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NE555 timer IC output drops to 0V ground pretty well when the output is low. But, the output always falls short of the positive supply when it is high by itself. Therefore, there is usually no problem switching an NPN BJT on and off completely with an NE555 timer, but the IC may not be able to completely turn off a PNP BJT.
Using a resistor to connect the output to the positive supply (pull up resistor) will result in the output voltage rising to the full positive supply voltage when the output is high as long as nothing else needs current from the output node.
The output can still go to 0V almost perfectly while low. Causing just a little extra current to be wasted through the pull up resistor during that time. Using a relatively high value pull up resistor limits that wasted current. A 10,000 ohm (10k) pull up resistor often works out well.
When 5V is powering the NE555 timer circuit, then a pull up resistor can raise the output to 5V if little to no current is demanded of the output. When the NE555 timer output is switching a PNP BJT, the transistor will stay solidly off.
Without the pull up resistor, the NE555 timer output will lose some of the positive supply voltage while it is high. Just me touching the output resistor can get the PNP BJT (and LED load) to conduct a little when there’s no pull up resistor at the NE555 timer output. But the PNP BJT and LED load stays off completely when I touch the output resistor while there is also pull up resistor.
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