PTT CIRCUIT

IF YOU USE A ' SOUND CARD ' PROGRAMME IT WILL MOST LIKELY HAVE A ' PTT ' OPTION - IN WHICH CASE YOU WILL NEED TO BUILD THE BELOW CIRCUIT - OR A SIMILAR ONE - WHICH IS A LOT EASIER THAN TOGGLING  THE SWITCH ON THE TRANSCEIVER EVERY OVER. HOWEVER IF YOU ALSO RUN PROGRAMMES THAT UTILISE A HAMCOMM TYPE CIRCUIT ON THE SERIAL PORT OF YOUR PC  - THEN THE PROBLEM IS ALREADY TAKEN CARE OF - AS THE HAMCOMM CIRCUIT HAS A SIMILAR BUILT IN CIRCUIT TO THAT OF BELOW.  FOR AN ALTERNATIVE PTT ARRANGEMENT TO OVERCOME RF FEEDBACK OR EARTH HUM YOU CAN USE EITHER A RELAY OR AN OPTO-COUPLER, SEE THE BELOW CIRCUITS.

TRANSISTOR PTT CCT1

TRANSISTOR PTT CCT2

ALTERNATIVE CIRCUITS TO OVERCOME FEEDBACK/HUM PROBLEMS

IF YOU ARE USING TWO TRANSFORMERS TO ISOLATE THE TRANSCEIVER OUTPUT TO THE LINE IN AND THE LINE OUT OF YOUR COMPUTER AND YOU ARE STILL HAVING PROBLEMS WITH THE PC AND TRANSCEIVER INTERACTING - THEN CHECK YOUR PTT ARRANGEMENT. IF YOU ARE USING THE SIMPLE ONE TRANSISTOR ARRANGEMENT ABOVE, OR YOU HAVE A HAMCOMM CIRCUIT ATTACHED TO THE SERIAL PORT AND ARE USING ITS TRANSMIT / RECEIVE FACILITY YOU ARE MAYBE  RE-INTRODUCING THE PROBLEM YOU THOUGHT YOU WERE CURING BY USING THE TWO ISOLATION TRANSFORMERS ( I.E YOU STILL HAVE GOT A COMMON GROUND CONNECTION ) YOU NEED TO USE A CIRCUIT TO ISOLATE THE PTT CIRCUIT FROM PC TO TRANSCEIVER. A SUGGESTED CIRCUIT IS FOUND BELOW - YOU OBVIOUSLY MUST STILL USE THE TWO ISOLATION TRANSFORMERS - YOU WANT NO DIRECT CONNECTION BETWEEN COMPUTER AND TRANSCEIVER. IF YOU ARE STILL HAVING PROBLEMS - THEN MOVE THE PC FURTHER AWAY FROM THE TRANSCEIVER IF POSSIBLE OR REDUCE YOUR OUTPUT POWER ! ANOTHER POTENTIAL SOURCE OF FEEDBACK COULD WELL BE DOWN TO YOUR POWERED PC SPEAKERS I HAD THIS PROBLEM WHICH WAS OVERCOME SIMPLY BY CHANGING MY SPEAKERS TO THE NONE POWERED TYPE ( I.E NO AUDIO AMP IN THE SPEAKER CASE.)

OPTO-COUPLER PTT

ALTERNATIVE CIRCUIT USING THE SFH618-2 ( AVAILABLE FROM MAPLINS )

IF THESE CIRCUITS SEEM NOT TO WORK CHECK YOU ARE USING THE RIGHT COM PORT PIN ( SEE BOTTOM OF THIS PAGE ) SWOP FROM PIN 7 TO PIN 4 - IT WILL DEPEND ON THE SOFTWARE AS TO WHICH ONE IS BEING USED ( PINS DIFFER FROM THE DB9 TO DB25 SOCKET ALSO )

PLEASE NOTE  - NO COMMON GROUND CONNECTION BETWEEN THE COMPUTER AND THE TRANSCEIVER/TRANSMITTER!

RELAY PTT

RLA SHOULD BE AS HIGH A VALUE AS POSSIBLE - AND PULL IN AT ABOUT 9 - 12 VOLTS ( THE COMPUTER HAS GOT TO SUPPLY THE ALL OF THE CURRENT FOR THE RELAY - SO BE CAREFUL IN RELAY CHOICE ) IF YOU CAN GET HOLD OF A 'DIL' TYPE RELAY - THE ABOVE CIRCUIT CAN BE FITTED INTO A 25 PIN ' D ' PLUG AND FITTED DIRECTLY TO THE COMPUTER SERIAL PORT - MAKING FOR A VERY COMPACT INSTALLATION. MAPLINS DO ONE WITH A 500 OHM COIL & 1 POLE CONTACT WITH A COIL PULL IN AT 3.7 - 7.5 VOLTS, WHICH SHOULD BE OK.

COM PORT PIN NUMBERS

	KEYING CIRCUITS
	ISOLATING THE AUDIO SIGNAL