I have a Relaxation Oscillator here. What I'm trying to do is to change the duty cycle of ca 50% to 10%.
What I thought of and tried to change R1 and C1 values so the time the capacitor loads and unloads changes. But it stays at ca 50%.
What I am trying to find is a way to change the duty cycle to 10%.
Charge the capacitor faster (or slower) than you discharge it.
For example, replace R1 with this:
simulate this circuit – Schematic created using CircuitLab
Now, when discharging (Vout low), D1 conducts putting R1 and R2 in parallel. This will reduce the period, but also increase the duty cycle (by decreasing the low period). If you wanted to reduce teh duty cycle, reverse the diode.
Instead of a single resistor R1 use different resistance values for charging and discharging the capacitor. You can use diodes to separate the current directions.
Adjustable duty cycle without changing the frequency very much can be achieved by replacing R1 with this:
47 kOhm potentiometer has been chosen because it's generally available. Have a linear scale version.
You have only plusminus 5V operating voltages. The voltage drop in 1N4148 is a little unpredictable and it's variance can be too large, if you expect high precision results. In that case you should use mosfet switches to charge and discharge C1 through resistors.
Instead of grounding R2, return it to a large negative voltage.
If you trust your -5V DC supply, use that. You can split R2 into two resistors for this purpose: their ratio affects duty cycle. Perhaps not the best way, because this method includes the op-amp saturation voltage.
