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Circuit for second PTT that will mute Mic but allow line-in...

Rob French (KC4UPR)
 

Hello!

So I want to have a separate line-in for my uBITX, separate from the Mic input.  I also want to have a separate PTT.  The intent is to have the regular mic/PTT plugged into the front of the radio, and then also have the computer hooked up in the back.  

Requirements/Constraints/Desirements:
- Always keep the computer plugged in.
- Always keep the mic plugged in.
- If the mic PTT is pressed, don't transmit computer audio.
- If the computer PTT is asserted, don't transmit mic audio.
- Don't require a mechanical switch between the inputs (I have a fairly compact case and don't have room for more switches).
- Don't require a relay*. (* This is not a hard requirement, but I'd rather not use a relay if I can help it.)
- Don't require use of any Arduino pins (I'm using parallel display, don't have anything available.).

I was looking at schematics for some old Ten-Tec Omni V rigs, and saw the diodes in the transmit audio line, where a pair of diodes was alternately forward/reversed biased to route the audio either to the speech processor, or not, and I thought, maybe I could use a similar diode arrangement to enable just the audio from the mic or the line-in... except that they switch the bias voltage to the diodes via a front panel switch, so still a problem to solve.

So my thought is, in the "default" configuration, only the Mic audio would be routed to the uBITX.  But, IF the rear panel PTT was activated, then only the Line-in audio (for as long as the rear panel PTT is activated).  Any thoughts on a circuit--preferably simple :-)--to achieve this?

Thanks!
-Rob KC4UPR

_Dave_ AD0B
 

I have successfully used a dpdt arrangement. The center goes to the radio and the switched inds going to the mic/soundcard speaker and the other to the speaker/sound cardmic.

One uBITX used a regular pushbutton dtdp switchsalvaged from something and the other a relay controlled by a spst switch.
Neither used any electronics
--
73
Dave
ADOB
Raduino bracket and Ham_Made_Keys

Evan Hand
 

Interesting idea.  After some quick sketching, I think it could be done with 3 capacitors, 3 resistors, 4 diodes, and a Nan gate (Not And).  I have not worked out the details, but here is a rough diagram in the attached PDF.

I do not have a selection for the diodes.  You do want diodes with low forward voltage drop and high reverse resistance (Schottky?).   The resistors are to provide enough forward current to ensure that the AC signal will not be clipped.  I would check your diode selection and adjust accordingly. 

If the Nan gate cannot handle the current, then use the third gate as an inverter driving a transistor or MOSFET to switch the computer audio diode.

A potential reference :
https://hackaday.com/2016/12/09/diodes-the-switch-you-never-knew-you-had/

Please verify before building, as this was early morning musing, and I am not confident that I have thought through all of the requirements.
73
Evan
AC9TU

John (vk2eta)
 

Hello Rob,

I have the same situation and was looking at a simple solution too. Here are my thoughts based on your idea.

Have a look at this page for examples: http://www.k7tty.com/development/electronicprojects/Diode Switching Circuits.htm

Instead of using +/- voltages for switching I would use a voltage divider, say 1/2 of Vcc (say 47k and 47k) off the 5v from a regulator on the common output side and use the ptt signal from each ptt source for switching through a 22 to 47k biasing resistor to ground on the input side. On the input side I would connect each diode to the +5v via a 100k to 220k resistor.

That way with all PTT off you get reverse bias of +5 - 1/2 of +5 = 2.5v of reverse bias, and with ptt on you get +5 - 22/122 * 5v/2 = 1.6v of forward bias (if using 100k and 22k resistors on the input side).

Since you are dealing with max audio signals of around 100mv the biasing should keep you cleanly in the on or off region.

The output impedance of the electret is a few k ohms so small enough compared to the biasing circuit values.

For the logical OR of the ptt inputs I would use a pair of schottky diodes (e.g. Bat46) since the ptt input on the nano is an analog input (A3 pin) and the threshold for ptt on can be changed in software.

I would take care of stray RF which would cause issues with these circuits (screened cables, good ground, decoupling capacitors and ferrite sleeves if required).

Hope that helps,

73, John (VK2ETA)

Ted
 


Hey, all.

I have two 6-pin (RJ14) ports on the front of my portable setup, one for a microphone and one for JS8, etc. It's really simple and requires no extra parts.

