Switching between Microphones with IR proximity sensor

Thread Starter

extremee

Joined Mar 9, 2018
8
Hi all


I want to build circuit that can control which Microphone will be selected as input, with IR proximity sensor. I could use some kind of IR circuit like FC-51 (or if you have better idea) that has digital output low/high and use it as logic for mux. But i dont know which mux to use, or something other as a switch, that is very fast at switching without "poping" noise and that can handle alternating signal from mics. I found MAX4734 CMOS analog mux, that maybe could be used for this. Here is simple schematic of my idea, so if anyone have some ideas, i'm listening. Thanks all for help. :D

sema.jpg
 

sghioto

Joined Dec 31, 2017
2,373
Looks like it should all work as in the schematic. The only problem (or may not be a problem) I see is one of the mics will have priority over the other. Output on the FC 51 module is normally high so from the logic table mic 2 is normally on until an obstacle is detected.
SG
 

Thread Starter

extremee

Joined Mar 9, 2018
8
Yea i forgot about that, but it's not a big deal, a can switch mics (easier than putting not gate). My biggest problem is mux ic, will it work good enough, or if there is some better solution?
 

Veracohr

Joined Jan 3, 2011
757
That mux will cause a pop unless you include some kind of zero-cross detection circuitry to modify the control signal. Or you could use something like the circuit below, which creates a quick cross-fade if the control signals are slow.

Screen Shot 2018-03-09 at 5.15.33 PM.png


Here's the circuit that creates the control signals. You could use your sensor output signal in place of the hardware switch I put on the CLK input. For the sake of reducing clutter I omitted some DC offset parts on the opamps. I'll credit Douglas Self for this because that's where I learned it. He suggests driving the G-S voltage just slightly positive to be fully on, and that's what the DC offset is for.

Screen Shot 2018-03-09 at 5.23.42 PM.png
 

sghioto

Joined Dec 31, 2017
2,373
As far as a "pop" goes when switched on will depend on the type of mic used and the input of the amp. If using dynamic mics the common line from the MAX4734 going to the amp input just needs a resistor to gnd to maintain a load on the input of the amp.
SG
 

MisterBill2

Joined Jan 23, 2018
9,025
As long as there is no DC offset- you probably will not hear any clicks or pops.

Eliminating the offset is key
With the XLR connectors shown, these are probably balanced output and fairly low impedance microphones. So to avoid hum and noise problems you also need to switch both leads. A CD4066 quad analog switch will work, since it has 4 switches. Then, from each IR sensor, enable the two switches from one mic or the other. At that point there is no microphone selected by default, which may be useful. In addition, you can add any number of microphones, selecting them with the IR sensors, one for each microphone channel.
 

Veracohr

Joined Jan 3, 2011
757
You will get a pop any time you switch quickly between signals with different instantaneous levels. If the signals are low it may not be very obtrusive.
 

sghioto

Joined Dec 31, 2017
2,373
The SM58 is a 150 ohm balanced output mic. You would only need to switch both leads if the audio input of the transmitter is also balanced.
SG
 

Sensacell

Joined Jun 19, 2012
2,852
If there is significant signal level, the click will be swamped by the signal itself.
Clicks are most noticeable against quiet passages.
No signal - no difference, no click.
 

Thread Starter

extremee

Joined Mar 9, 2018
8
Mics will be 20-25cm(7.8 to 9.8 inches) apart, so they will pickup almost the same signal. It's mic setup for accordion, but when they are both on, there is some noise and they are a quiet(probably because signal from mics are not in sync, but have slight offset), so you need to boost gain up on transmitter, but then there is problem with audio feedback. That's why i need mux to switch between mics, so only one is on, based on hand position on accordion, that i can detect with IR obstacle sensor (upper and lower part). If hand is on upper part, activate upper mic, else lower.

Also, i have few more questions. If i use MAX4734, it has 4 inputs, if i use only two, do i need to ground other two so they don't float? And on that FC-51, can i use multiple ir diodes and ir receivers, or if someone have better circuit for that purpose?
 
Last edited:

sghioto

Joined Dec 31, 2017
2,373
Also, i have few more questions. If i use MAX4734, it has 4 inputs, if i use only two, do i need to ground other two so they don't float? And on that FC-51, can i use multiple ir diodes and ir receivers, or if someone have better circuit for that purpose?
Ground unused inputs. Not sure why you would need multiple ir diodes and receivers. You can connect the output of several FC-51 modules together through diodes to a single input if that's what you mean.
SG
 

MisterBill2

Joined Jan 23, 2018
9,025
The SM58 is a 150 ohm balanced output mic. You would only need to switch both leads if the audio input of the transmitter is also balanced.
SG
A balanced system that has one side tied to another microphone is no longer balanced. The main purpose of going to the effort of having a balanced system, with equal capacitance to ground and noise sources on each side, is to reduce noise pickup. THAT is the reason to have things balanced.So switching only one side will have the potential of adding noise pickup that would otherwise be cancelled.
 

sghioto

Joined Dec 31, 2017
2,373
If the input is single ended you have no choice. We don't know what the transmitter the TS is using at this point. Apparently the SM58 is very popular with musicians, it has an internal isolation/impedance matching transformer so you are not connecting mic elements together directly if one side needs to grounded.
SG
 

MisterBill2

Joined Jan 23, 2018
9,025
If the input is single ended you have no choice. We don't know what the transmitter the TS is using at this point. Apparently the SM58 is very popular with musicians, it has an internal isolation/impedance matching transformer so you are not connecting mic elements together directly if one side needs to grounded.
SG
I also own an SM58, and what I think you are wanting to do is select one or the other microphone going into a single input channel, instead of feeding them into two separate channels. I am guessing that you only have one input available for this application. So here is an alternate concept, which is to put the two microphones in series feeding the one input, and then also have two photo-resistive sensors, each sensor shunting one microphone. Then which ever sensor had it's light source blocked would have a high resistance and that microphone signal would get through. The benefit is that with no active electronics connected there could be no thump at all, and also that the switch would be very smooth because the light beam controlling the resistance would take several milliseconds instead of being almost instant. Some of those resistive light sensors go from a hundred ohms lighted to several thousand ohms in the dark. CLAIREX is the source that I recall made a line of those sensors.
 

Thread Starter

extremee

Joined Mar 9, 2018
8
Yes, transmitter only have one input channel, so i want to switch between mics.

That's not bad idea but that adds more resistance to mics circuit and also i don't know how it will work in dark, and hand will not cover sensor 100% so there will always be some light, and it also depends on ambient lighting. It is simpler, but also have its disadvantages.
 

MisterBill2

Joined Jan 23, 2018
9,025
Yes, transmitter only have one input channel, so i want to switch between mics.

That's not bad idea but that adds more resistance to mics circuit and also i don't know how it will work in dark, and hand will not cover sensor 100% so there will always be some light, and it also depends on ambient lighting. It is simpler, but also have its disadvantages.
The resistance would be across the microphones, not in series with them. And because of it being changes in resistance instead of on/off like a regular switch, it could also leave both microphones working if you chose. And there is still that advantage of being totally click free, no potential for clicks. so it is at least worth thinking about.
 
Top