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A PTT mute for WebSDR use


What's this for?

It is common to use a WebSDR as an "auxiliary" receiver (or main receiver if your local noise is really bad!) and this means that when you transmit, your signal may be received by the WebSDR and get back into your transmit audio - usually via acoustical coupling - and cause an "echo".

Other than wearing headphones - which doesn't spare you from your own, delayed audio in your ears that can be very distracting, there are several ways to avoid this:
Figure 1:
A diagram of two circuits that can mute the receiver when you transmit to avoid the "echo".
Click on the image for a larger version.
PTT-triggered audio muting circuit
All of these require manual intervention and become cumbersome if you are engaged in quick and/or frequent band-and-forth, but there's another way:  Audio muting keyed by your transceiver.

The (simple!) circuit(s):

The circuits depicted in Figure 1 are "old school" using relays:  Relays were chosen because they are cheap, simple to use and offer good isolation at DC, audio and even RF.  If you don't happen to have a pair of DPDT relays, you can use several SPDT relays or even a 4PDT relay - it really doesn't matter.

The advantage of using relays is that it will worth with practically any amplifier, regardless of its keying voltage.  The only point of concern might be if you get a slight "pop" in the audio when you un-key the amplifier:  If your amplifier has a relay, it may be that its coil's electromagnetic field is collapsing and generating a high voltage/arcing across RY2's contacts and the a bit of this can couple into the computer audio:  The optional "snubber" network mentioned on the diagram should minimize this.  Similarly, diode D1 is important as it suppresses the (high!) voltage from the relay coil(s) when they are de-energized and the magnetic field collapses:  Do not omit it!

There are two circuits shown in Figure 1 and the one on the left is about as simple as it can get:  You key your radio and its external amplifying circuit (often a relay) keys the relays in the circuit with one relay disconnecting the audio and the other keying the amplifier.  If you don't own an amplifier and/or don't plan to use one, you can forget about the circuit at RY2.

The circuit on the right side shows the use of one of the relay contacts disconnecting the audio "ground" between the computer and the speaker.  This may be useful if the electrical connection between the two, while transmitting, results in your (usually distorted) transmit audio from being picked up along this lead via a ground loop and/or RF circulating currents - but other than that, it's exactly the same.

Comments on radios' keying circuits:

While most "base station" radios have relays in them to key an external amplifier, some "portable" (usually QRP) radios use transistors instead, so it would be a good idea to check the voltage and current ratings on that output.  If its keying circuit can handle 15 volts or more and more than the current drawn by the relay(s) in the circuit, you are all set.

Note that transistor-based keying circuits are usually polarity-sensitive and ground-referenced (e.g. a positive signal on the external keying output gets "grounded" when you key the radio) so you would want to make sure that the keying line is connected appropriately (e.g. one side of the relays is tied to V+ and they are activated by "grounding" the other side of the coil.)


Circuit power:


The relay requires an external source of power - 11-15 volts being fine for a typical "12 volt" relay.  Out of convenience, the prototype was powered from a transformer (not switcher-type) 12 volt "wall wart".  Because the relay only consumes a few 10s of milliamps when keyed, the current rating of this wall transformer could be as low as 100 milliamps:  It does to have a capacitor-filtered output (but no need for regulation) or else the relay will chatter.

If one side of the radio's PTT relay is (or can be) grounded (which is usually the case) it is possible to power this circuit from the same "12 volt" power supply as that which is running the radio.  If you do this, be sure to include a fuse (no more than 1 amp) in the power lead, near its connection to the rig's power supply to protect against accidental short-circuits.


"But I don't have any 12 volt relays handy!"


If you have different-voltage relays on hand, you can use those - just make sure that your power supply voltage is appropriate for them.   If all you have is 5 volt relays and your only convenient voltage source is 13.5 volts from your radio, you can use series dropping resistors, calculated as follows.
  1. Measure the resistance of your 5 volt relay's coil.  Let's assume for this example that your relay's coil measures out at 185 ohms.
  2. Calculate the current that the relay would draw at 5 volts.  Using Ohm's law, we divide the voltage by the resistance - so 5/185 = 0.027 amps (27 milliamps).
  3. Calculate the amount of voltage drop that you need by subtracting the rated relay coil voltage from our power supply voltage .  If we have 13.5 volts available, we need to drop (13.5 - 5 = )  8.5 volts.
  4. Calculate how many ohms it would take to pass the current calculated in step #2 at the voltage calculated in step #3.  In this case, we divide the voltage by the resistance, so ( 8.5 / 0.027 = ) 314 ohms.
  5. The two most common resistor values close to our example 314 ohms are 330 ohms and 270 ohms - but because we can get away with +/- 20%, either value will be fine.
  6. Now calculated the power dissipation in that resistor.  For this we multiply the current by the voltage drop, so we do (8.5 * 0.027 = ) 0.229 watts.  This is pretty near the maximum rating of a 1/4 watt resistor, so we would use a 1/2 watt resistor if we have one on-hand, but because it's being used only intermittently (e.g. while you are talking) a 1/4 watt resistor will probably be fine.
To minimize the possiblity of the relay's coil causing a "click" in the speaker audio when you unkey, it would be a good idea to put the reverse diode (D1 in the drawings) across each relay coil, directly.  If you are building the 2 relay version, unless both of your relays are identical, you'll want to do the calculation for each relay.  If both of your relays are identical, and they are both 5 volts, you can wire them in series for 10 volts total, and then do the calculations for that.  


Conclusion:

When tested, the circuit described above worked perfectly.  One thing to note is that any web-based receiver will have a slight delay, meaning that when you un-key, you'll probably hear the last syllable or word that you transmitted coming back to you.  While this may be slightly distracting, this can never be heard over the air because this circuit guarantees that your speaker(s) will be muted any time that you key your radio.




Additional information:
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