Why is this LED needed?

bertus

Joined Apr 5, 2008
22,270
Hello,

The led on the base of the PNP transistor will create a voltage drop.
Depending on the used led this can be from 2 to 4 volts.
A single diode will have a voltage drop of about 0,7 volts.

Bertus
 
I think the trick here is to note the output pin-3 of the 555 is about 1.7V lower than the supply voltage. So when pin3 is high, there would still be enough voltage drop at the base of the PNP to conduct. The LED causes a voltage drop sufficient to prevent current flow. A typical diode however only has a voltage drop of 0.7V, so there is still a 1V drop at the base, causing current to flow. The LED voltage drop is much higher. Try replacing the LED with 2 or 3 diodes in series.
 
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