Timing Circuit

[Jimbo]

OPPS... OK I think I understand what you saying. By using a
Microcontroller there is probably a lot of other logic I can replace as
well. I'll have to sleep on that one. Perhaps I'll design the product
both ways and then make a comparsion. I don't think I have enough
experience at this point to simply make a "judgment call".

 

[Barry Lennox]

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

I built quite large numbers of a semi-smart battery charger that used
the SMD 4541 timer to generate 14 hours on time. With a decent R and C
in the oscillator, the end time never varied by more than +/- 3-4
minutes, or better than 1%. The divider chain lets you select divide
by 256,1024,8192 or 65636, and it has a couple of neat control
features, dirt cheap too, IIRC, about $1 each.

Barry Lennox

 

[Rich Grise]

OPPS... OK I think I understand what you saying. By using a
Microcontroller there is probably a lot of other logic I can replace as
well. I'll have to sleep on that one. Perhaps I'll design the product
both ways and then make a comparsion. I don't think I have enough
experience at this point to simply make a "judgment call".

Well, you've got a lot of options:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=74

And they show the list prices in their parametic charts, too.

Have Fun!
Rich

 

[Spehro Pefhany]

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?

There are sub-dollar parts (qty. 100) that can exceed your accuracy
spec with no external parts (internal calibrated RC oscillator) and
(most important) NO manual trimming or accuracy testing. By using an
8-pin part with an external resonator you could get 0.5% accuracy for
about 65 cents (qty. 100)- eg. ATtiny11. You'd have a few I/Os left to
play with, can add additional functionality cheaply, and the circuit
will become more difficult to copy. Seems like a no-brainer. If the
startup will pay you something for the couple of weeks it will take
you to figure this stuff out (almost all learning curve), you'll both
be ahead.


Best regards,
Spehro Pefhany

 

[Winfield Hill]

Spehro Pefhany wrote...

There are sub-dollar parts (qty. 100) that can exceed your accuracy
spec with no external parts (internal calibrated RC oscillator) and
(most important) NO manual trimming or accuracy testing. By using an
8-pin part with an external resonator you could get 0.5% accuracy for
about 65 cents (qty. 100)- eg. ATtiny11. You'd have a few I/Os left to
play with, can add additional functionality cheaply, and the circuit
will become more difficult to copy. Seems like a no-brainer. If the
startup will pay you something for the couple of weeks it will take
you to figure this stuff out (almost all learning curve), you'll both
be ahead.

I second that.

 

[Peter Bennett]

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.

But an ECC 83 is a European 12AX7, which has a centre-tapped 12 volt
filament.


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Ken Smith]

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.

Use a thru hole film cap. After it is installed, cycle the whole PCB
between the hot box and the freezer a few times befor you adjust the pot.
The quick warm up and cool down work the stresses out of the parts and get
the initial drift mostly over with before you tweek the pot.

If you do 90% of a resistor's value with a fixed resistor and the last 10
with a pot, you can reduce the tempco effects in the pot and make the
tweeking easier.

 

[Phil Hobbs]

If you do 90% of a resistor's value with a fixed resistor and the last 10
with a pot, you can reduce the tempco effects in the pot and make the
tweeking easier.

The wiper connection is liable to become unreliable, so it's usually
better to combine pots and fixed resistors in parallel, or better,
series-parallel, to confine the adjustment range. Either way, the thing
will still work at some level if the wiper opens.

Cheers,

Phil Hobbs

 

[Ken Smith]

The wiper connection is liable to become unreliable, so it's usually
better to combine pots and fixed resistors in parallel, or better,
series-parallel, to confine the adjustment range. Either way, the thing
will still work at some level if the wiper opens.

It depends on whether a little out of spec is better than completely
stopped.

You could also use one of those EEProm based digital pots. In 555 like
circuits, changing the effective voltage source for the resistor also can
be used to trim the frequency.

 

[Jimbo]

What's the "wiper connection"?

 

[Don Bowey]

What's the "wiper connection"?

A new spy novel title? Unless you post a quote of what you are referring
to, it's hard to tell.

Don

 

[Spehro Pefhany]

What's the "wiper connection"?

The wiper is the metal bit wot touches the resistance element.
There are actually two sliding contacts involved in most (not all)
pots, a sliding low-resistance connnection to the moving contact and
the wiper contact to the element. Good quality pots and trimpots are
actually extremely reliable in most situations-- billions of them are
made every year. In production situations it's nice to avoid the
labor of adjusting them, but even that is a mixed blessing as much
of the adjustment process can be an essential part of the test
procedure.


Best regards,
Spehro Pefhany