The data transmit audio (and the rx data audio) lead to a $6 sound card dongle. The computer burps and beeps out of it's own speakers, not out the dongle; when the dongle makes noise that the radio might transmit, it's because my digital modes (FT8, JS*, etc) are doing what I told them to do. If I'm not doing anything that utilizes "USB Sound Card 2", that input remains silent.

The microphone audio can be clipped by judicious wiring of the mike element. Take the negative lead of the eletric mic element and join it with the PPT line and allow the push switch to ground them both together when using SSB. That, or if your microphone chassis of choice has a double-pole switch, use one path for audio cut-in and the other for PTT functions. The rest of the time, the mic line makes no sound.

At least, the parts count is low.



Ted
K3RTA










Justice will not come (to Athens) until those who are not injured are as indignant as those who are injured.
Thucydides



On Saturday, April 11, 2020, 09:12:24 AM EDT, John (vk2eta) <vk2eta@...> wrote:


Hello Rob,

I have the same situation and was looking at a simple solution too. Here are my thoughts based on your idea.

Have a look at this page for examples: http://www.k7tty.com/development/electronicprojects/Diode Switching Circuits.htm

Instead of using +/- voltages for switching I would use a voltage divider, say 1/2 of Vcc (say 47k and 47k) off the 5v from a regulator on the common output side and use the ptt signal from each ptt source for switching through a 22 to 47k biasing resistor to ground on the input side. On the input side I would connect each diode to the +5v via a 100k to 220k resistor.

That way with all PTT off you get reverse bias of +5 - 1/2 of +5 = 2.5v of reverse bias, and with ptt on you get +5 - 22/122 * 5v/2 = 1.6v of forward bias (if using 100k and 22k resistors on the input side).

Since you are dealing with max audio signals of around 100mv the biasing should keep you cleanly in the on or off region.

The output impedance of the electret is a few k ohms so small enough compared to the biasing circuit values.

For the logical OR of the ptt inputs I would use a pair of schottky diodes (e.g. Bat46) since the ptt input on the nano is an analog input (A3 pin) and the threshold for ptt on can be changed in software.

I would take care of stray RF which would cause issues with these circuits (screened cables, good ground, decoupling capacitors and ferrite sleeves if required).

Hope that helps,

73, John (VK2ETA)

Bill Cromwell
 

Hi Rob,

Check the switching and circuits in your mic. I don't know what is inside ALL mics, especially not the el cheapos. My mics have always had a separate pole on the switch for the mic audio. If the mic switch is not pressed then mic audio does not get to radio. If you have to modify anything it might be easiest/best to modify that switch even if you have to buy a new switch.

73,

Bill KU8H

On 4/11/20 1:49 AM, Rob French (KC4UPR) wrote:
Hello!
So I want to have a separate line-in for my uBITX, separate from the Mic input.  I also want to have a separate PTT.  The intent is to have the regular mic/PTT plugged into the front of the radio, and then also have the computer hooked up in the back.
Requirements/Constraints/Desirements:
- Always keep the computer plugged in.
- Always keep the mic plugged in.
- If the mic PTT is pressed, don't transmit computer audio.
- If the computer PTT is asserted, don't transmit mic audio.
- Don't require a mechanical switch between the inputs (I have a fairly compact case and don't have room for more switches).
- Don't require a relay*. (* This is not a hard requirement, but I'd rather not use a relay if I can help it.)
- Don't require use of any Arduino pins (I'm using parallel display, don't have anything available.).
I was looking at schematics for some old Ten-Tec Omni V rigs, and saw the diodes in the transmit audio line, where a pair of diodes was alternately forward/reversed biased to route the audio either to the speech processor, or not, and I thought, maybe I could use a similar diode arrangement to enable just the audio from the mic or the line-in... except that they switch the bias voltage to the diodes via a front panel switch, so still a problem to solve.
So my thought is, in the "default" configuration, only the Mic audio would be routed to the uBITX.  But, IF the rear panel PTT was activated, then only the Line-in audio (for as long as the rear panel PTT is activated).  Any thoughts on a circuit--preferably simple :-)--to achieve this?
Thanks!
-Rob KC4UPR
--
bark less - wag more

Rob French (KC4UPR)
 

Evan (looking at yours specifically in this reply),

Can you help me understand what is happening in the "both PTT open" state?

