Series E · The Duty Cycle Made Physical

The Diode Trick

555 astable · one diode · independent on/off · any duty cycle

The single most common 555+diode circuit. Without the diode, the duty cycle is locked above 50%. Add one diode across R2 and you split the timing into two one-way paths — charge through R1, discharge through R2 — so you can set on-time and off-time independently. The duty cycle, made physical.

◆ the circuit

◆ the scope

output → LED
toggle to split the path
10kΩ
47kΩ
—frequency
—duty cycle
—t_high
—t_low
555 ASTABLE · DIODE ACROSS R2 · CHARGE THROUGH R1, DISCHARGE THROUGH R2 · INDEPENDENT ON/OFF
NO DIODE → DUTY LOCKED >50% · DIODE → ANY DUTY (R1<R2 UNDER 50%, R1=R2 AT 50%, R1>R2 OVER)
ONE DIODE SPLITS ONE BIDIRECTIONAL RC PATH INTO TWO ONE-WAY PATHS = THE ASYMMETRIC CLOCK
THE DIODE TRICK · A PURPLE PAPER · SERIES E · JUNE 2026