Basic SSTV keying circuit-how it works.

This is your basic transmitter keying circuit. From within the MMSSTV program you click on OPTIONS tab. Then click SETUP, then click TX tab. Under the PTT port you will select the RS232 COM port used to key your transmitter from within the SSTV program. Usually this will be COM1.

This is how it works. Pin 7 on the COM port is the “RTS-Request to send line.” Normally the voltage at this point is –12 VDC. Current cannot flow to the base of 2N2222A because of the 1N4148 diode. When you press the TX key from within your SSTV program the voltage on pin 7 changes from –12V to + 12V. Current flows through the 2.7K resistor and diode to the base of transistor causing the transistor to saturate and pull the collector to ground which keys your transmitter. At the end of your picture transmission pin 7 changes back to –12V, un-keying your transmitter.

  The transistor, diode and resistor can be easily housed inside the DB9 plastic housing. I usually use a shielded conductor from the 2N2222A collector to the transceiver keying circuit. One word of CAUTION. Don’t buy your 2N2222A transistors from Radio Shack. On several occasions they package pnp transistors as 2N2222A NPN units.