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PC -> Transceiver Circuit

cspencer

Aug 4, 2010
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Hi all,

I want to connect my PC sound card to my radio transceiver. This means connecting the received audio to the line in, the audio to be transmitted from the line out, and the PTT (push to talk) control from a pin on a serial port. I've spent the last couple of days designing the circuit with guidance from here and here, and I would like to have it sanity checked before I buy the parts; I'm very new to electronics.


Larger version


There are four electrically separate circuits here to prevent ground loop problems. Top left is the PTT circuit. If the RTS line is on then the PTT line on the output is to be grounded. The resistor is to reduce the current arriving at the opto-isolator; I'm not sure how the 1.2k value was arrived at, that's just what is used in both the documents I linked above. The diode is a shunt to prevent damage to the LED when the RTS line is off (-ve V).

Next is the TX audio, which is connected to the sound card line out. The voltage divider is to get the 2.5V max output from my sound card to < 40mV which my radio requires. The audio transformer is a 600:600 ohm transformer. Below that is the RX audio. The radio outputs ~1.5 V, which is fine for the sound card line in, so nothing more is needed here.

The pinout to the radio was found by testing with my voltmeter. It's completely different to the pinout listed in the manual weirdly..

Thanks, Chris.
 

cspencer

Aug 4, 2010
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Aug 4, 2010
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The simple solution is to use a resistive divider to reduce the voltage on your sound card's output to a value suited to the trahciever and then to plug it in to the TX Audio.
It is a resistive divider. There's no way to directly connect the TX audio to the rig, it's either through the Mini-DIN 6 data plug (which I'm using), or though an RJ12 connector in place of the actual mic unit. Both ways are pretty much the same, but with the Mini-DIN I can still use the mic unit while leaving the PC connected.

Next step is to push the PTT button. I would use the forefinger of my dominant hand.
It's called PTT, but when it's connected to the PC you don't actually push anything. This circuit is for using digital transmission modes. The PTT is controlled by the application. When it wants to transmit it activates the RTS line on the serial port; if the RTS line is off then it is receiving.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Damn, you got in before I deleted my message. I noted you had done all of that.

It looks reasonable to me. The only issue would be that the opto can reliably trigger the PTT, and that it's th e right way around. But I assume you've tested that.
 

NickS

Apr 6, 2010
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It looks good except for the rig out side of that opto. I always like to use a series resistor there to limit the current flow possible through the transistor. The datasheet should tell you the max current it can handle.

Now there is almost certainly a resistor in your transceiver doing this very function already to ensure if PTT is tied directly to ground that it will not damage the internal circuitry. If you can verify that the current through the PTT line(when grounded) does not exceed what your opto can handle then all is good in the world and you don't need the resistor but if the current from PTT exceeds the opto's rating then your opto may die in the line of duty.

Good luck

-Nick-
 

cspencer

Aug 4, 2010
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It looks good except for the rig out side of that opto. I always like to use a series resistor there to limit the current flow possible through the transistor. The datasheet should tell you the max current it can handle.

Now there is almost certainly a resistor in your transceiver doing this very function already to ensure if PTT is tied directly to ground that it will not damage the internal circuitry. If you can verify that the current through the PTT line(when grounded) does not exceed what your opto can handle then all is good in the world and you don't need the resistor but if the current from PTT exceeds the opto's rating then your opto may die in the line of duty.

Thanks Nick. I've measured the current through the PTT line when grounded and it's only 1mA, so I guess that's ok. This is the opto-isolator I have ordered. I'm not really sure what value I'm supposed to be looking at in the datasheet, but there are only two current values in the absolute maximums section: average forward current (80mA) and collector current (100mA).
 

cspencer

Aug 4, 2010
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It looks reasonable to me. The only issue would be that the opto can reliably trigger the PTT, and that it's th e right way around. But I assume you've tested that.
I'll be sure to when it arrives. :)
 

NickS

Apr 6, 2010
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Thanks Nick. I've measured the current through the PTT line when grounded and it's only 1mA, so I guess that's ok. This is the opto-isolator I have ordered. I'm not really sure what value I'm supposed to be looking at in the datasheet, but there are only two current values in the absolute maximums section: average forward current (80mA) and collector current (100mA).

It would be the collector current so you are just fine on that.
 

cspencer

Aug 4, 2010
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The finished product:
10iQG.jpg


Not bad for my first real circuit :)
 

cspencer

Aug 4, 2010
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Yup, it works perfectly. I bought various spare components, but I didn't break any. I'm very happy with how it's turned out. :)
 

davenn

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Sep 5, 2009
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Yup, it works perfectly. I bought various spare components, but I didn't break any. I'm very happy with how it's turned out. :)

well done :)

in the words of the "A" Team .... "I love it when a plan comes together"

Dave
 
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