555 Max and Min Speeds

Thread Starter

zane9000

Joined Mar 16, 2007
19
Anyone know the max and min semi-accurate frequency output of the 555? I guess the min frequency would be dependent on how much the cap you choose leaks, but what is the slowest you have been able to make it go?
 

SgtWookie

Joined Jul 17, 2007
22,219
555 timers are pretty versatile with a wide operating range. But still, even a great thing has limits.

500KHz is about the max frequency you'd want to try to run it. Beyond that, look for a different technology.

For the low end, yes - you could use a large resistor and a big cap. Big caps are expensive though. You'd be better off to use an inexpensive CMOS counter or divide-by circuit to get really long delays; your resulting circuit will have a much smaller footprint.

What is it that you're trying to do?
 

GS3

Joined Sep 21, 2007
408
Years ago I designed a simple timer to control a watering system for plants. It had a 555 with a 42.2 second period and a 4020 binary divider which provided watering every 8, 4, 2 or 1 days and watering length of 11, 22, 45 and 90 minutes (both were selected with rotary switches or could be hardwired if you wanted to save the cost of the switches). It had two solenoids which were actuated successively so you could water two areas successively. It was not a high precission thinbg but it was simple and it worked.
 

Nomad

Joined Oct 21, 2007
43
There's many different versions. some cmos versions go as high or even exceed 2 MHZ. those most common, are 500 khz as stated previously. I've seen time periods greater than a day. i wouldn't want to test longer. lol as for accuracy they will drift with supply voltage and temperature, so communications applications are out.
 

Thread Starter

zane9000

Joined Mar 16, 2007
19
Thanks for the info, that pretty much answered my question. Its not for any particular project, just something I have been curious about but couldnt find any infor. The question popped up because I am working on something that needs a delay of one or two seconds. While I know this should be within the abilities of the 555 it got me wondering just how long of a delay I could get. And wondering about that got me wondering just how short of a delay is possible.
Thanks again!
 

Distort10n

Joined Dec 25, 2006
429
An interesting tid-bit about the 555 is that you need to keep in mind parasistics on the PCB, and between the pins that can affect timing. Especially when you start to use small cap values.
Spice certainly did not help, but building a circuit a customer was having trouble with certainly confirmed her lab testing.
Also...die shrink played a part. There are better equations in certain 555 datasheets that take other affects into account.
One day, I shall derive the 555 timer equation just because.
 

GS3

Joined Sep 21, 2007
408
Thanks for the info, that pretty much answered my question. Its not for any particular project, just something I have been curious about but couldnt find any infor. The question popped up because I am working on something that needs a delay of one or two seconds. While I know this should be within the abilities of the 555 it got me wondering just how long of a delay I could get. And wondering about that got me wondering just how short of a delay is possible.
Thanks again!
Up to about a minute, no problem. More than minutes I would not recommend it and would rather use dividers or something else.
 

Nomad

Joined Oct 21, 2007
43
I know this is old, but i wanted to add that since i started playing with pics, i wouldn't even consider using a 555 as a timer anymore. can't beat it for accuracy, they're cheap, not as cheap as a 555 but still. and the ones with precision internal clocks theres zero external parts needed. or you could set with switches, or up down buttons, send it a specific time via serial input or even read a pot for variable. for timing purposes, forget 555's. just my opinion.
 

SgtWookie

Joined Jul 17, 2007
22,219
What is the widest range of frequencies anyone can get out of a circuit using just one 555 and no decade switching?
Sorry, that's classified.

You'd have to beat it out of a datasheet.

Really, it's unrealistic to attempt a really broad frequency range using just a single pot. You wouldn't have the necessary resolution. If you were looking for a particular frequency, and you had a really broad range of frequencies from a 3/4 turn pot, you'd skip right over it. It would be like throwing darts in a windtunnel; you'll never know where they might end up.
 
Last edited:

thingmaker3

Joined May 16, 2005
5,084
I know this is old, but i wanted to add that since i started playing with pics, i wouldn't even consider using a 555 as a timer anymore.
I can wire up a 555 without having to boot up any computer or learn any software or buy (or make) any programming interface. Instead of de-bugging code, I can simply tweak a trimmer pot.

Each technology will have its own advantages and limitations. I would never say never. Just my opinion. ;)
 

KL7AJ

Joined Nov 4, 2008
2,225
Anyone know the max and min semi-accurate frequency output of the 555? I guess the min frequency would be dependent on how much the cap you choose leaks, but what is the slowest you have been able to make it go?
Yes...you can make a 555 with a time constant of DAYS...but the accuracy is going to suffer, because large capacitors are not that stable. On the high end, 555s start losing performance at 1-200k. (Though I've seen them pressed a bit harder)

If you need accuracy at low frequencies, it's much better to run a 555 in its "sweet spot"....the low 10s of khz, and divide down from there. It's much easier to get SMALL precision capacitors....hence high clock frequency. Also...when you divided down the frequency, you divide down the ERROR!

eric
 

mik3

Joined Feb 4, 2008
4,843
A timer capable for time delays for several days is the 4047 IC timer. It has the counters for frequency division GS3 mentioned built-in.
 
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