The above circuit should be placed in the family jewels category as this is what makes the CW transceiver work! The NE555 first introduced in 1972 is considered one of the most popular ever integrated circuits with billions being produced. Urban legend has it that the name of the device as suggested by the Product Manager was based in large because internally it has three 5K resistors.
Keep in mind there is no MCU (Arduino) in this rig but were there one much of the circuitry could be done within the Arduino. Likely a lot of the current Extras Class Hams never played with the NE555 timer so we will take a bit of time to "splain" what is happening. I say this because many early Code Practice Oscillators were built using the NE555. Thus, no code requirement no NE555 CPOs.
The NE555 is basically a programmable timer consisting of two comparators, a flip-flop and an output amplifier. The theory behind its operation is to use the principle of charging/discharging a capacitor fed by a resistor, thus an RC time constant. This is an analog timer and how accurate the timing is in large depending on the external quality and accuracy of the R and C. For more accurate timing requirements then a digital timer is the choice. But for this project, RC is "good enough".
A rudimentary explanation of operation is when the diode connected to Pins 2 & 6 is grounded (with the key) this shorts (discharges) the timing capacitor and this starts the (Charging) timing cycle and output appears on Pin 3.
The duration of the cycle with a fixed resistor is principally set by the Capacitor also connected to Pins 2 & 6 and is shown as 4.7Ufd. When the Cap is fully charged this toggles the flip-flop and the Output on Pin 3 stops. Make the Capacitor a larger value and the cycle is longer. Conversely smaller values mean a shorter cycle.
The timing cycle essentially puts a Square Wave voltage output on Pin 3 close to the 12VDC as shown connected on Pins 4 & 8. Pin 4, the Reset Pin is always tied to Vcc. A long cycle Square Wave simply looks like a DC Voltage being turned On and then Off. For those pondering -- one grounding of Pins 2 & 6 produces but a single square wave and not a continuous stream of square waves. The output is a function of how many times you hit the key and if you key too slowly it will time out depending on the value of the capacitor.
For me the 4.7Ufd Cap lets me send a complete character and then it opens up. I do this as a way of sending good CW and make it easy for the op on the other end to copy what I am sending -- this comes to about 13 wpm.
In this rig the timed 12VDC on Pin 3 feeds two locations. One location is the diode switching network and the other is a three-terminal regulator.
Thank W7ZOI and shown in SSDRA for the NE555 TR switch. This same NE555 jewel was used to create a Solid-State Power Switch as used in several of my transceivers like the KWM4. There instead of diodes I use a SN74S00 and power transistors to create a Solid-State Power Relay for supplying transmit and receive circuits power without having back emf issues.
With the key up and the timing cycle completed, Pin 3 looks like ground and the diodes forward biased with 12VDC and above ground via the 15uH RF choke. This causes the signal from the Low Pass Filter to pass on to the Receiver section. Since Pin 3 looks like ground the three terminal regulator is inactive.
Now when the timing cycle starts the two diodes are back biased (via Pin 3) making it like an open circuit and no signal is sent to the Receiver. But since the power is applied to the transmit circuits, and the diodes back biased, the IRF510 output is connected to the Low Pass Filter.
At the same time the three terminal regulator is hot, thus you are supplying a regulated timed voltage to the Transmit Mixer.
Now quite separately there is a PNP keying transistor that operates during the timing cycle so as long as you keep "keying" the cycle remains on. The second you stop keying the cycle times out based on that capacitor value and the radio returns to Receive. Some capacitors added to the TIP42 transistor can make it "hang" and thus you can soften the keying and shape the CW pulse.
This is a good place to stop for today.
73's
Pete N6QW
PS. The original designer of the NE555 is quoted as saying he never dreamed of all of the applications he saw for his design.
A cousin of mine in the mid-1970's had photography as a hobby. He contacted me regarding designing a long delay timer that could also operate a camera shutter. I used a NE555 and a car door lock solenoid as the shutter actuator. It was about $20 in parts, He told me a commercial unit was about $300 and for his hobby use my device was perfect!