![]() (R1 + R2):R2 tells us 50:50 will be impossible unless R1 = 0. The origin of this myth seems to be early 555 datasheet application notes that illustrated the topology shown in Figure 1 for astable oscillation. A case in point is the oft stated (yet mistaken) notion that it’s complicated to coax the original (bipolar) version of the 555 to oscillate with a symmetrical square wave 50:50% duty cycle. Most of the former and some of the latter is justified, but sometimes a supposed shortcoming will grow to the status of “common knowledge” just because a simple fix has been overlooked, even for a part in such long, wide, and popular use as the 555. More than a half-century old, the ubiquitous and mind-bogglingly useful 555 analog timer has become a perennial object for both kudus and criticism. ![]() One resistor makes the classic 555 timer generate a symmetrical square wave 50:50 duty cycle.
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