# helping pick resistor values for 555

Discussion in 'The Projects Forum' started by campeck, Sep 15, 2010.

1. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
I need a 555 to go from .6Hz to 30hz only being able to change one of the resistors by 1k. duty cycle is irrelevant. I'm having trouble getting these numbers with calculators and since I suck at math I'm not good at flipping formulas around the way I need them. Could someone help and explain how you got the values?

Thank you!

PS. I am actually not sure 30Hz is what I want for max. Is there a website where I can visualize or hear a 30hz clicking/flashing?

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
The standard form astable circuit using the 555 timer probably won't give you workable practical values for such a small control change of 1000Ω in one of the two usual timing resistors [normally designated RA & RB].

Perhaps you might explain why you have this constraint.

The problem lies in satisfying the condition for the required astable frequency range of 0.6 to 30 Hz based on the astable formula ...

f=1.49/((RA+2RB)C)

Whether you change either RA or RB by a mere 1kΩ leads to unrealistic values.

3. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
Could it be done with 10k 50k or 100k?
I am using a digi pot to control it.

I thought the ad5220 was 1k but it's 10k. So thats what I have right now.

4. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
10K or better would be more in line. The reason for staying away from low value resistances in the timing circuit is that in most cases that equates to a a very large timing cap. High value electrolytics usually exhibit a fairly decent (and unpredictable) amount of leakage current so you end up with a rather inaccurate &/or unpredictable circuit.

The eye can't see anything that flashes much over 10 Hz nor can you really hear frequencies that low (much less easily reporoduce them) - they become more of a physical feeling. http://en.wikipedia.org/wiki/Audio_frequency

There are tons of online java calculators for 555 ICs, here's one of the better simple ones:
http://freespace.virgin.net/matt.waite/resource/handy/pinouts/555/index.htm

Last edited: Sep 16, 2010

Mar 24, 2008
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6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
If you have installed Linear Technology's LTSpice (free download), you can run the attached 555 simulation with different values for resistors and caps, and get a 5-second .wav file that you can play on your computer's speakers.

In LM555.zip, there are two files:

LM555.sym goes in \Program Files\Linear Technology\SwitcherCad\lib\sym\misc
LM555.sub goes in \Program Files\Linear Technology\SwitcherCad\lib\sub

Your directory structure may not be exactly that, but it will be similar.

Here's a neat piece of software for you to download that works in freeware mode:
http://www.schematica.com/555_Timer_design/555_Timer_PRO.htm

It takes the grief out of calculating your values of R and C. Just tell it what frequency and % duty cycle you want, and click the "calculate" button.

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7. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
Too good actually!

I am setting up one 555 at a set frequency to move the wiper of the digipot that will change the freq of another 555 triggering a 4017. So that the speed of chase will ramp up. I need the max speed to be around 30Hz. And since the digipot will hit it's limit and stop I don't really need to worry about stopping it or turning off the 555 controlling it once I hit 30Hz.

I need to draw a schematic for you guys to look at. Because once the pot hits it's limit I will feed that voltage into an opamp that will then turn on a relay which will make the second 555 bypass the 4017 and go straight to my 4 relays which will make all my lights flash at once. Schematic to come later tonight. And I will try those calculators. Also ordering some higher value digipots.

8. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
Free status appears to have changed:
555 Timer PRO and 555 Timer PRO EX (extended) are combined in the same demo download. Simply toggle between modes to view the features of each. Most resistor values are masked in the demo; some circuits are electronically blurred; some menu items are disabled.

9. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
Ok so Rb of an astable 555 will be replaced with a DS1804-50
A 50k ohm digi pot. The max wiper current is 1ma

using. a .022uF cap and Ra of 2200 and a protection resistor of 1k in series with the digipot I am at 3ma when the cap needs to be discharged through pin7. That happens at 3.33v giving me 3.33/1000 = .00333A

Can I use pin 5 to bring that voltage down to 1v?

heres a quick look at what I have.

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10. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
Ok...I give up for now.
I'm going to buy a \$25 Basic stamp 2 to control this. I just drew out a crude schematic of what I would need and I just don't have the time to do this. I may still make it for practice. But I think I am going to work smarter, not harder on this one.