Capacitor C 1 charges through resistor Rl, and when the gate level established by potentiometer R2 is high enough, the SCR is triggered. Current flows through the SCR and earphones, discharging Cl. The anode voltage and current drop to a low level, so the SCR stops conducting and the cycle is repeated.
Resistor R2 lets the gate potential across Cl be adjusted, which charges the frequency or tone. Use a pair of 8-0 headphones. The telegraph key goes right into the B + line, 9-V battery.