Hi guys, first post here so hoping I don't inadvertently break any rules.

I'm driving the Set and Reset inputs of a latch (a CMOS 555 chip in bistable mode, operating at 8V) from the outputs of the the two comparators in the LM393 dual comparator chip.

The comparators give sharp falling edges, but because of their open collector outputs I get a slow rising edges. In fact the rising output seems to enter a narrow linear domain before reaching saturation.

I don't want to use a low value pullup resistor because it will be powered from a very small solar panel so I have to conserve energy.

Any suggestions on how I might interface these two devices?

Thanks.

 

Choose a comparator with push pull output.

 

Choose a comparator with push pull output.

A comparator with a push pull output may not be available for a reasonable price. The output of an open collector circuit looks like an RC network. This can be a problem on the rising edge of a signal. The falling edge is not a problem since there is an active device pulling the output low. To make the rising edge faster you can reduce the capacitance, or you can reduce the resistance of the pullup. You cannot reduce it to far or you will force the output to sink more current than it is capable of. If you can look at the rising edge on a scope you should be able to estimate the total capacitance that goes along with the value of the pullup resistor.

 

Choose a comparator with push pull output.

Some interesting devices available but they all seem to be SMD.
Maybe it's time for me to bite the bullet and make the transition.
I have no idea how to breadboard with SMD devices.

 

I have no idea how to breadboard with SMD devices.

You would use something called SurfBoards.

https://www.digikey.com/en/product-highlight/c/capital-advanced/surfboards-9000-series

https://www.digikey.com/en/product-highlight/c/capital-advanced/surfboards-6000-series

 

Something I came up with a very long time ago:

LM393 Rail to Rail​

 

You would use something called SurfBoards.

https://www.digikey.com/en/product-highlight/c/capital-advanced/surfboards-9000-series

https://www.digikey.com/en/product-highlight/c/capital-advanced/surfboards-6000-series

Fantastic. This is opening up a whole new vista for me. Major re-think ahead.

 

You could add a totem-pole, pull-up output circuit (sim below):

 

Something I came up with a very long time ago:

View attachment 304779
LM393 Rail to Rail​

OMG that's brilliant. In all likeliness I'll now go the SMD route with a push-pull comparator, but I'm so impressed with that.

 

You could add a totem-pole, pull-up output circuit (sim below):

View attachment 304780

This looks interesting. I've got all the bits to do this so I'm going to play. Thanks.

 

You would use something called SurfBoards.

https://www.digikey.com/en/product-highlight/c/capital-advanced/surfboards-9000-series

https://www.digikey.com/en/product-highlight/c/capital-advanced/surfboards-6000-series

Excellent.
I've used these before.
https://schmartboard.com/4-pack-of-schmartpatch-for-sot23-6-203-0003-01/

 

I have no idea how to breadboard with SMD devices.

A breadboard works slowly because of all the stray capacitance between its rows of contacts ands its wires all over the place.

 

OMG that's brilliant. In all likeliness I'll now go the SMD route with a push-pull comparator, but I'm so impressed with that.

Note that it takes two comparators for each channel, so you would need to use the quad LM339 for two channels.

 

Something I came up with a very long time ago:

I believe pin 3 needs to be biased at some small positive voltage to insure that its output goes high when the other comparator output goes low.

 

This looks interesting. I've got all the bits to do this so I'm going to play. Thanks.

You also get similar results if you use a PNP in place of the diode:

 

Hi guys, first post here so hoping I don't inadvertently break any rules.

I'm driving the Set and Reset inputs of a latch (a CMOS 555 chip in bistable mode, operating at 8V) from the outputs of the the two comparators in the LM393 dual comparator chip.

The comparators give sharp falling edges, but because of their open collector outputs I get a slow rising edges. In fact the rising output seems to enter a narrow linear domain before reaching saturation.

I don't want to use a low value pullup resistor because it will be powered from a very small solar panel so I have to conserve energy.

Any suggestions on how I might interface these two devices?

Thanks.

Just add some hysteresis to each opamp.

Can you post your comparator circuit?

 

I have no idea how to breadboard with SMD devices.

SOIC8 to DIP adapters are readily available.

This is another option (I use it for programming 93C46R in SOIC8):
Be careful with these. There are two widths and the sellers don't seem to comprehend that.

Neither of these sellers offered free shipping.

When I only had SOIC16 adapters, I cut one in half:

 

Just add some hysteresis to each opamp.

They are comparators.
Don't see how that will help the rise-time(?).

 

They are comparators.
Don't see how that will help the rise-time(?).

Nevermind....I misunderstood the issue.

 

Hi guys, first post here so hoping I don't inadvertently break any rules.

I'm driving the Set and Reset inputs of a latch (a CMOS 555 chip in bistable mode, operating at 8V) from the outputs of the the two comparators in the LM393 dual comparator chip.

The comparators give sharp falling edges, but because of their open collector outputs I get a slow rising edges. In fact the rising output seems to enter a narrow linear domain before reaching saturation.

I don't want to use a low value pullup resistor because it will be powered from a very small solar panel so I have to conserve energy.

Any suggestions on how I might interface these two devices?

Thanks.

What size pullup are you using currently?

How slow are they currently rising and how fast do you need them to rise?