On the proliferation of badly designed bike light circuits here on CL.

I have been watching the many, many copies of the simple bike light circuit, first seen here:


and am surprised that no-one seems to have noticed that there is a major design flaw - i.e. the lack of any current control resistors in series with the LEDs - which is commented upon here:


The simulations for this circuit may appear to show that it works but that is only because all 3 LEDs are absolutely identical and so share current equally.

In practice this will not happen. Even if the LEDs were identical, their forward voltages are temperature dependent. Any small temperature difference between them will change their relative forward drops. The warmest one will have the lowerst drop so it will draw more current and so will get hotter and so on.

This is a poorly designed circuit so to avoid disappointment and a lot of blown 2N3906s, please don't copy it unless you understand how to fix the design flaw!

by signality
August 13, 2015

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