Red Phone Line A

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Retire 2 Vfe/Töiflranscei^er /n/erface schematic.

procedure- When you have finished the 5 volt and ground connections, make this connection on the bottom of this same boa rd. Free the board by removing the two large self-tapping screws from the edges of the metal cover that shields the foil side of the board. You don't have to take out all the little screws that hoid this shield to the board. Fortunately, you make the connection on the exposed edge of the board, and not the part of the board covered by the shield.

The negative-going trigger pulse is applied to pin 2 through a 1 mF capacitor. The combination of the 10 mF capacitor (tantalum, please) and the 1 meg pot make up the RC network to set the time delay on the output at pin 3. Before pin 2 is triggered, pin 3 is low. When pin 2 is triggered, pin 3 goes high and turns on the 2N2222 transistor via the 2.2k base resistor. With the transistor emitter at ground and the collector connected to the transceiver push-to-talk, the transceiver is

"You're better off with more shades of gray and less pixels than vice versa..."

Remove the board by lifting sideways, disengaging the head pins from the connecting strip on the top side. You will see two white wires connected to tne board which you do not need to remove. Tack-solder a wire to the send switch terminal as shown in Figure 2. This is a negative-going puEse which will trigger a 555 timer chip configured in the rnonostable mode Bmg this wire out with the +5 volt and ground wires. Reinsert the header pins in the connecting strip and fasten the two self- tapping screws. Before buttoning up the cabinet again, turn on the VisiTel and check for -5 volts, ground, and a negative-going pulse on rhe last wire tacked to the send switch. <A scope or logic probe will be helpful.) Close up the case, and you're ready to connect the interface as shown in Figure 2,

Operating the Interface

The operation of the interface is quite simple. You don't have to switch the red and green telephone imes.The push-to-talk circuit is just one way of using the miraculous 555 timer-

turned on for a period of time set by the RC network.

Because picture transmission lime is 5.5 seconds, set the pot so that the transceiver is keyed on for about 6 seconds and then drops out. This setting is close to the center of the pot or around 500 kO. A little expert menting with pot settings will get the delay just right.

After you've made all the connections, plug everything in, open the VisiTel doort and get a picture on the monitor. Press the send button on the VisiTel and your push-fo-talk wilt be keyed, and you'll be transmitting your picture At the end of transmission, your push-to-talk relay wiil drop out and you'll be ready to receive.

Since the picture information is sent on a bandwidth of 3 kHz or less, you can transmit on any of the amateur frequencies permitting SSTV transmission, including HF. A little DIN plug or some such connector on the leads coming out of the VisiTef might be handy for quickly disconnecting the VisiTel when your XYL wants to use it to see her grandchildren in Sheboygan! Have fun? EH

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