How the led_brightnesses Script Works

Remember from the Connect and Blink a Light activity’s On-Off Signals section that a blinking light has a period, frequency, and duty cycle.  The period is the total on/off time for a single blink, abbreviated T and measured in s.  The frequency is the blinks per second, abbreviated f and measured in Hz.  The duty cycle is the percent of the period that the light is on.

The write_analog() method makes the light blink at 50 times per second (f = 50 Hz).  That’s faster than the eye can see, but the eye does detect the high time of each blink as brightness.  If the light is on for ½ the time, it appears to be half as bright.  On for ¼ of the time -¼ as bright, and so on…

Remember that the period is 1/f.  In the case of write_analog, the micro:bit sets the light's blink period to T = 1/50 Hz = 20 ms.  You have 1024 options for how long the light stays on during the 20 ms period — from 0/1024ths to 1023/1024ths of the time.  For 50% duty cycle (and 50% brightness), use pin14.write_analog(512).  That’ll give you 512/1024ths of on time, which is 50%.

Example: Set the brightness to 25%.
Solution: For 25% brightness, we need 25% duty cycle.  0.25 x 1024 = 256, so use 256 in write_analog, like this: pin14.write_analog(256).

Try This: Brightness Fade-in

Here is an example that uses a for loop inside a while loop to repeatedly fade-in brightness from 0 to max (1023).

• Enter the led_brightnesses_loop_up script (python.microbit.org/v/2 matches the screencaptures).
• Set the Script Name field to led_brightnesses_loop_up.
• Click Load/Save, and then click Download Project Hex to save your work.  Close the Load/Save dialog box after downloading.
• In the python.microbit.org editor, click Connect, Flash, and then Disconnect.   
• Watch the LED gradually go from completely off to fully on.