#563 Dual Variable Mode Oscillator
Testing a 555 timer astable oscillator circuit that can be switched between variable duty cycle and variable frequency.
Notes
This is a very effective circuit that I found as an example in the Multisim circuit simulator samples. I’ve made a few modifications to the circuit, primarily to reduce the clock to a very low (visible) frequency.
The circuit can be manually switched between variable duty cycle and variable frequency, and it performs impressively:
- variable duty cycle mode gives almost full range control (4-96% in this build) with no significant impact on a fixed frequency
- in variable frequency mode, the duty cycle remains stable at around 50% for the entire frequency sweep
Construction
I selected VR1 = 50kΩ, and C1 = 10µF for the build in order to keep down to very low frequencies so that the output LED indicator can be followed by eye.
Performance
With VR1 = 50kΩ, and C1 = 10µF, frequency can be adjusted between 1.2Hz and 32 Hz, or duty between 4% and 96% at a fixed frequency of ~2 Hz. Here are the output scope traces for the minimum and maximum VR1 setting for variable duty (upper) and variable frequency (lower)
Some ranges I’ve measured with various VR1 component values:
| VR1 | f(min) | f(max) | duty(min) | duty(max) | f(fixed duty) |
|---|---|---|---|---|---|
| 10kΩ | 5Hz | 35Hz | 14% | 86% | ~6Hz |
| 20kΩ | 3Hz | 34Hz | 8% | 92% | ~5Hz |
| 50kΩ | 1.2Hz | 32Hz | 4% | 96% | ~2Hz |
| 100kΩ | 2Hz | 34Hz | 2% | 98% | ~1Hz |
NB: I should have switched C1 for a higher frequency range in order to capture more accurate measurements. Perhaps later?
Multisim
I only had a brief look at Multisim Live, but it seems quite capable, and is very intuitive.
Here are the results and a glimpse of the user interface for the simulation of the circuit similar to the one shown here:
