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 Post subject: 555 Timer
PostPosted: Jan 8th, '12, 05:38 
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Anyone care to discuss how to use a 555 as a timer?

Pause for a 15 minute flood and 45 minute drain. The timer I'm thinking of would control a small dc motor for a rotating indexer. When a stop trips a limit switch, the circuit would pause for 15 minutes then the indexer would rotate to the next trigger spot.

I've used a micro in a nested do loop but don't like the overhead that is required on the micro.

Somewhere I've got pics of the 4 port, 2 inch outlet indexer I've developed. I'll have to look around.

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 Post subject: Re: 555 Timer
PostPosted: Jan 8th, '12, 08:42 
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I'd love to discuss it, but I have no idea about 555 timers.

I see them everywhere in projects of every kind all over the net, so they must have something going for them.

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 Post subject: Re: 555 Timer
PostPosted: Jan 8th, '12, 09:12 
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They are not stable for cycle times above 10 minutes.
I suppose you could have a 7.5 minute cycle time and use a decade counter ic after it to stay on for 2 cycles, then off for 7 cycles and connect reset to the 9th output pin...


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 Post subject: Re: 555 Timer
PostPosted: Jan 8th, '12, 09:29 
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it IS possible to use them for a long period, however I know of a design that uses a 555 timer to time 30mins and it is not that reliable. The manufacturer of the device has to test the capacitors used to make sure they are of an exact value, otherwise the time is way off.

using a micro is just such a simple solution...

like werdna mentioned you can use additional ICs like decade counters or similar to multiply the time out, but you are then multiplying the inaccuracy and adding complexity.
In the mean time you could be writing code for your micro :)

If you were still set on using a 555 timer make sure you use one with a CMOS input. They draw much less current and therefore can be used for longer times (because the discharge the capacitors slower)

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 Post subject: Re: 555 Timer
PostPosted: Jan 8th, '12, 16:14 
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Is the time unreliable but predictable. ie can you just adjust the count until it operates properly, or is the error variable on an ongoing manner with temperature changes or something?

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 Post subject: Re: 555 Timer
PostPosted: Jan 8th, '12, 16:29 
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As the values of R and C increase, you will get a bigger variation due to temperature changes.
As Andrew says, best to stay away from them for times above a few minutes.

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 Post subject: Re: 555 Timer
PostPosted: Jan 8th, '12, 20:54 
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Thanks to all for your insight. I think I'll stay with a micro for timing.

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 Post subject: 555 Timer
PostPosted: Jan 8th, '12, 21:17 
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There's a circuit I drew up on here somewhere. It uses 2 555's set to small increments, decade counters and gate logic to compare the counters with dip switches.
I agree. Micros are better, but it is possible to do it with a 555.

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 Post subject: Re: 555 Timer
PostPosted: Jan 8th, '12, 22:52 
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Thanks KudaPucat for the heads up.

I did a quick search on 555. Saw TC's schematic and build, on her thread.

http://backyardaquaponics.com/forum/vie ... 07#p206407

Also quickly scanned some of your remarks. A lot of stuff to take in.

I'll have to spend more time reading the entire list of references.

Thanks again!

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 Post subject: Re: 555 Timer
PostPosted: Jan 8th, '12, 23:01 
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Bob H wrote:
I'll have to spend more time reading the entire list of references.


Or get a micro starter kit and spend the time learning that, time much better spent.

I'll be completely honest with you, the company I worked for 25 years ago used a 555 timer to generate -12V for the RS-232 supply voltage.
If I had 1 pound for every return we had due to 555 timer failure, I'd be a rich man.
IMO they are hateful little things, the mosquito of the electronics world.

That was 25 years ago and silicon is much much better these days, but I think you get my drift.
:)

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 Post subject: Re: 555 Timer
PostPosted: Jan 8th, '12, 23:31 
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Bob H wrote:
Thanks to all for your insight. I think I'll stay with a micro for timing.

Sorry I didn't see that one before I posted the previous post.

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 Post subject: Re: 555 Timer
PostPosted: Jan 9th, '12, 06:03 
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yep to anyone considering this stuff.
Your time will be definitely best spent getting a development kit of some sort of micro.
And if you haven't already got one, get one you just write C code for !
Most flexible, most widely used, most powerful.
It might SEEM a little harder at the start because you have to remember a few extra brackets and semi colons but it isn't harder at all.
And you will be able to program every micro in the market, instead of just being stuck with pickaxe or aduino type things...
My 2c :D

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 Post subject: Re: 555 Timer
PostPosted: Jan 9th, '12, 06:57 
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It's not just the programming.

PICAXE and Arduino offer shields of pre-built stuff, that is plug and play. For instance, making a project wirless is just a case of spending 40$ for a pair or boards that transmit and receive, and literally plugging them in,

But perhaps more importantly, the only device you need to program a picaxe is the software (free), and a USB - 3.5mm headphone jack cable $10 or so. (mine was a $2 non standard one from China).

Once I spent the $2 on the cable (actually it was given to me - Thanks Pete) a device to check the temperature and set off an alarm at given limits takes 3 lines of code, 6 components, your alarm can be a telephone ring tone, and the entire project costs $5.

Add $40 for a pair of wireless transmitters and put the alarm on your desktop, or connect it to your pc and have it send you an email.

net server modules- gps modules - code support for temperature and sonic range finders and other robotics stuff etc etc

So there are a few advantages. But I agree, the learning C part shouldn't even come into the decision. All languages are easy to learn once you make a start, especially if you only learn the bits you need when you need them.

And you cant program them all if you learn C, because you cant program PICAXE ! :bootyshake: :)

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 Post subject: Re: 555 Timer
PostPosted: Jan 9th, '12, 07:57 
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Fair point.
I think there are other solutions with "PLUG IN" functionality.. not sure though, microchip have some stuff, probably not as diverse as the things you mentioned BW
At the end of the day there is a reason why the industry does it a certain way.
Im just saying thats the BEST way to learn, even though the learning curve is a little steeper :)

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 Post subject: Re: 555 Timer
PostPosted: Jan 9th, '12, 09:56 
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Quote:
I'll be completely honest with you, the company I worked for 25 years ago used a 555 timer to generate -12V for the RS-232 supply voltage.
If I had 1 pound for every return we had due to 555 timer failure, I'd be a rich man.
IMO they are hateful little things, the mosquito of the electronics world.

That was 25 years ago and silicon is much much better these days, but I think you get my drift.
:)

That is interesting. Akai used to use them in TVs a while ago, it was the cheapest, crappiest designed power supply I had ever seen, just a high value resistor off the smoothing cap to drop the 340v down to around 10, and into a 555 being used as the switching ic and driving a chopper transistor.
It used the typical optocoupler to adjust the output
As simple as it was, we never had to change 555s unless there was a lightning strike etc.

I would say that the 555 wasnt failing from quality, but possibly more from not enough isolation, causing static damage. They are known for being incredibly forgiving. One of the reasons they are used alot in basic circuits.


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