First effort at a better amplifier relay control output.? ?
Previous work with an older vacuum tube SB-200 amplifier consistently showed the tuned circuits in the amplifier actually improved the signal!? (With the amplifier, you can get 200-300 or so watts output.? ?There is no need for ALC from the amplifier, because full output from the sBITX simply isn't enough to saturate the SB-200 amplifier.? ?The same sort of result should generally apply with commercially provided 811A or 3-400Z 3-500Z etc amplifiers.? ? This will likely also work with solid state amplifiers, but those don't provide much in the way of signal "cleanup," like the older vacuum tube amps with their tuned circuits do!? ??
However, learned the hard way NOT to use the switched +12V analog since this is involved in small signal stage power....
Much safer to utilize the digital TX signal from the Raspberry Pi.? ?However, this is a modest current +3.3V output.??
To avoid significant loading on this signal, used a 2N7000 enhancement mode N-channel MOSFET (same idea as Ashhar's design uses repeatedly) to turn on current through a commercially available optocoupler PC817 available as a small breakout board from Amazon:? ?
This optocoupler doesn't have much "gain".
The typical modifified SB-200 with solid state input (instead of the high voltage -170VDC input that would destroy most newer transcerivers if connected by the unwary ham!) -- open circuit relay voltage of 1.58 V, and short circuit current of 4.8 mA.? ?Most of that current is required to turn on the T/R relay in the SB-200.
I changed the 1000 ohm resistor in the breakout board to a 200 ohm so it could be driven by a 5V supply with about 10mA -- so that the output optotransistor can conduct 5 mA without any problem.
Tapped into signals:
TX Digital -- pin 5 of an IC controlling the relays
Ground -- pin 8 of the same IC
+5.1 VDC -- scraped the +5V trace to gain this voltage, soldered.
Haven't fully tested, but it does go ON and OFF properly and it did positively and correctly control the T/R relay system in my SB-200.? (Remember:? this has a SOLID STATE MODIFIEED input)
There is less room available in the harrdware of the V2,, so this breakout board idea may have to be disassembled a bit, but the basic idea should still work.? ?Will test this more thoroughly to verify spectrum is unaffected and operation OK with full RF flowing.? ? Those tests haven't been done yet.
73
Gordon KX4Z
