Nothern Utah WebSDR Logo - A skep with a Yagi Northern Utah WebSDR
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.

First circuit:

The first circuit, depicted on the left-hand side of Figure 1 is "old school" using two relays:  The stereo audio leads from the computer and to the speaker are wired to RY1's "normally open" contacts and with its coil in parallel with RY1 is RY2,  a single-pole, single-pole relay.  When the radio is keyed, both relays are activated:  RY1 disconnects the speakers and the (optional) RY2's contacts close, keying the amplifier.  If you don't have an amplifier - or don't plan on using one - you can just omit RY2.

The advantage of this circuit 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 should prevent this.

Second circuit:

Just "because" it could be done, the second circuit was built, seen on the right-hand circuit of Figure 1.  This is slightly more complicated and it has the limitation that it if it is used with an RF amplifier, that the amplifier has low-voltage positive DC keying.

Relay "RY1" is a double-pole, double-throw relay with the computer audio wired in a "pass through" fashion on its "Normally Closed" (N.C.) contacts - in other words, when the relay was energized, audio would pass.  As mentioned previously, his amplifier put about 5 volts on the keying line, so a simple voltage clamp consisting of two ordinary LEDS in series - LED1 and LED2 - provide about 3.5-4.5 volts (not critical) bias:  This voltage is lower than the keying voltage on the amplifier's input - and current-limited - so there is little or no chance of causing any damage to the circuit by backfeeding this voltage.  You don't need to use LEDs:  A 3-5 volt Zener diode or 6-8 ordinary silicon diodes in series would work just as well, but the LEDs are cheap, convenient, and provide a nice, visual cue that the circuit is being keyed (e.g. they will turn off when the radio is keyed.)

When un-keyed, both the amplifier's internal circuit and the R1+LED circuit pull the voltage up - but diode D1 blocks the voltage from the amplifier from being "seen" by Q1.  With the voltage on the base of Q1, it is turned on which means that the voltage at the junction of Q1's collector, Q2's base and the bottom of R2 is low, turning Q2 off with the relay being de-energized and the audio output from the computer is connected to the speaker.

When the amplifier is keyed the radio's relay grounds the connection, causing the voltage on the keying line to be brought to ground.  When this happens, a small amount of current flows through diode D1, turning off transistor Q1. This causes the voltage at the junction of Q1's collector, Q2's base and the bottom of R2 to go high, turning on Q2 and energizing the relay, disconnecting the computer's audio output from the speaker, muting the audio.  As you may have figured, diode D1 prevents the amplifier's keying voltage from being "seen" by transistor Q1 - and this is important as it is possible that such current could cause the amplifier to be keyed all of the time.

The circuit may seem to be unnecessarily complicated with the addition of a few other componentes - but they have their purpose.  D2 is located in the emitter of Q1 to take into account the fact that when the radio's PTT line is grounded, the voltage at the base of Q1 doesn't completely disappear:  It actually goes down to about 0.6 volts - which is right about the same voltage to turn on Q1.  What this means is that in some circumstances, Q1 may not actually turn off when the radio's PTT line is activated.  In some radios, there isn't a relay at all, but rather a transistor - typically an open-collector or open drain - and this may not go all of the way to zero volts:  If it were just a few tenths of a volt above ground, Q1 would probably not shut off at all and the circuit wouldn't work!  Adding D2 raises the emitter-base voltage threshold, allowing a few tenths of a volt drop in the radio's keying line and still permit reliable keying.  Because we've raised the "on" voltage of Q1 by a "diode drop", we must do the same with Q2 or else Q1 wouldn't be able to shut it off.

Diode D3 is a necessary component as well, suppressing the back-emf from the relay coil's magnetic field collapsing when Q2 turns off:  Without it, it is possible that this voltage could be quite high and back-conduct through Q2, damaging it over time.  Capacitor C1 is present to minimize RF pick-up on the keying line (which is probably not much of an issue, really) and also to snub other voltage transients that may show up.

As noted on the diagram, this circuit will work only with low-voltage, positive DC keyed amplifier circuits:  It should easily handle from 4-5 volts to, perhaps, 24 volts of DC.  This circuit will NOT work properly on an amplifier keying circuit that uses AC voltage or a negative DC voltage - and it should be used with care on an amplifier that has a positive DC voltage greater than 24 volts.  It is possible to build a circuit to work with AC/negative/high voltages, but that is beyond the scope of this article.  If you have such an amplifier you might consider building the "simplified" version of this circuit (e.g. just the relay) and using two relays - one to switch the audio in/out and another to key your amplifier.

Even if the amplifier's keying circuit is low voltage, it would be a good idea to check to see if the PTT line is connected to the coil of a relay:  If it is, make sure that a snubber diode is present across its coil - but if not, or if you are not sure, be sure to add the "Dsnub" diode depicted in the diagram to suppress the back-emf from this relay when it is de-energized.

Circuit power:

Either circuit 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.

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 user 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.  
How to connect:

Many amateur transceivers and/or amplifiers provide their external keying via a "phono" plug (a.k.a. "RCA" jack).  If this is done, a simple "Y" connector may be used inline with the keying as the circuit in Figure 1:  One side of the "Y" would connect to the amplifier and the other side to the circuit in Figure 1.


When tested, either 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 might 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.

There are many ways that this could be accomplished - and this may not be the easiest, but it does work!

Additional information:
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