Specifically, I'm trying understand the status of the "Mic In" and "Computer In" diodes in this condition.  If the "Computer PTT" switch is open, then the output of the logic circuit is HI (+5V), which is present at the cathode of the "Computer In" diode.  Meanwhile, Vcc (+5V) is applied at the anode.  It's not clear to me what the bias state of the diode is here.  Now, if the "Computer PTT" switch is closed, then I think the "Computer In" Diode is biased to essentially 1/2 Vcc, so I think I understand that path.

In general, I think I understand how either input is passed (forward bias of +2.5V, I think), but I don't understand how either path gets blocked (i.e. I don't see any reverse bias being applied across the input diodes).  

What am I missing?

Thanks!
-Rob KC4UPR

Evan Hand
 

ROB,

The gates are NAN gates.  A NAN Gate logic is

A   B    out
========
0    0     1
1    0     1
0    1     1
1    1     0

For the input, we take the PTT of the Mic direct and invert the Computer.  

When the logic gate is High the diode is off
When the logic gate is Low the diode is on

The only time the computer diode should be forward biased is when Mic PTT is open (logic 1) and the Computer PTT is low (which is inverted to High or a logic 1).  When the gate output is LOW it would turn on the diode to be forward biased.

As stated in the text of the message, if the logic family selected cannot drive the diode, then a transistor or MOSFET needs to be used, and the inverted output needs to be inverted again.  I guess an alternative would be to use a PNP transistor but did not look into that in detail.  I did go back and look at TTL specs, and a standard 7400 quad 2 input nan gate IC should work.

I may have the thinking messed up, but that was what I came up in the spur of the moment.

73
Evan
AC9TU

Rob French (KC4UPR)
 

Hey Evan, thanks for the quick response.  I follow the logic of the NAND gate, and also the forward biasing.  What I'm confused about is the reversing biasing, i.e. turning off a given input.  My thought is that when a given input is supposed to be off, the corresponding diode needs to be reverse biased, but I don't see that happening.  Or are you assuming that the forward voltage drop across the diode will be sufficient to block that input?

Thanks again!
-Rob KC4UPR

Evan Hand
 

Rob,

I was following the examples in the attached.
https://hackaday.com/2016/12/09/diodes-the-switch-you-never-knew-you-had/

Not sure if that will work moving both of the diodes to a single resistor to Vcc, however it is just slightly different from the 2 resistors with positive voltage switching.  Again, not sure if it will work, I would breadboard and try first.

73
Evan
AC9TU

Evan Hand
 

Rechecked and it will not work reliably.  Will need to look at it some more.  As stated, I just did a quick  sketch.

Sorry,
73
Evan
AC9TU

Rob French (KC4UPR)
 

John, you mentioned 100mv for the audio signals, is that ballpark for both computer audio and mic audio?  I guess I'd assumed it was around 1 V p-p or line level or somesuch.  But I haven't measured it.  Is +/- 100mv what I can expect for both sources?

Rob French (KC4UPR)
 

I have refined my requirements a little.

- The default input (enabled, even if neither PTT line is activated) is the computer line-in.  Why?  So that I can do PTT via CAT control if desired.
- However, the mic input can only be active if the mic PTT line is activated.
- I trust the computer to be quiet while the mic is in use.
- Mixing the two inputs may be desirable (i.e. if the mic PTT is being depressed, it might be desirable to also inject something via the computer line in).

Thus, the only input line I need to be able to switch is mic input, using the mic PTT.  While that input is enabled, the computer line in can also remain enabled.  I think this will greatly simplify the transistor/diode logic I need to implement...

Rob KC4UPR

_Dave_ AD0B
 

I would say the diodes you saw were to prevent any signals greater than the forward bias voltage, rather than signal routing. 
--
73
Dave
ADOB
Raduino bracket and Ham_Made_Keys

John (vk2eta)
 

Hello Rob,

Replying to your first question: a mic output is generally in the 10s of mV. On top of my head I have 50mv as a typical mic input voltage for radios, so 100mv was king of a safe value for a mic output.

This means you will have to reduce the audio voltage from your computer line out (a trim pot would be the easiest).

Here is a hand sketch based on you new requirements. Only difference in that circuit is that it will cut the computer audio when the mic PTT is pressed.

I think that is better as it prevents sound card noise and other feedback from the computer audio circuit.

If you want to mix the computer audio with the audio coming from the mic, remove R1, R2, substitute D2 by a resistor (100K trim pot?).

I haven't built that circuit yet, so it is purely theoretical at this point.

