Timing Circuit

All depends on how accurate that you need to be really. A simple timer circuit - the CMOS version of the good old 555 is quite capable of this sort of delay - could be set up fairly precisely in monostable mode, to do 10 minutes or thereabouts, say give or take a few seconds.

If you really need an accurate 10 minutes, then you need to use a counter. You would start with a clock generator, probably a standard watch crystal at

32.768kHz, then divide it down to get your 10 minute interval. If you don't want to divide as far, you can start with a lower frequency, but it's usually convenient to use a ' binary ' frequency. A 16 bit counter on the end of a 32k clock, will get you down to a second. You then need a further 10 bits of counter and a bit of AND-ing to get you to the 10 minutes.

You could also pick a ' custom ' frequency for the clock to suit your final time, which saves the gates at the end.

Horses for courses really

Arfa

buy a cheap cooking timer

Arfa Daily spake thus:

Since it looks like 32.768 kHz is the lowest-frequency oscillator available (checked both Digikey & Jameco), I'd say all you need would be one of them (a complete oscillator, not a crystal) and 2 counter chips (like the 74HCT4040), one set to divide/16 as you explained, the other /10. This would give you a TTL pulse at the end of the countdown cycle.

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James-

A simple timer might consist of a resistor charging a capacitor, with a level detector used to determine when the desired time has been reached. A more sophisticated timer would use flip-flops to count pulses, with logic gates set to output a pulse when the state of the flip-flops reaches a predetermined value.

A "counter" usually refers to an arrangement where counting is done only during a precise period called the timebase. The result is counts per unit time, usually expressed in Hertz (Hz), or counts per second. A free-running counter would continue counting until told to stop, and the results would just be the number of pulses received since the count was started.

There is an IC that might get you started, the NE-555. Other sources make it, but always with the "555" number.

This chip can be configured to start timing on command, and charge-up a capacitor through a resistor, until a preset time has past. At that point it should reset and generate an output pulse or level shift.

The trouble with this approach is that it is best for short time delays. Ten minutes is a relatively long time, so circuit leakage may cause the delay to be longer than what you would calculate based on the R-C time constant.

Another approach to timing, is to count a clock, such as the NE-555 being used as an oscillator or using the 60 Hz powerline frequency. The counter would be started by your digital signal. When the counter reaches a predetermined value, it should reset itself and generate an output pulse or level shift. This method is better for long delays, and is less affected by circuit tolerances.

Does this help?

Fred

If you use the CMOS version - usually 7555 - the low current allows times of much more than 10 minutes.

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The problem is that you end up needing absurd component values - ten minutes delay would need 1.2Mohms and 470mf, for example, and the latter is likely to be inaccurate and probably variable.

You can use two of them (or a 556), or there used to be a chip that would work up to many hours delay, the number of which I cannot recall; I think it was a TDA- something.

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LAurence

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Laurence Taylor spake thus:

Why would you want to mess with a 555 at all? Not precise, and more complicated. I already posted a possible solution using an oscillator (1MHz) and two 74HCT4040 chips, which will give you an *exact* time and uses fewer components. (Like no resistors or capacitors, except for maybe one 0.01 uf bypass cap.)

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Well letcher laurence.... Does everyone know that you are a pedophile? laurence likes to f*ck children..... alt.support.boy-lovers