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DIY Optogate
Old 2nd November 2019
  #1
Here for the gear
 

DIY Optogate

I need something like an Optogate PB-05 to use with one of the bands that I am playing with, but I am trying figure out if there is a way I could possibly build one for significantly less money. I ran across these on Amazon:

https://www.amazon.com/FTCBlock-Infr...%2C176&sr=8-11

Could something like this be used to develop a similar type of unit? I understand that the voltage would need to be dropped from 48v to 5v if I was using phantom to power the unit.

I appreciate any ideas/feedback. Thanks!
Old 2nd November 2019
  #2
I think the moderator should move this to the Geekslutz forum where most of the folks who can help you tend to hang out.

There are three elements you need to get right.

Sensing

You've already found a plausible IR proximity sensor, but you need to get an actual data sheet for it to find out how much current it draws.

Mic muting

There are lots of DIY mute switch schematics you can study if you search online. Note that most of them work only with dynamic mics; hopefully that's ok. The basic idea is that you put equal resistors in series with pins 2 and 3, then short across them with a switch. "Pop free" designs usually have a capacitor in series with the switch, and a high value (100k or more) bleed resistor across the switch so that the cap has the same dc voltage in either state.

You want to switch to be electronically actuated, and since you're running the circuit from phantom power, an isolated switch is necessary. A good choice might be the On Semiconductor H11F1 optically-isolated FET. This has an on-resistance of about 200 ohms, so you'd need to use 3k series resistors to get 30 dB of signal reduction. With that source impedance, you'll want to use a low capacitance mic cable and keep it as short as possible.

Getting the specified 200 ohms shunt resistance requires 16 mA of LED drive current, which is quite a lot. That leads us naturally to the next topic.

Power

The circuitry envisioned needs way too much current to use a linear regulator; you'll need a switch mode step-down converter to get enough power out of a phantom power connection. Most of the high voltage ones I saw on Amazon were based on the LM2596 and that part draws too much current to work from phantom power. Something based on the Monolithic Power MP4568 or the Linear Technology (now ADI) LTC3255 would be ideal, but I'm not finding commercial modules incorporating those chips. Both manufacturers offer demo boards, but they are quite expensive.

I see a lot of dc-dc converter modules based on the MP1584, but these are not rated for 48V. But since the source impedance of a P48 output is 3.4k (pin 2 and pin 3 in parallel) the voltage will drop to a lower value depending on the load. What you need to do is figure out what your minimum 5V current consumption is going to be, and then translate that into current at 48V. A reasonable estimate is:

Supply Current = (5/48) * Circuit Current / 0.75

The voltage seen at the input of the DC-DC converter will then be

Vin = 48 - (3400 * Supply Current) [express the current in Amps]

Switch mode converters often have difficulty starting up from a resistive supply, so it would be good to put a large capacitor (50V minimum) across the input.

Good luck with your project!

David L. Rick

Last edited by David Rick; 2nd November 2019 at 08:09 PM.. Reason: typos
Old 5th November 2019
  #3
Here for the gear
 

David--Thank you for your amazing reply! Please pardon my delay in getting back on this--I have been traveling, and just now catching up.

As I guessed, this project is going to be a little more involved than what it seemed like on the surface. That said, I am looking forward to taking your notes and diving into it. I anticipate that this is going to be a bit of a learning experience for me, which I am looking forward to.

And, yes, the moderators can feel free to move it into a more relevant forum. I didn't see the Geekslutz forum until about 30 seconds after I posted this.
Old 2 weeks ago
  #4
Here for the gear
 

Hello, Jbarlow and David.

I was considering the same approach for a need I have to a live show I'm handling monitors for.

I have 4 musicians with talkback microphones to talk to me at monitors or each other on stage, since they are all using inears.

Although two of them are able to press a pedal to open and close the microphone, the other two are not able to do it while playing.

My first approach was to use an IR proximity sensor that would either close a relay or an optocoupler in order to have the signal go through.

I was afraid that phantom power wouldn't be able to have enough current to be able to open a relay, but I'm a bit rusty in my electronics theory classes from 20 years ago.

I'd be very appreciated if you'd like to board on this quest with me in order to find a diy solution, since the Optogate PB-05 is way over the budget for the purpose we require this for.

David seems to have a well established thought of how to accomplish this in a effective way and I would love to dig more into this.

I haven't considered shunting the pins and I do need to get my maths and electronics theory refreshed.

Do you mind keeping me in the loop of new developments on this idea?

Thank you.
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