73, John (VK2ETA)

Rob French (KC4UPR)
 

John,

Thanks for the schematic!  That all makes sense to me, and it matches the math in your previous post, so I appreciate putting a picture to the words.

I know in the past I have connected my sound card output directly to my mic input without a trim pot any sort.  But I also haven't done any of the TX preamp adjustments to my uBITX that I've seen, so perhaps my audio, even if higher than the mic, was still fine getting into the TX stages.

As far as (potentially... haven't made my mind up) removing R1/R2 and replacing D2 with a trimpot (essentially a blend control, is what I think you are proposing?), I'd actually considered using an op amp mixer to blend the two inputs.  Your version would certainly be simpler.

If I wanted to ensure I had headroom for higher amplitude audio signals, could I replace the +5V with +12V?  Here's my thought.  Down where you have the Mic PTT, of course switching that to ground is how the PTT would work.  But, I was thinking I could have a diode connected between the Raduino and the switch.  The cathode end would be connected to the top of the switch, as would the "Mic PTT" input in the schematic you created.  Voltage present when the switch is open (+12V I think) wouldn't exceed the reverse voltage of the diode.  When the switch is closed, that would ground out both the Raduino PTT pin (via the diode) as well as the Mic PTT line in the circuit above.  Forward voltage drop across the diode would still land me in logical-zero land for 5V logic.   I'm sure I'm missing something here, but it makes sense in my head...

Thanks again for putting this together!

Regards,
-Rob KC4UPR

John (vk2eta)
 

Hi Rob,

Yes regarding the blend control that was what I had in mind.

Regarding feeding 12V to the circuit and connecting the PTT via the diode it should work fine as the Arduino sketches I have seen use the digitalread function for the PTT (although it is an analog input) and therefore will read a logical zero for anything below 2.2V (at 5VDC supply).

BUT, if you feed the circuit from the boards' main supply I would decouple the DC biasing voltage very carefully with resistors and capacitors otherwise you could have bad feedback through the variations in supply voltage.

Personally I would stick with 5V (or anything regulated below your supply line voltage) and well decoupled. Remember the audio voltage to the mic input is normally in the tens of millivolts so a) you don't need more than say a volt to be quite safe from spurious diode conduction and b) small variations in supply voltage will happily feedback in your mic input signal. Just my 2 cents.

Happy home-brewing.

Let us know what you come up with please.

73, John (VK2ETA)

Rob French (KC4UPR)
 

Will definitely keep you in the loop with my progress.

Were you envisioning a dynamic mic with this, or electret?  I'm planning on putting together a little electret mic driver, but then I was thinking, could I just use the bias from the PTT circuit???  But I guess that would be a pretty low voltage (0.9V?) available there at the junction of R3 and D1, so probably not enough to drive the electret element...

John (vk2eta)
 

Hi Rob,

I use an electret, a low cost Baofeng style microphone with the capsule replaced by the one supplied with the uBitx kit and larger holes in front of the capsule.

I use an SSM2167 speech compressor module between the Mic and the radio Mic input fed through a trim pot to bring the max volume to acceptable levels. See http://ubitx.net/tag/ssm2167/ for details if you are interested. It really increases the average power on SSB.

Not sure about using the bias circuit voltage.

For powering the additional circuits in my first build I added a connector on the Raduino to pickup the 5V. On my second build I simply added another 5V regulator and that's a simpler option IMO.

Hope that helps.

73, John

Rob French (KC4UPR)
 

Hey guys, as a follow-up... so I ended up not pursuing any of these routes.  Nonetheless, I do appreciate everyone's inputs.

What I ended up doing was resurrecting a little DSP processor I had been previously working on using a Teensy 3.2.  I have two PTT's wired to it, and then the following inputs:

- Mic connected to (and powered by) Teensy Audio Adaptor mic input.
- Line in connected to right channel of Teensy Audio Adaptor stereo input (left side is used by receiver audio).
- USB audio input from PC.

Whenever the Mic PTT goes active, I switch the audio to the mic input, and use that.

Whenever the rear panel (line) PTT goes active, I use the right channel audio input for the transmit audio.

I've added DGU and DGL (digital USB/LSB) modes to my Raduino, which cause the rig to use the USB input/outputs instead, activated only via CAT.

Thanks again for the inputs, I know I didn't go any of the suggested routes, but this was definitely all part of the learning experience for me.  Thanks!

Rob KC4UPR