Timing Circuit

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

 

Go to http://www.ti.com and enter 74HC4060 in search box.

 

THANKS! This chip looks perfect. Good ol TI always seems to come
through for me.

 

Jimbo wrote...

The 74hc4060 is an attractive chip with its on-board oscillator,
but you should be aware cmos inverter-oscillators aren't very
predictable (from IC to IC), nor very stable after the timing
components are adjusted for your time delay. If you're only
looking for a rough 10 minutes, fine, but if you want a semi-
accurate 10 minutes you may prefer to use a second oscillator
IC, such as a cmos 555, etc. Also, the 'hc4060 isn't the most
attractive choice, because its 2^(14-1) = 8192-cycle timing
requires a pretty low oscillator frequency for 10 minute timeout
(13.65 Hz). Other parts have a higher divider ratio, such as
Motorola's (now ON Semiconductor) mc14536, with 24 bits.

The mc14536 also has an onboard oscillator, in case you still
want to give that a try. I first started using the '4536 in
the early 70s, and shipped thousands of instruments that relied
on its cool 4-bit divider-select feature. We also featured the
'4536 in our "hour of power" circuit in AoE (page 972), although
I'm not happy with circuit we show there. $0.76 at DigiKey,
very nice. http://www.onsemi.com/pub/Collateral/MC14536B-D.PDF

BTW, TI also makes the '4536, as their cd4536B, if you prefer
them. ST calls it the hcf4536B. The TI and ST datasheets,
which were modeled after the Intersil/Harris datasheet, do a
better job of showing how to use the powerful timer functions,
http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=cd4536b
http://www.st.com/stonline/products/literature/ds/4901/hcf4536.pdf

Alternately, if you want to stick with a 16-bit IC, ON Semi's
mc14541 is more attractive than the '4060, because it features
an output flip flop with a polarity selection feature (and it
also has an onboard oscillator), only $0.50 to 0.58 at DigiKey.
http://www.onsemi.com/pub/Collateral/MC14541B-D.PDF This IC is
also heavily manufactured and used. Here're Fairchild and TI's
datasheets, http://www.fairchildsemi.com/ds/CD/CD4541BC.pdf
http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=cd4541b

BTW, you can get free samples of both ICs from ON Semi. And TI.

 

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.

THANKS FOR YOUR HELP!
Jimbo

 

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?

 

That is only 5%.

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.

Once you get close, the differences in implementation become negligible.

 

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

 

Fred Bloggs wrote...

Acckk!! You're right, I mentally added in an extra factor of
60 and came up with a moderately-painful 1:1200 design. Hah!

 

Jimbo wrote...

==========
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?

 

Check out the 4536 as well. But a single 8-pin microcontroller can
give you the required accuracy with no external parts, or 10x better
with one external part. You would have to program it.


Best regards,
Spehro Pefhany

 

There is next to no sense at all in it.

The first few numbers/letters may tell you something but not a lot.

Graham

 

The first 1 or 2 digits is usually the heater voltage.

Regards
PN2222A

 

The first 1 or 2 digits is usually the heater voltage.

Regards
PN2222A

 

so you have a 300mA heater??

 

Actually the first letter is the heater voltage and the next one or two are the construction. For
example ECC83 is a 6.3VAC heater dual triode and ECF86 is 6.3VAC heater triode/pentode.

 

Winfield Hill wrote...

Actually, I take that back, Jimbo no doubt will need a pot, even
if he does use a 2% capacitor. The chip's timing factor of 2.3
has a tolerance as well, although none of the manufacturers will
give you a hint as to what it is. NSC's cd4541B datasheet does
have a frequency-deviation graph from which one can see about 2%
drop vs supply from 15 to 10V, and another -8% for 5V, along with
a 1% increase for a 50-deg C increase in temperature. One useful
thing we can take from this graph is that for operation near 5V
one should use a constant of 2.5 instead of 2.3 in the equation.

 

---
The easy way out in order to essentially remove the power supply and
temperature dependency of the chip, (without resorting to a crystal
or ceramic resonator) is to use a 7555 an an astable to determine
the clock frequency. With an initial worst-case accuracy of +/- 5%,
and your suggested 2% cap and 1% resistor, that comes out to +/- 8%.

However, using a low tempco rheostat and cap to set the center
frequency takes out the uncertainty due to the chip's initial
accuracy, leaving only the cap's tolerance and the pot's tempco to
deal with (assuming the cap's tolerance includes its tempco), so you
wind up with, essentially, a 2% machine.

 

microcontroller? I've programmed them before but I don't know of any
that are as cheap as these other chips. Also, I'm doing this to pay my
way through grad school and I'm working with a very small upstart so
any development boards/software is out of the question becuase of the
price. This is going to be a "large volume"/"low margin" product so
every extra dollar we can squeeze out of the design will have a huge
impact on the bottom line. Are you thinking about something like the
6811 or 6812?