A Swiss Army Knife in Disguise!
This post today is in special recognition of the segment of the blog readers who are averse to things like the Arduino MCU and programming them. This specific circuit we soon will discuss could easily be done today with an Arduino Pro-Mini or Nano. But in 2009 when this was 1st started the MCU's were not as prolific as they are today. This circuit will have appeal for those that have a non-Arduino comfort zone.
Perhaps with the exception of the P3ST 20M SSB Transceiver, where I purposely set out to not build a warmed over Bitx, EMRFD rooted or TIA based rig, often what I design is in response to a specific problem or issue. The above circuit is exactly that scenario. Soon you will also see its connection to the Swiss Army Knife.
If you check my Tri-band Transceiver found at
https://www.jessystems.com this was the starting point. That particular rig has many circuit modules and power is switched to these circuits in Receive and then to Transmit. The switching was substantial and needed a rather large relay. Yes, relay spiking that even with a snubber caused a frequency shift in the PTO (from a Ten Tec Triton IV).
To respond to that issue, I designed an equivalent of a Solid-State relay which is the upper half of the schematic. The Opto-isolator was just a way of debouncing an "ON" signal to the 7400 wired as two inverters. Pins 1,2 and 3 is one inverter and 4,5 and 6 the other.
The 10K pull up resistor with the PTT disengaged provides +5 to Pins 1 and 2 which means Pin 3 is inverted to "0" and Pin 3 now causes an inversion on pins 4 and 5 which means Pin 6 is 5 volts firing the 2N3904 and we have +12VDC for the Receive circuits.
When you hit the PTT the 10K Pull Up is set low through the 4N35 and that toggles the 7400 so that Pin 3 is now 5 Volts turning on the Transmit 12 VDC and Pin 6 is low turning off the +12VDC to the receive circuits.
[There will be a bunch of blog readers who say PFET's and why all that old stuff. Hey, I had all of the parts in the junk box, and this is a solid design. That solved the issue we saw with the relays.]
Fast Forward to 2012 when I built the KWM-4 and again lots of circuit modules and the need for extensive voltage switching. But on purpose the KWM-4 was to be both an SSB and CW transceiver. The addition of CW created the need to have some effective way of creating CW while switching in the CW only circuits. We also needed a way to Tune the Transmitter with a control on the front panel.
Those with a keen eye will see that the lower half of the schematic is an independent circuit which essentially is keying an NE555 timer IC and the input comes from the CW Key or the panel mounted Tune Switch. The outputs include a keyed voltage to key a Buffer Transistor, a circuit to trip the PTT via a small relay and voltage outputs to various parts of the circuit that are only used for CW. One of those is a CW oscillator at 455 kHz, used only on transmit. BTW the 4.7Ufd cap sets the timing for how long the timed voltage stays on. Those who do 25wpm CW likely will want a smaller value of hold time.
The 1st 455 kHz IF amp stage (bilateral) in normal operation is connected to the Collins Mechanical Filter via a relay that is only activated on CW, removing the connection to the Mechanical Filter. The NE555 produces a timed output voltage and one connection to that Pin is the relay. When that relay is energized it is sending the signal to the stage in the reverse direction. Thus, the output from the keyed Buffer stage is fed to this relay and on out to the Transmit circuits. On CW the mechanical filter is not used in transmit.
This solves a problem with the KWM-2 that uses an audio tone dumped into the Balanced Modulator. To meet FCC requitements that tone is 1350Hz which is useless today for CW.
Along the way our control board also provides a relay closure to control an external linear amp.
Hopefully as you understand what this control circuit does that you will identify its Swiss Army Knife comparison.
Lucky me the design is a CNC program and if I want another Board, I just punch the start button.
73's
Pete N6QW