# Explanation of 555 astable circuit

Discussion in 'The Projects Forum' started by jubsy, Mar 5, 2012.

1. ### jubsy Thread Starter New Member

Mar 5, 2012
1
0
I'm trying to get into electronics and I'm starting with a simple 555 timer circuit. I'm reading up on general circuit design but I find most websites tend to just give circuit diagrams with no explanation of the purpose of the components. I've not got any intuition for the various components yet so things are a bit confusing.

The circuit I'm using is a 555 astable one from here:
http://www.kpsec.freeuk.com/555timer.htm#astable

I want a 50% duty cycle. I can see examples with a diode across R2 and presumably with R1 and R2 equal it would be 50% (more or less). With my limited knowledge though I wonder why two resistors are required in that case (ie. why not remove R1 and the diode). The only reason I can think of is that R1 is doing something else in the circuit other than affecting the charge/discharge time. I've also read that it's not accurate to use a single resistor. Could anyone enlighten me as to why I need R1? Or is there a good resource that explains the 555 astable circuit thoroughly?

2. ### Experimentonomen Member

Feb 16, 2011
331
46
flush pin 7 and wire a resistor from pin 3 to pin 2 and 6, will give u nice 50% duty cycle.

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540

Make R1 as large as you can, and make R2 as small as you can (1KΩ typical minimum). You can not hit 50%, but you can get very very close.

This circuit will cover the entire duty cycle spectrum, but is a bit unstable if the power supply voltage varies.

If you use a CMOS 555 and a large resistor this will be a very stable 50%. If you use a conventional 555 it will not be, for the reasons explained in this article.

555 Hysteretic Oscillator

The above schematics are from my cookbook...

My Cookbook

Bill's Index

If there is something I can help specifically with just ask.

For an explanation of how a 555 astable circuit works look at the theory of operation in these articles, the first deals with 555 specifics, the second on the astable configuration.

555 Schmitt Trigger

CMOS 555 Long Duration Minimum Parts LED Flasher

The 555 Projects