Hello all. I've lurked for a long while but am coming out of the shadows to ask a question about a 555 circuit. I've been around long enough to know about the 55.5kHz "easter egg"
but I have a different issue.
I have implemented a circuit like this
one that uses steering diodes to allow for a variable duty cycle controlled by a pot (P2). I breadboarded the circuit using a CMOS 555 powered at 12 volts via a bench supply. Everything appears to work correctly. Changing the common pot P1 varies the frequency and changing P2 varies the duty cycle (I've designed it to run at about 60Hz).
However, I am a bit surprised to see that changing the duty cycle also changes the frequency. I thought that
Th = .0.693 * Rc * C
and
Tl = 0.693 * Rd * C
where Rc is the resistance during the charge cycle and Rd is the resistance during the discharge cycle. The total cycle time then would be
T = Th + Tl
If I "move" some resitance from Rc to Rd via P2 it seems to me that the total cycle time should remain unchanged.
In the circuit I built
Rc = R1 + P1 + (1 - k) * P2
and
Rd = R1 + P1 + (1 + k) * P2
where k is the position of pot P2 and varies from 0 to 1. So varying P2 "moves" resistance from Rc to Rd and vice versa.
Obviously I'm missing something and my math is wrong since my Oscilloscope measurements tell me that varying P2 also changes the frequency.
I would be very grateful if someone could enlighten me.