Timing Circuit
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From: Jimbo on 29 Jan 2006 16:36 Win, Is there a "system" for the naming of the IC out there? I keep reading things that tend to "hint" that there is a systematic way of naming these things. Isn't the formation usually "LETTERS" + "NUMBER" + "LETTERS". Have any idea as to the method these things are named?
From: Fred Bloggs on 29 Jan 2006 16:54 Jimbo wrote: > Thanks for the info. As far as accuracy, I'm looking for something > that will work with a +/- 30 seconds or so. That is only 5%. > I'm assuming that any > change in performance based upon manufacturer and/or temperature > fluctations will not be a worry. Also, I don't really care as to the > number of bits the chip has for an output. I'm thinking I'll just run > the desired outputs through an AND that sets a S-R flip/flop. You don't need to do that unless you want the timeout to be programmable, in which case none of the suggested parts are that good. > I'm sure > there is a better way of doing this and once I start with the design > maybe I'll come across some features that can help me cut down the need > for extra circuirty. Perhaps that's what the mc14541 does. Doing this > stuff in actual practice is quite a bit different than what I did for > school. It seems that there are so many permutations of doing the same > thing it's dizzing. It seems almost impossible to find the absolute > cost effective and most efficent way of doing something. Too many > options and not enough time. Once you get close, the differences in implementation become negligible.
From: Pooh Bear on 29 Jan 2006 17:18 Jimbo wrote: > I need to design a circuit that I can tell it to start (digitally), and > then it will tell me when 10 minutes have passed. I just need to be > pointed in the right direction. What's the difference bewteen a > timer/clock/counter. I have a good understanding of circuit theory but > does anyone know of a good IC that can be surface mounted (and is > cheap) that I shoulod use. Any help or tips are greatly appreciated This has to be a classic job for a baby microcontroller. Typically a tiny PIC. That involves learning about microcontrollers and doing programming though. Doing it in hardware is 'easier' for a beginner but will use lots of parts in comparison. It *won't* be 'just an IC' although the microcontroller method can be. Graham
From: Winfield Hill on 29 Jan 2006 17:47 Fred Bloggs wrote... > Jimbo wrote: >> Thanks for the info. As far as accuracy, I'm looking for >> something that will work with a +/- 30 seconds or so. > > That is only 5%. Acckk!! You're right, I mentally added in an extra factor of 60 and came up with a moderately-painful 1:1200 design. Hah! -- Thanks, - Win
From: Winfield Hill on 29 Jan 2006 18:01
Jimbo wrote... > > Thanks for the info. As far as accuracy, I'm looking for something > that will work with a +/- 30 seconds or so. I'm assuming that any > change in performance based upon manufacturer and/or temperature > fluctations will not be a worry. Also, I don't really care as to the > number of bits the chip has for an output. I'm thinking I'll just run > the desired outputs through an AND that sets a S-R flip/flop. I'm sure > there is a better way of doing this and once I start with the design > maybe I'll come across some features that can help me cut down the need > for extra circuirty. Perhaps that's what the mc14541 does. Doing this > stuff in actual practice is quite a bit different than what I did for > school. It seems that there are so many permutations of doing the same > thing it's dizzing. It seems almost impossible to find the absolute > cost effective and most efficent way of doing something. Too many > options and not enough time. ========== Oops, I made a mistaken post, with mental calculations off by 60x. Let's leave it here, below, but for sake of discussion let's assume that Jimbo's requirement was for a 0.5-second, 0.1% accuracy. :-) In practice, with his 5% spec, he can use a '4541 IC with a 294k 1% resistor and a 27nF 5% film capacitor. Or better, 2% caps like the nice Panasonic parts offered by DigiKey. That way tolerances will be in the IC, and Jimbo may be able to dispense with adjustments. =========== Jimbo, your 30-second spec is one part in 1200, within the range of analog relaxation-oscillator stability with RC components, provided they are properly chosen. A 16-bit divider (15-bits in timer mode) like a '4541 will have its oscillator running at 32768/600 = 54.6Hz, which isn't bad. Using f = 1 / 2.3 RC, and R = 249k plus a 100k pot, we get C = 0.027uF, a moderate-value capacitor for which small stable versions may be practical. Still, I'm tempted to suggest instead an 8-pin uP with a 32kHz resonator. Have you considered that possibility? -- Thanks, - Win |