Help understanding a few basic electronics questions

[Ulnarian]

Hey all, I am an extreme beginner to electronics. Anyways, I came across the following schematic and I have a few questions regarding it:



First off, C=Capacitor and R=Resistor (sorry for the crappy diagram )

Anyways, this is a circuit which creates a flashing LED.

My questions are this:

1) Why isn't the LED always on? Does the capacitor block the current flow as it charges? If so, what causes the capacitor to discharge? Does it discharge because the supply voltage is brought low when the capacitor is charged triggering the output of the 555?

2) My book says that R1 and R2 control the timing and duration of the flashes. However, why is it necessary to have two resistors here, wouldn't one work just as well?

3) As I understand it, current flows from the positive to the negative, if this is the case, then what happens when the current flowing from the positive terminal of the battery through R3 smacks head on with the current coming out of the output pin (pin 3).

Thanks!

 

[karthikkrv85]

Hai

Have you gone through the datasheet?

Please refer the pdf, i hope it will give you more details about the application

https://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lm555&reg=en&fileType=pdf

Thanks

 

[mister_rf]

Some circuits are not always as simple as they may appear at first look. Difficult to learn about 555 serie circuits reading datasheets…
To get some basic idea about the 555 circuits see for example this site:

555 Timer Tutorials


But you need to begin by learning the basics of electronics: what a resistor and a capacitor are, what diodes and transistors are, how to calculate voltage and resistance, what a circuit is and later how to read a circuit diagram.

There are many different websites that offer tutorials in these subjects that cover these things for the absolute beginner all the way in this field. Some tutorials have lesson plans that include practical lessons for building circuits and other basic experiments. It helps also to have a couple of good books to reference while you're working.

A Rough Guide to Electronics
101 Electronics Links - www.101science.com
**broken link removed**

 

[Syncopator]

Here are two more links to tutorials on the 555 timer.


555 and 556 Timer Circuits

 

[Ulnarian]

Thanks for the links! (even if I didnt really get an answer to any of my questions...). Most 555 tutorials Ive seen just tell you how to use a 555 but dont really do a good job explaining the basic mechanics of how they actually work (which is what Im really interested in at this point). However, I will check out links and see what I can learn from them.

 

[gabexhun]

Hi,
I would advise to read/study some basic electronics, but I'll try to make some points clear, briefly.
The 555 "measures" the time by charging and discharging the capacitor. As if you would charge your mobile phone's battery to measure 2 hours 30 minutes until it's fully charged, and then would discharge it to measure 5 days 14 hours. Same happens here, one resistor charges the capacitor to 2/3 level, and the other resistor (or both together, I'm not sure) discharges it to 1/3 level. The output of the 555 is showing in which state (charge/discharge) it is. If the two durations equal (unlike your mobile phone hopefully), then the LED on time will be equal with the LED off time (similarly to a car's turning light, where it's 0.6s on, 0.6s off).
And your third question: the LEDs anode is connected to the supply voltage, let's say 5V (I can't view your schematic correctly). If the output of the 555 is zero volt, then current will be sinked by the output pin, the same current that lights the LED up. If the output of the 555 jumps to 5V (meaning it started the other state of charge/discharge), then the LED and R3(?) will have 5V on both ends, so no current will flow, since there is no voltage difference.
In short: the output is not a current source, it is more of a voltage source.

I hope it helps, good luck with your studies

Gabor