Foot operated mute

[buggyboy]

Moving to using a lav has given us a great increase in room volume, but it has it's downsides. The sound guy must ride the mute on the lav as the service opens and the pastor stands and sits, speaks, sings, prays, etc. Some or which is supposed to go through the foh, and some which needs to be muted. This goes for the remote pa mix as well.

Also, the pastor must indicate to the sound guy if he has to cough, sneeze, etc.

So, My hope is to build a cough mute for the pastor. I know some wireless systems have built in mutes, and some even have mute buttons that can be placed at the wrist. But these mics are out of are range right now.

And the truth is I'd rather have a foot operated mute for ease of use anyway.


The way my pastor stands, the lav cable trails off to his right and connect to the stagebox. This keeps the cable away from his feet so he doesn't trip on it.

If I were to use a standard cough drop mute box, the lav cable would have to go down to the mute box, then over to the stagebox. This isn't ideal.

I think I've come up with a method to allow a mute swtich right near his feet, but allow the cable to stay the way it is.


A muting unit is placed between the end of the lav's cable and the stagebox. Then a line is run to the desired position of the floor switch. When the floor switch is activated, it sends current to an electromagnet at the mutebox which operates a reed switch and activates the mute.

At the same time a big red LED is lit on the foot switch to indicate a mute condition.

As a bonus, the foot switch is easy to replace if it gets smashed to bits.


So, questions...
1. Can phantom power supply enough power to operate a small electromagnet? Or must I use batteries.
2. Will the electromagnet cause any interference or noise in the mic cable?

I'm open to any other ideas for how to connect a remote foot switch to the mic cable.

thanks.

[buggyboy]

Please correct me if I got this wrong.

Phantom power provides:
48vdc into pins 2 & 3
0vdc to pin 1
It will supply around 14 milliamps


Question: How would one tap off dc power to run something? We want to make use of the phantom power, without disturbing the audio signal.

Question 2: The shure 839 has a battery compartment to allow operation without phantom power, and it has enough circuitry to automatically switch to the battery if phantom power fails. If we ignore the battery power, and also the switching circuit, can the lav capsule be connected directly to the xlr jack? Could a headset mic do the same?


I though of another way that might work for a remote foot switch. Could a microcontroller be rigged to sense the foot switch action, and operate the mute switch?
Would a transistor do the job? sensing current on one leg and connecting the other 2?

Would the audio suffer by being passed through a transistor or microcontroller?

[Tim Padrick]

"Popless mic mute switch" from Crown:

[buggyboy]

Thanks Tim,

The crown design is quite similar to the Pro Co cough drop I had found.


I think I've solved another of my questions myself. Passing audio through a transistor happens all the time in amps. So using a transistor instead of a physical switch should work fine. However, would the connection only work one way? So transistors behave like diodes?

Here is proof that phantom power can be used to power an led. http://www.proavmax.com/Horizon-Musi...or-pflblox.htm
And here is a description of how to do it. http://www.gearslutz.com/board/geeks...indicator.html
Basically they propose attaching resistors of equal value to the wires for pins 2 and 3. the out put from the resistors would be joined and provide the +v to the led. I presume pin 1 provides the Ground. Any thoughts?

P.S. must I always upload an image to an external host before posting it here?

Thanks

[buggyboy]

All the info I've been able to gather so far assembled into a diagram.


A two wire lead provides phantom power to a remote led and switch. with the switch open, no current flows and all 48v should be available to the mic. Close the switch to activate the mute and also complete the led circuit.

Two transistors are shown, if the transistor bridge only works in one direction, two may be needed. If one will do the job, I'd only use one.


Any thoughts?

[Tim Padrick]

Why involve semiconductors when you can do it with passive components?

[buggyboy]

What passive components do you have in mind that will allow the switch to be located away from the mute box?

[Frank DeWitt]

Pin 2 and 3 are at exactly the same potential so there is no voltage or current to light the LED or turn on the transistors.

Frank

[buggyboy]

I'll preface all my ideas with the fact that I'm just a novice here.

My thought was that pins 2 & 3 are joined (after resistors, typically 6.8kohm) and together they provide voltage (P48v) for use.

I know I'm missing some components (I'm posting in other forums to figure out what is needed) but that voltage and current is then applied to the base of the transistor(s) to saturate them and allow a route to open which bypasses the resistor so pin 2 & 3 are connected by a capacitor.

Obviously 48v would smoke a transistor, so some parts are needed to step the voltage (and current?) down.

As always, I appreciate any ideas.

[Frank DeWitt]

Pin 2 and 3 both have 48 volts DC with respect to Pin 1.

The DC voltage between pin 2 and pin 3 is zero volts.

[buggyboy]

I suspect the answer to this part of the project is in this page somewhere: http://www.rason.org/Projects/transwit/transwit.htm

But I don't really understand what I'm reading.

The transistor needs current passing from base to emitter. So tapping off some phantom power and feeding it the the base makes sense... but that current flows out the emitter (which would be on pin 2 or 3) so, yeah... taking current from

pin 2 --> base --> emitter --> pin 2 (3)

doesn't make sense. There is no voltage difference, so no current would flow.

This idea may not be possible... but something must exist that can connect pins 2 & 3 based on an external signal.

Any ideas what might do that?

[Gracetech]

I have no idea why you would put a mute in the hands(or in this case feet) of the pastor but i'll see if i can simplify things here.

First off don't tamper with the Phantom power. If you don't cause issues with the lapel then you may with the mixer. All you need is a triple pole/ double throw latching push button switch. connect - and + to 2 of the poles and then have a power circuit attached to the extra pole on the secondary throw. When you click the button the mic line is broken and the power circuit completed.

crt