Capacitor CI charges through resistor Rl, and when the gate level established by potentiometer R2 is high enou
gh, the SCR is triggered. Current flows through the SCR and earphones, discharging CI. 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 CI be adjusted, which charges the frequency or tone. Use a pair of 8- headphones. The telegraph key goes right into the B + line, 9-V battery.