Pull up resistor helps NE555 timer output turn off PNP BJT circuit

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.

Donate: Via Patreon

Video below goes live 6am CST on 1 May 2023.

Good topics to check out next:

List Of More Complex Electronic Demo Circuits

P Channel Enhancement Mode MOSFET Transistor Basics

Low power voltage tripler circuit using Schottky diodes 555 timer MOSFET inverter and capacitors

Bipolar Junction Transistor basics – BJTs

NPN BJT switch circuit

PNP BJT switch circuit

LED on when it is dark circuit using PNP BJT LDR – 2N3906 and Light Dependent Resistor

Light controlled NPN BJT switch – LDR high and low side

2N2222 TO-92 datasheet google search difficulties – NPN BJT 2N2222A PN2222A P2N2222A

 To support this site, check out the following links:

• Check out my YouTube videos! https://www.youtube.com/c/Electronzap/videos
• Products I used in my videos or otherwise think look like a good buy. As an Amazon associate, I earn from qualifying purchases. https://www.amazon.com/shop/electronzapdotcom

Home page

• Information on this site is not guaranteed to be accurate. Always consult the manufacturer info/datasheet of parts you use. Research the proper safety precautions for everything you do.
• Electronzap is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.