- using a trimmer we can manually set a voltage level;
- using the PWM of the Arduino we can set the level by software.
For the manual selection, a multi-turn potentiometer put between +5 V and GND is sufficient.
While for software selection we need a low-pass filter that filters a PWM signal coming from the Arduino. PWM signals (more on this to follow) are square signals with a constant frequency but a variable pulse-width. This variability brings a variable mean value of the signal that can be extracted with a low-pass filter. A good cutoff frequency for the filter is about one hundredth of the PWM frequency and I chose about 560 Hz.
After the two threshold sources I inserted a couple of pins that allows to select, with a jumper, which source I wanted. After the selection I also added an emitter follower to decouple the sources from the Arduino pin.