm electret mic.
output: tape cassette recorder so it can records whatever coming from the signal.

No speaker.


Want to switch between input and output.

ahgu

 

[jourval]

OK
It need different solution.
Principle will stay the same - to detect resistor value connected
in parallel to known resistor, but ...
electret as a rule have open drain FET as his output,
so it need a few hundred microamp DC current flow and
tape recorder have 10-100k input AC connected.
BTW, what it mean 700 Ohm? Do you have internal connection
circuilt of your microphone, as a rule you must connect external load resistor and take AC signal from this point. And this resistor can be
much bigger...
I don't see any effective solution just now, but I'll think
it must be possible.

 

[jourval]

One additional question:
Is your signal - stereo ?
if not - you can use stereo connector for mono signal
and connect second unused channel to the ground into plag only in one of connectors it is enough for separate them....

 

[ahgu]

input

consider the mic input is already amplfied and it is a output of an OPAMP with low output resistance with AC coupling. NOT a bare electret mic.

I cannot do any stereo plug because RCA jack is standard, I want to use any RCA plug/jack you can purchase. Not force uers to use certain plug.

so I need to detect 2 cases:
1. output to 10k AC resistance.
2. input from low-output resistance opamp.

 

[artem]

The idea was already mentioned .
If your microphone has internal resistance
for DC, give to the output small current which
is supplied from bipolar transistors Base
with sequentially connected resistor (defines test current value ) ,emitter has to be connected to the positive voltage
,if no microphone inserted , transistor is not opened and you can check it via its collector voltage . Its collector voltage will be sued to relay manipulation . When microphone will be connected , there will be current going thorugh base and trans wil be opened and switch relay to input .
If Microphone has to supplied through jack
starting currency could be very small , but when relay is triggered permanent supply voltage can be added .

 

[artem]

For AC coupling you should need supply some ultrasonic freqeuncy (which is out of your working sound ) and measure its value on jack . If your microphone has low AC resistance (connected thourhg capasitor )
freq level will be decreased . You can use bandpass filter to measure just this frequence
. If it is quite high (freq) filter design can be degraded to RC circuit . It is also possible to to make interval checks - test signal can be sent to the input/output with 1 sec interval to test input resistance - so continuos freq supplying can be avoided .