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loicalset (Electrical)
2 May 07 3:21
Hello All,

I'm in need of a retriggerable one shot delay. The application is that I will receive a series of external triggers and each time I receive a trigger (5v signal), I will close a low power (12VDC/1A) relay for 1-10 minutes (adjustable) in order to power-on the target equipment. If I receive additional triggers during this delay period, the timer will reset and begin counting to 1-10 minutes again before opening the relay and shutting off the equipment.

I was looking at the venerable 555 IC, and to get delays into the minutes range I would have to use a rather large value of capacitor - I am worried that this will significantly reduce accuracy due to leakage currents.

My question is, has anybody had experience using the 555 for long (10 minute) delay periods and has it proven accurate to +/- 30 seconds? Are there any alternative IC's I should consider? Thanks in advance for your help!
itsmoked (Electrical)
2 May 07 4:10
A 555 would be a lousy device for that.  Your concerns are justified.  These days everyone uses micros for that as they are superior and can even cost less.

What are you doing? A one-off? Or a product? or what? as the correct answer will vary significantly.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

OperaHouse (Electrical)
2 May 07 8:52
Look at the MC14541 timer chip.  This is an oscillator and counter that is quite repeatable and can give delays of many hours.  
loicalset (Electrical)
2 May 07 9:47
Keith,

Thank you for confirming that. I could use a small uC for the timing but really didn't want to get involved with programming one for a one-off prototype for the customer. I will definitely go down if I can't find a small IC though.

OperaHouse, Thanks for the lead. That looks like it may do the trick.

I appreciate the help from both of you.

-LA
loicalset (Electrical)
2 May 07 9:58
err... I will definitely go down that road if I can't find a small IC though.
Skogsgurra (Electrical)
2 May 07 10:15
Thanks for the correction - I was really worried for an hour.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

melone (Electrical)
2 May 07 10:38
Would a programmable relay work?
loicalset (Electrical)
3 May 07 2:37
Argh. So the MC14541 counts to the highest possible value of 65536 and then it flips the output low again. The minimum count frequency is 1kHz. So, the maximum output pulse time is 65 seconds (t = 1/f * counts). I was looking for something that could go up to at least 5 minutes. Adjustable 1-10 minutes, even better.

Anybody know of a higher-bit counter (bigger than 2^16) off the top of their heads? Searching through Digi datasheets now, but no luck. Hmm, maybe I could daisy chain ten MC14541's together to get my 10 minute delay, if power draw was not a concern. Otherwise it looks like I will go to a uC. I really liked the MC14541 though because it was low power. Power is an issue because this will be running on batteries.
loicalset (Electrical)
3 May 07 2:38
OperaHouse - How do I get a delay in the range of >1 min with the MC14541 IC? I might not be reading the datasheet correctly. Thanks.
richs (Computer)
3 May 07 3:52
Hiya-

If battery operation is desired, and you want to go to a uC, then you might like some of the features of the PIC.

I recently looked at using a 16F886 (a "mid range" PIC) that has a feature that is common with many of the PICS.

They have a "low power" oscillator that uses a watch crystal, 32.7KHz, to drive their 16 bit timer1 register.

The PIC sets itself up, enabling the timer1 with a value of say 0x8000 and then puts itself to sleep.  When timer1 overflows, (one second later), the PIC comes out of sleep and determines if enough time has elapsed.  If not, then it goes to sleep again.  This way, while in sleep most of the time, the PIC draws very little power.

The 10F220 parts don't have them, but the 12F629 would fit the bill.  This is an 8 pin part available in DIP or other packages and is dirt cheap.

How are you planning on adjusting the time.  If simple reprogramming of the part is all that you need, then the 12F629 would be fine.  However, if you are planning on adjusting from some sort of human interface control, then this might not be the chip for you.  They don't have a usart on them for rs232, nor do they have the pin count for some sort of display (at least IMHO).  They have only 8 pins, two for power, one an input only and the rest is general purpose.  You could whomp up some sort of extra controller to "bit bang" in a new value, or you could do a software uart for rs232 input. Looking at the data sheet it says that the EEPROM can be programmed from Vmin to 5.5v allowing EEPROM writes with as little as 2.0V.  I didn't see anything in the errata sheet that disproves this information.

Hope that this little tidbit of information helps.

Of course, this same feature might be available in the ATMEL chips, but I don't know that much of that family.

  Cheers,

   Rich S.
Skogsgurra (Electrical)
3 May 07 3:59
There's an example on page 5 in this data sheet. "Digital Timer"

http://pdf1.alldatasheet.com/datasheet-pdf/view/11988/ONSEMI/MC14541B.html

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

richs (Computer)
3 May 07 4:17
Hiya again-

Well after writing the above post, I looked at the MC14541 datasheet.

It certianly looks like you can cascade them.  By fiddling with the RC time constant you should be able to get what you want with the +/- 30 second accuracy that you desire. The PIC is overkill.

*I'd* do it with the PIC.  I keep a stock of 12F675s on hand, have the programmer, and enough of the code already debugged so it's an off the shelf design and part. I can turn a single sided PC in a couple of hours and have it to the customer the next day. Well, truth be told, I'd have to go out to the all night drug store, buy a cheap digital watch and rip the crystal out of it smile

  Cheers,

    Rich S.
itsmoked (Electrical)
3 May 07 4:48
MacGyverS2000 (Electrical)
3 May 07 7:34
I didn't read the MC14541 datasheet, but if they can be cascaded you should be able to get 65 seconds squared, or over 70 minutes of delay time.  Of course, programming it for different times each go around isn't going to be pretty, but it's doable.

Dan - Owner
http://www.Hi-TecDesigns.com

OperaHouse (Electrical)
3 May 07 8:31
I'm not going to go back to read the data sheet, but we made a product with a single chip and pot that would adjust from 12 seconds to 20 minutes.  That is a 100 to 1 range with a single  pot.  That same product was offered in many ranges from 0.5 second to 24 hours.  No need to cascade.  You are not reading something correctly, this is a very versatile timer. Repeat accuracy is easily better than .5% and timing can be calibrated with a frequency counter.
jimkirk (Electrical)
3 May 07 8:55
According to the Fairchild spec for the 4541, the oscillator range can go well below 1 kHz.  Maybe the accuracy suffers.  "Oscillator frequency range ? DC to 100 kHz."  Also you can input your own clock at pin 3.
loicalset (Electrical)
3 May 07 10:02
Ah ok. I will go back and reread the sheet. Thanks!
logbook (Electrical)
6 May 07 12:03
ON-semiconductor data sheet: MC14541B

Clock conditioning circuit permits operation with very slow clock rise and fall times.

Clock pulse width  - no maximums specified.
Clock pulse frequency -  no minimums specified.

Figure 5 slows a graph with clock frequency down to 100Hz.

There is a formula for clock frequency giving a range from 1kHz to 100kHz, but that is just because the formula gets inaccurate above and below these values. However, this is just because the curve is not ideally straight.

I have used these chips for 30 minute delays and they are ideal for your application.
cbarn24050 (Industrial)
6 May 07 12:30
The HEF4060 will go down to at least 30hz, at 14 bits that just about gets you there.
loicalset (Electrical)
7 May 07 0:30
Perfect. Thanks for everyone's help!

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