Image: This unit I build several years ago and used it to work digital modes. I eventually sold it.
(Click on image for larger view.)
Here's a very versatile PTT Controller to key a transmitter for remote or repeater operation. Does the following sound familiar? You needed to key a transmitter when a signal came into your receiver, but there was no way that you were going to risk voiding the warranty on your new radio. Years ago, accessing the COS line (carrier operated squelch) was fairly easy, allowing simple control of transmitters for repeaters, base extenders, etc. However, recent advances in technology have produced radios that make such tap-offs very difficult, even for the experienced radio amateur. And the possibility of voiding a warranty is no linger something to scoff at.
I needed a way to key a transmitter upon receiving a signal, without invading the insides of my transceiver. Adding a permanent dangling wire was not the way to go. The are circuits but they all seem to need voice audio (VOX) to develop a keying signal. The above project will be especially useful for putting together base/mobile/portable/cross band repeaters or remote operations. It can also be used as a high-tech soundcard interface. It will allow the use of equipment without you having to get inside and do circuit surgery. Also, this circuit will provide a switching signal for various devices or secondary units which need to operate when a signal (with or without audio) is received.
The circuit functions on the noise level difference between a full-squelched audio output and the audio noise level present when the squelch opens upon receiving an un-modulated signal. For convenience a transmit audio feed is included as an option. Test conducted showed that this controller can be used to relay from HF onto VHF without the need to TX the two meter set all the time. Another bonus is to use this contoller to PTT digital modes. No need anymore for a serial port or USB port for digital mode keying. I have tested a few digital modes that worked flawlessly with the PTT Controller. I am sure that there are many other uses for this very versatile PTT Controller.
- 73 Amateur Radio Today
Ed. The image underneath reflect my current test bed unit. The veroboard to the right is used to control the external speaker audio coming from the radio and then goes to the microphone in of the USB Soundcard Dongle. The unit worked as stated in the article that appeared in 73 Amateur Radio Today, January 1995. (Link to PDF file) I however opted to do the necessary circuit "surgery" for COS on the linked Motorola radio as not to have another unit that needed to be boxed and search for a place to store the controller. I would like to play around with it more in future as to test all the versatile functions as described in the article. Who know I might just find a use for the unit in future.
(Click on image for larger view.)
Source: 73 Amateur Radio Today - Roland Burgan KB8XI
