How 555 Timers Work -- The Learning Circuit 62

The Learning Circuit: 62: How 555 Timers Work

The Learning Circuit | element14 presents | Project Videos

The 555 timer is probably the most common and popular IC to be used in hobby circuits. There are A LOT of projects out there using the 555 in various ways and it’s easy to find schematics to make a project that has already been proven. But rather than just taking plug and playing circuits with the 555 timer, Karen wants to give you the chance to understand the why of what’s happening when you use the 555 timer. In this episode, Karen breaks down what is happening inside the 555 that makes it function. Learn how the inputs interact with the supply voltage to trigger and reset the output high and low. Find out which pins can be used to adjust the threshold at which that change happens. And see which familiar components can be found within the 555.

Covering what happens in the 555 timer, Karen walks through connecting the IC to voltage and ground, following the path through the voltage divider to the 2 comparators, which in turn set and reset the flip-flop, that controls the 555’s output signal. She also reviews the basic function of the control voltage, reset, and discharge pins.

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Parents

beacon_dave over 5 years ago

The Learning Circuit continues to go from strength to strength.

Regarding the section on the reset pin 4 (at 6:20 into the video) though, am I missing something or is the on/off operation of the PNP transistor illustrated here as being the wrong way around ?
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beacon_dave over 5 years ago

The Learning Circuit continues to go from strength to strength.

Regarding the section on the reset pin 4 (at 6:20 into the video) though, am I missing something or is the on/off operation of the PNP transistor illustrated here as being the wrong way around ?
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makerkaren over 5 years ago in reply to beacon_dave

Do you mean the emitter and collector are flipped?
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beacon_dave over 5 years ago in reply to makerkaren

Hi makerkaren - I was referring to the bit where you say "...if pin 4 is ever grounded, going low, the transistor is turned off..." and vice versa.

However with a PNP type transistor if the base is made more negative than the emitter, it turns on. In the diagram the emitter is tied to +Vcc so pulling pin 4 to ground would presumably turn it on ?

Pin 4 needs to be pulled high for the 555 to function (reset is active low), however by doing this it presumably turns the transistor off ?
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jc2048 over 5 years ago in reply to beacon_dave

The emitter doesn't go to Vcc. The base-emitter diode forms part of a wired-OR circuit internally to force the state of the flip-flop (and the collector goes to the discharge transistor base so that it triggers the discharge at the same time).
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jc2048 over 5 years ago in reply to beacon_dave

This is from the ST datasheet for the NE555.
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makerkaren over 5 years ago in reply to beacon_dave

It's very possible I got them flipped. I didn't compare my diagram with the in depth schematic that shows all the transistors. I was focused more on the transistor's role in the 555 than ensuring I had the pins laid out correctly. Pobody's nerfect?
If my diagram is wrong, I would be happy to be corrected. It's all part of the learning process.
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