voltage comparator question

Hi All
I am controlling the output of a bicolor LED using a dual comparator
(393), however when the input voltage hovers around the ref voltage,
the LED starts to flicker between colors, and understandably so. Now I
was wondering if there is a way to tie down the LED output during the
first instance of the crossover between input and ref. Also since the
voltage swing can be large sometimes (+/- 1.5v), the only way to
restore the original LED output would be to reset the chip. I hope
i've made myself clear.

 

On Apr 11, 12:16 pm, 10y...@gmail.com wrote:

I am controlling the output of a bicolor LED using a dual comparator
(393), however when the input voltage hovers around the ref voltage,
the LED starts to flicker between colors, and understandably so. Now I

Google "hysteresis resistor comparator" and read the app note which
heads up the results.

 

[Jamie]

10yrcc@gmail.com wrote:

Hi All
I am controlling the output of a bicolor LED using a dual comparator
(393), however when the input voltage hovers around the ref voltage,
the LED starts to flicker between colors, and understandably so. Now I
was wondering if there is a way to tie down the LED output during the
first instance of the crossover between input and ref. Also since the
voltage swing can be large sometimes (+/- 1.5v), the only way to
restore the original LED output would be to reset the chip. I hope
i've made myself clear.
You need to have a little feed back from the output to the (+) input

of the comparator.
For single supplies, normally a voltage divider is used as the fixed
ref at the (+) input to set the threshold point for the (-) input
sense. You can place a high value R from the output to this point so
that the threshold reference value changes when the output switches.

You can either experiment or do the basic ohms law to calculate the
value needed for R verses the hysteresis value of interest, this will
stop your flickering.


http://webpages.charter.net/jamie_5"

 

[Jasen Betts]

On 2009-04-11, 10yrcc@gmail.com <10yrcc@gmail.com> wrote:

Hi All
I am controlling the output of a bicolor LED using a dual comparator
(393), however when the input voltage hovers around the ref voltage,
the LED starts to flicker between colors, and understandably so. Now I
was wondering if there is a way to tie down the LED output during the
first instance of the crossover between input and ref. Also since the
voltage swing can be large sometimes (+/- 1.5v), the only way to
restore the original LED output would be to reset the chip. I hope
i've made myself clear.

positive feedback, it's hard to suggest a good arrangement without more
details.

 

[Gareth]

10yrcc@gmail.com wrote:

Hi All
I am controlling the output of a bicolor LED using a dual comparator
(393), however when the input voltage hovers around the ref voltage,
the LED starts to flicker between colors, and understandably so.

Have you considered using a Schmitt trigger? This is basically a
comparator with a couple of extra resistors to provide positive
feedback. This causes the low-to-high threshold to be higher than the
low-to-high threshold, which should prevent the problem you describe

If you think this will solve your problem, there are two different
configurations (inverting and non-inverting) and a couple of calculators
here:

http://www.random-science-tools.com/electronics/schmitt-trigger-calculator.htm

http://www.random-science-tools.com/electronics/inverting-schmitt-trigger-calculator.htm

I suggest that you make only one of your comparators the schmitt trigger
and use the output of this to trigger the other, or you could end up
with a dead zone where both outputs are in the same state and the LED is
off (unless that would be useful?).

Now I
was wondering if there is a way to tie down the LED output during the
first instance of the crossover between input and ref. Also since the
voltage swing can be large sometimes (+/- 1.5v), the only way to
restore the original LED output would be to reset the chip.

You can use a couple logic gates to create a Set-Reset Latch if that is
what you want. Use the comparator to drive the Set input of the latch
and a switch to drive the Reset input, connect the LED between the Q and
Q_bar outputs. Note, the logic gates may not have sufficient output
current to drive the LED so you may need to buffer the outputs with two
more comparators.

There is more about the SR latch here:

http://en.wikipedia.org/wiki/Latch_(electronics)

I hope
i've made myself clear.

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To reply to me directly:

Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris

 

[Eeyore]

10yrcc@gmail.com wrote:

Hi All
I am controlling the output of a bicolor LED using a dual comparator
(393), however when the input voltage hovers around the ref voltage,
the LED starts to flicker between colors, and understandably so. Now I
was wondering if there is a way to tie down the LED output during the
first instance of the crossover between input and ref. Also since the
voltage swing can be large sometimes (+/- 1.5v), the only way to
restore the original LED output would be to reset the chip. I hope
i've made myself clear.

There is no reset on an LM393. What you seem to be experiencing is the
effect of noise on the input. The classic solution is to use some small
positive feedback to give some hysteresis betwen the coulour change
points. See app notes.

Graham

 

Thanks for the reply. I'll try the latch circuit as you suggested.

Gareth wrote:

10yrcc@gmail.com wrote:
Hi All
I am controlling the output of a bicolor LED using a dual comparator
(393), however when the input voltage hovers around the ref voltage,
the LED starts to flicker between colors, and understandably so.

Have you considered using a Schmitt trigger? This is basically a
comparator with a couple of extra resistors to provide positive
feedback. This causes the low-to-high threshold to be higher than the
low-to-high threshold, which should prevent the problem you describe

If you think this will solve your problem, there are two different
configurations (inverting and non-inverting) and a couple of calculators
here:

http://www.random-science-tools.com/electronics/schmitt-trigger-calculator.htm

http://www.random-science-tools.com/electronics/inverting-schmitt-trigger-calculator.htm

I suggest that you make only one of your comparators the schmitt trigger
and use the output of this to trigger the other, or you could end up
with a dead zone where both outputs are in the same state and the LED is
off (unless that would be useful?).

Now I
was wondering if there is a way to tie down the LED output during the
first instance of the crossover between input and ref. Also since the
voltage swing can be large sometimes (+/- 1.5v), the only way to
restore the original LED output would be to reset the chip.

You can use a couple logic gates to create a Set-Reset Latch if that is
what you want. Use the comparator to drive the Set input of the latch
and a switch to drive the Reset input, connect the LED between the Q and
Q_bar outputs. Note, the logic gates may not have sufficient output
current to drive the LED so you may need to buffer the outputs with two
more comparators.

There is more about the SR latch here:

http://en.wikipedia.org/wiki/Latch_(electronics)

I hope
i've made myself clear.



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To reply to me directly:

Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris