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Cheap USB interface to connect Ham radios and other devices to the PC October 14, 2011

Posted by GuySoft in Crictor, diy, Electronics, Hamakor, ITU, linux, open source, wireless.
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Ham radio PC interface

Hi all,
As promised in my last post, I have finally finished designing and building a cheap interface to connect ham radios or any other audio device that does not share audio common ground to a PC, including a PTT interface. Price should be around $17 or 60 NIS.  The control is done using a USB interface, so unlike most ham radio hardware that uses serial connections, this one is plug and play in modern PCs. The neat thing is that it uses them same commands as a RigBlaster. So this interface works out of the box with common ham radio programs like Echolink and Svxlink. The interface should also work with soundmodem which can be used for X.25 packet digital communications, without any extra hardware (I am still trying to get this to work). This interface could also be used to connect to other devices that don’t share a common ground. For example like phone lines.

Parts you will need

    1. TTL USB Serial interface – The main part is a simple CP2102 $4 controller that can be percussed from ebay. It simulates a serial device, meaning you can still work with older pieces of software with it. Note though you need to solder a cable from the RTS pin on the board, because normally they don’t come with a pre-made pin.
    2. Audio line isolation transformer – This can be easily salvaged from any old dial-up modem. That’s how I get mine, it seems to be much easier to get them off old modems than finding them in electronics shops, plus they are so easily identifiable (just make sure not to break the contacts when you take them out, happened to me twice). If you can’t find any old dial-up modems you can get it from ebay.
    3. 3.5mm stereo and 2.5mm mono earphone connectorsYou need one of each, PCs use a 3.5mm stereo jack and the radio’s microphone is usually a mono 2.5mm jack (consult your HT’s manual, they tend to have a schismatic for the mic). I personally like to have two 3.5mm phone plugs on each side and an adapter, so in the future I can use this interface for other things than ham radio.
    4. (recommended) Isolated audio cable – Ham radios can produce a lot of interference and using isolated audio cable close to the transformer is a good idea, you won’t need more than half a meter, unless you want to put a larger distance between the radio and the PC.
    5. Prototyping Perfboard – So you have what to solder the parts on.
      Now comes the difference between the two circuits, you MUST have either of the two options 6 or 7 below:
    6. (option 1) Reed relay – If you manage to find this part number HE722A0510, then you will really get a simple board, with just 3 parts on it. Its also quiet compared to a normal relay.
    7. (option 2) 6 pin 5V mechanical relay and two NPN 2N3904 transistors – Adding two more parts to the board and a louder relay, this would be a lesser choice, but still fully functional interface. I am putting this as an option because these parts are so common I doubt anyone would have a problem finding them.
    8. (recommended) 14 pin socket – If the relay breaks, it would make it easily replaceable.
    9. A 3.5mm audio splitter and a set of cheap earphones – Most sound cards today have a circuit that checks if a device is plugged in to the card, otherwise they would not play anything though it. To overcome this you can split the outgoing audio signal, connect one side to the radio interface, and the other to a set of earphones. This is not needed if you want to plug a device like an MP3 player, where there is no such check.

The Schematics

I am going to supply two schematics, the first is the one I built which uses a reed relay, which is more quiet, faster but slightly less common and costs around $3 more. The second, I built and tested on the breadboard. It uses a normal 8 pin mechanical relay and two NPN 2N3904 bipolar transistors.

The finished interface looks like this:

PTT Interface option 1 using a reed relay and the CP2102 USB to UART controller

And here is the schematic:

PTT Interface1 using a reed relay (click image to enlarge)

Note that the reed relay I used can handle the strange 3.3V to 5V connection used (pin 2 and 6 on the relay). This saves the need for an inverter, since for some reason, the TTL’s logical high is 0V and logical low is 3.3V and not the other way round (still trying to figure out if this is the spec, or the Chinese assemblers fault).

If you can’t find part number HE722A0510, then you could use two NPN 2N3904 transistors and a 8 pin mechanical relay. The NPN transistor on the left inverts the signal, while the transistor on the right amplifies the signal to 5V. This is the schematic:

PTT Interface2 using a normal 8 pin relay (click image to enlarge)

Will build ham radio interfaces for food – If you want I would build you one

I am willing to build these for people  and get paid via paypal, price is negotiable, but I don’t think I’d be willing to build this for under $55 (or 200 NIS). If I find cheaper ways to get parts, I’ll lower the price. This depends on how much call there is, and if I find a cheap way to print PCBs for this project.

I  hope this interface brings use to people.  The one I built I hope to donate to the Jerusalem Amateur Radio Society operations.


As usual, I could not have done this alone, thanks to 4Z5UG, Eric, who helped me out with the inverting NPN circuit design, and for 4X1GM for suggesting the reed relay.

That’s it, Looking forwards to hear your thoughts on this post, 4Z7GAI wishing you 73, best regards.


1. geoff - October 14, 2011

You can avoid the headphone splitter by putting a resistor (about 30 ohms) across the audio. This will take the place of the headphones which usually have a 36 ohm DC resistance,

2. alexander - November 7, 2011

i think in interface2 something is wrong, the relay don’t have + power, only negative

GuySoft - November 9, 2011

The relay is connected to the +5V from the TTL chip, it gets it from the collector (pin marked c) of the second NPN transistor Q2.

3. Craig Nicholson - October 4, 2012

Ok so I was thinking of using the same usb interface to do the same thing but here’s my ideas. I was going to ride on the txd and connect an optocoupler to the ptt circuit on my ht. That way when the data is transmitted the optocoupler closes the ptt, when the transmitted data ends the ptt opens. Do you think there’s enough voltage of the txd pin to power an opto coupler (i was thinking of a homemade job with a 1.8-2.2v ir LED. Whatcha think? my e-mail is kf5qgf@gmail.com and any help would be greatly appreciated. Thank you.


GuySoft - October 27, 2012

That should work, you have 3.3V from the pins, and should be enough amps to power an LED.
If you are just using the optocoupler as a relay it should work. There are plenty of ‘relay-like’ circuits this controller should be able to interface with.

4. Yonu - November 27, 2012

I am going to ask a very stupid question as I have never been good at diagrams on the interface 1 diagram what do ring and sleeve represent as I am trying to sort out the ends as I will be triggering a older HTX-242 if I can get it sorted out


GuySoft - November 28, 2012

The ring and sleeve refers to a PL audio connector.
Because they fit in to one, and as you can see the ground tends to be on the outside.
PL audio connector

I think you will need on the side of your HT a RS45 connector plug, since that is where your mic would go in. But I am not sure.

5. Yonu - November 28, 2012

Thanks that helps what I am doing and please tell me if this should work with the basic circuit Diagram is i took the ptt line from the relay and not routing it to the audio and will simply tie it to the ptt circuit the only issue I am working on sorting out is the htx-242 pulls to ground for ptt not positive

6. Rey Tambolero - April 14, 2013

what kind of usb are we going to use, is it available in the store?

GuySoft - April 14, 2013

The link in the post broke.
Yes, the part is avilable on ebay.
Here is a search for it

7. Eric - August 19, 2013

I too am not good with diagrams, and the cp2102 I have does not have the breakouts. Is #24 RST/NC on the schematic the same as the pin labeled RST on the cp2012? Also is #8 VBUS the same as VBUS on the USB connector?

GuySoft - August 19, 2013

Yes, there should be one RST, and it says that pin is both RST and NC.

8. etan - December 15, 2016

my fiend if i use 12vdc.. how much is resistor will be? 5vdc is 300 ohms. 12vdc is what? thamks! 73!

GuySoft - December 15, 2016

I don’t think you can use a 12 volt relay, because the USB TTL device needs to switch it. You might as well get the correct part

9. KALUGUMALAI Raja - March 15, 2018

Our club like to.provide echolink in our new repeater. Can you arrange to give one set for vhf 145.600 -.
My mail id.vu2klg@gmail.com

GuySoft - March 15, 2018

You can built it yourself.

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