555 Timer Variable Frequency, 50% Duty Cycle / Constant Frequency, Variable Duty Cycle Astable Multivibrator

This is a 555 timer astable multivibrator circuit configurable to operate with a variable frequency and 50% duty cycle or with a constant frequency and variable duty cycle. SPDT switch Smode sets the mode of operation. When Smode is thrown to the left the circuit is in Constant Frequency, Variable Duty Cycle mode. In this mode Rfdcadj controls the duty cycle. With the wiper at the center, the duty cycle is 50%. Adjusting the wiper to the left decreases the duty cycle, adjusting the wiper to the right increases the duty cycle. When Smode is thrown to the right the circuit is in Variable Frequency, 50% Duty Cycle mode. In this mode Rfdcadj controls the frequency of oscillation. Adjusting the wiper to the left decreases the frequency, adjusting the wiper to the right increases the frequency.