Re: trimpot

On Fri, 8 Feb 2008 09:50:48 -0800 (PST), bill.sloman@xxxxxxxx wrote:

On Feb 8, 3:53 pm, John Fields <jfie...@xxxxxxxxxxxxxxxxxxxxx> wrote:
On Fri, 8 Feb 2008 04:04:38 -0800 (PST), bill.slo...@xxxxxxxx wrote:
On Feb 8, 9:51 am, jchan...@xxxxxxxxxxx wrote:
I hope I do not sound stupid.  I am good at soldering kits together
but do not understand all what is going on with the electronics.  I
have a 5 led random/sequencer flasher kit I purchased and built.  It
works good but I want to slow the flash rate down quite a bit.  I know
the trimpot controls the speed so is it as simple as putting in a
different trimpot?  If so, how do I figure out which one?  The
schematic of the kit can be found athttp://www.kitsrus.com/pdf/k52.pdf

The kit doesn't identify the integrated circuit that contains the
oscillator and controls the LEDs.

It is probably some kind of CMOS chip - the Vss label on the circuit
diagram suggets as much - but you are going to have to take a close
look at the chip to see if you can see some kind of part number on it
before we could be all that much help.

---
That seems highly unlikely in that the first sentence in the link:

"This cmos VLSI single chip-on-board is designed for
electronic toy and warning light applications."

identifies "chip-on-board" (COB) as the means used to affix the chip
to the board.

In that method of construction the bare die is affixed to the traces
on the PCB and then covered over with a dollop of epoxy in order to
protect it mechanically and environmentally.

Consequently, there is usually no way to identify the chip or its
internal circuitry.
---

Not exactly true. There are solvents designed to soften epoxy resin,
and once you've got rid of the blob of epoxy, you can look at the
surface of the chip where there is usually some kind of mnufacturers
logo and a part number.

---
Hmmm...

As usual, you seem to have taken information which was provided you
after the fact in order to pretend that you knew what was going on
before the fact.
---

Back when I did this, a good optical microscope could give you enough
magnification to read the logo and the number. You might need an
electron microscope these days.

The OP probably hasn't got access to this sort of gear, so you aren't
really misleading him.

---
Misleading him?

Unlike you, my aim is to enlighten and never to mislead, even at the
cost of having to admit ignorance and learn everything all over
again.

Again, unlike you, who, with your:

"but you are going to have to take a close
look at the chip"

are implying that there are markings on what you thought was a
packaged, exposed integrated circuit on the board which would
identify it.
---

The situation doesn't look promising. If there was an explicit
capacitor on the schematic, it would be easy enough to replace it with
a higher-capacitance part which should then give a lower the
oscillator frequency. The absence of an explicit external capacitor
suugests that the circuit might rely on an internal capacitor,
possibly part of the integrated circuit, which you wouldn't be able to
get at.

A higher value potentiometer might lower the operating frequency, but
all the potnetiomenter ranges I know stop at 1M so this isn't a
practicable option.

---
Don't get around much any more, huh?

True,

They seem to be as common as dirt:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=262968;keywo...

http://www.mouser.com/search/Refine.aspx?Ne=254360+1447464&N=254360+1...

I couldn't find any in the Farnell catalogue, so I don't think "common
as dirt" is entirely accurate.

---
Regardless of your inability to find stock, I'm sure that Farnell
doesn't consider themselves above supplying "common as dirt" parts,
and the fact remains that your take on the rarity of rheostats with
a terminal resistance of > 1e6 ohms was flawed.
---

And dirt is a problem at that sort of
impedance - if you aren't careful about washing off residual flux, you
don't exactly see 5M between the pins of the device.

---
Got some data, Mr. bull***?
---

Besides, if he's not looking for being able to vary the flash rate
he can simply replace the rheostat with a fixed resistor.

It does look as if the pot is being used as a rheostat, but that is an
assumption.

---
Really?

With the arm of the pot being tied to one of the ends of the element
what, other than a rheostat, would call it?
---

Assuming, of course, that the value of the resistance doesn't exceed
whatever might be the chip's limitation.

Another assumption.

---
Geez, it seems you missed it, but that's what I said.

It might be that the chip can't take much more than a megohm across
its oscillator inputs and still generate a clock.
---

Easy way for him to find out is to try it.

Getting resistors over 1M is not entirely trivial - they are readily
available, but do have to dig through the catalogues to find them.

---
As opposed to what?

2200 ohm resistors which cry out "Please select me"?

Sloman, you're a loser.


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