voltage comparator question

[[email protected]]

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.

 

[[email protected]]

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]

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]

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.

 

[[email protected]]

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

 

[Eeyore]

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