LED Flasher

Jim Thompson · Aug 16, 2003

I was sitting here amusing myself this afternoon pondering how *I*
would design a high efficiency LED flasher.

The result may be seen at LED-Flasher-2.pdf on the S.E.D/Schematics
page of my website.

I didn't have a 3V LED model available but, since the circuit doubles
the rail voltage, it should easily drive a 3V LED with a 3V supply.

...Jim Thompson
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Mark Jones · Aug 16, 2003

Jim said:
I was sitting here amusing myself this afternoon pondering how
*I* would design a high efficiency LED flasher.

The result may be seen at LED-Flasher-2.pdf on the
S.E.D/Schematics page of my website.

I didn't have a 3V LED model available but, since the circuit
doubles the rail voltage, it should easily drive a 3V LED with
a 3V supply.

...Jim Thompson

Is it too much to ask for the theory of operation? Is the IC a
schmitt-trigger?

Kevin McMurtrie · Aug 16, 2003

Jim Thompson said:
I was sitting here amusing myself this afternoon pondering how *I*
would design a high efficiency LED flasher.

The result may be seen at LED-Flasher-2.pdf on the S.E.D/Schematics
page of my website.

I didn't have a 3V LED model available but, since the circuit doubles
the rail voltage, it should easily drive a 3V LED with a 3V supply.

...Jim Thompson

That's the biggest heap of chips I've ever seen to blink an LED and it's
not even efficient! It's worse than the old LM3909.

If you want to go nuts with chips, at least use a flyback booster. You
can even bootstrap the boosted LED voltage to drive the PWM circuit,
allowing operation down to a few mV after a starting threshold is
reached.

Watson A.Name - 'Watt Sun' · Aug 16, 2003

That's the biggest heap of chips I've ever seen to blink an LED and it's
not even efficient! It's worse than the old LM3909.

If you want to go nuts with chips, at least use a flyback booster. You
can even bootstrap the boosted LED voltage to drive the PWM circuit,
allowing operation down to a few mV after a starting threshold is
reached.

So show us a circuit. The one thing I don't care for is the
LMC67whatever it was that is probably hard to find. Other than that,
it looks a bit like Dave Johnsion's flashers at
http://www.imagineeringezine.com/e-zine/hcircuits.html#LED Circuits.

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Ian Stirling · Aug 16, 2003

In sci.electronics.design Jim Thompson said:
I was sitting here amusing myself this afternoon pondering how *I*
would design a high efficiency LED flasher.

The result may be seen at LED-Flasher-2.pdf on the S.E.D/Schematics
page of my website.

I didn't have a 3V LED model available but, since the circuit doubles
the rail voltage, it should easily drive a 3V LED with a 3V supply.

Hmm.
Bit bulky.
Currently pondering my design for a flashlight/compass/key-finder.
Take a bit of sapphire a centimeter and a half in diameter, and 5cm or
so long. Facet the outside a bit to knock the corners off and add a little
sparkle.
Now bore a hole and take a rod of sapphire of the appropriate size, and
carve it to accept a few dice of varying colours, which are run at an average
current of a few microamps, and pulsed every 30 seconds for a few milliseconds.
The battery is inside a decorative 'totem pole', and all covered by a
stainless/silver cap.

The flashlight is triggered (I think, design work continues) by a sharp knock.

Recharging happens due to a tiny bit of solar cell incorporated into the cap,
which is charged by an optical charger. (or sunlight)

This was triggered after seeing a nice bit of sapphire on ebay.
(pale blue)

Battery a 1/3 NiCd AAAA probably.

Jim Thompson · Aug 16, 2003

I was sitting here amusing myself this afternoon pondering how *I*
would design a high efficiency LED flasher.

The result may be seen at LED-Flasher-2.pdf on the S.E.D/Schematics
page of my website.

I didn't have a 3V LED model available but, since the circuit doubles
the rail voltage, it should easily drive a 3V LED with a 3V supply.

...Jim Thompson

480)460-2350 | |
| Jim-T@analog_innovations.com Fax

Jim Thompson · Aug 17, 2003

That '6762 takes a rapid nosedive to off for V+ <2.7V- doesn't seem like
it can run those batteries all the way to total depletion.

Like I say, I was just 'musing myself... but does the real part "die"
at +2.7V? The published model is still working at +2V. My experience
with CMOS says they'll usually work down to 2*VT and sometimes lower.

...Jim Thompson
--
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480)460-2350 | |
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Jim Thompson · Aug 17, 2003

I've never used it but the datasheet shows that at some point guaranteed
to be no greater than 2.7V, the supply current takes a steep nosedive to
off. Another catch to this type of comparator in applications with
extremely slow input rates is the steep increase of supply current as
input threshold is approached. This could jump to 100uA's or more, mA's
in the case of logic Schmitts. Maxim has a good low voltage comparator
specifically designed to suppress this effect. I notice the
corresponding graph is absent from National's datasheet:-(

I'll look up the Maxim part and place it in the circuit.

(You may note that I can now emulate parts with 8, 14, or 16-pin, or
whatever, representations, and call the equivalent SUBCKT ;-)

...Jim Thompson
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Watson A.Name - 'Watt Sun' · Aug 17, 2003

I've never used it but the datasheet shows that at some point guaranteed
to be no greater than 2.7V, the supply current takes a steep nosedive to
off. Another catch to this type of comparator in applications with
extremely slow input rates is the steep increase of supply current as
input threshold is approached. This could jump to 100uA's or more, mA's
in the case of logic Schmitts. Maxim has a good low voltage comparator
specifically designed to suppress this effect. I notice the
corresponding graph is absent from National's datasheet:-(

I have a question along this same line. On Dave Johnson's
http://www.discovercircuits.com/PDF-FILES/hourmtr.pdf, he has an hour
meter that uses a CMOS inverter as an amplifier. He shows the output
tied to input with a 22 Meg resistor, to make it linear. He claims
the whole circuit uses only 2 microamps, which seems low to me. I
think that the CMOS chip would draw more than that because it's
biasing itself right in the middle, halfway between + supply and
ground. The supply voltage is 3VDC. The inverter is a 4069B.
I don't have one laying around to try and see if it's really true.

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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
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Jim Thompson · Aug 17, 2003

Jim said:
Jim said:

[snip]
That '6762 takes a rapid nosedive to off for V+ <2.7V- doesn't seem like
it can run those batteries all the way to total depletion.

Like I say, I was just 'musing myself... but does the real part "die"
at +2.7V? The published model is still working at +2V. My experience
with CMOS says they'll usually work down to 2*VT and sometimes lower.

...Jim Thompson

Click to expand...

I've never used it but the datasheet shows that at some point guaranteed
to be no greater than 2.7V, the supply current takes a steep nosedive to
off. Another catch to this type of comparator in applications with
extremely slow input rates is the steep increase of supply current as
input threshold is approached. This could jump to 100uA's or more, mA's
in the case of logic Schmitts. Maxim has a good low voltage comparator
specifically designed to suppress this effect. I notice the
corresponding graph is absent from National's datasheet:-(

Click to expand...

I have a question along this same line. On Dave Johnson's
http://www.discovercircuits.com/PDF-FILES/hourmtr.pdf, he has an hour
meter that uses a CMOS inverter as an amplifier. He shows the output
tied to input with a 22 Meg resistor, to make it linear. He claims
the whole circuit uses only 2 microamps, which seems low to me. I
think that the CMOS chip would draw more than that because it's
biasing itself right in the middle, halfway between + supply and
ground. The supply voltage is 3VDC. The inverter is a 4069B.
I don't have one laying around to try and see if it's really true.

A 4069B is old-style high-voltage CMOS. At 3V rail you are probably
at or below 2*VT, thus low consumption.

Since my simulation uses a device-level model (*not* behavioral) I
just re-checked the results and zoomed in to the LMC6762 current... At
V+ = 2V, I(V+) is nominally 10.25uA, pops up to 1.7mA at the start of
the capacitor dump but has fallen back to 10.25uA before the cap is
half dumped. Average power consumption of the whole contraption is
150uA at 2V, 250uA at 3V.

At 2V operation I'm smacking the LED with 6.7mA, decaying in 225ms.
I've haven't played enough with LED flashers to know how long the
duration needs to be to be noticeable.

From my LMC6762 data sheet, the fall-off of current seems to occur
below 2.2V unless you're fond of -40°C ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice

480)460-2350 | |
| Jim-T@analog_innovations.com Fax

Ross Matheson · Aug 18, 2003

[ ... ]
: ... unless you're fond of -40°C ;-)
:
: ...Jim Thompson

The neat thing about 40 below is that it's the same in °C or °F ;-)

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