DIYEFI.org Forum

Electronically knowledgeable people, what is out there? Most schmitt ICs seem to have about 1V hyst, which while useful, isn't ideal. If there was something that had a high trigger of 4V and a low trigger of 1V that would be MUCH better, provided it was fast. If a small single part can do this job, I'd like to find it, if not....

Abe posted these three related/same/similar threads over the years:

viewtopic.php?f=9&t=65
viewtopic.php?f=9&t=757
viewtopic.php?f=9&t=412

LM339 is a comparator, and it's pretty straight forward to generate such a circuit using one, much like Abe did above.

There are other ways to do this with discretes too, but if there is an IC, it'd be nice to use it.

Fred.

Give this a 10V supply (use a small 10V reg) and you will get what you want.

http://au.element14.com/texas-instrumen ... dp/9589520

there are a couple more that I know that I know of, cant think of them right now.

Cheers,

Dan

Cunning solution! I think it needs to work from 5V, though. Optionally being able to work from battery voltage could be useful too, but not much.

you will struggle to find a unit that has large hysteresis with only a 5V supply.

Dan wrote:you have already struggled to find a unit that has large hysteresis with only a 5V supply, hence this thread.

Fixed ;-)

BUT, the comparator + passives setup should work just fine, which is likely why Mazda used it in Abe's ECU.

Also, someone tested a hall input through a normal schmitt IC with 1V hyst and it cleaned up their signal perfectly as far as I could tell. However, that's not to say that it would clean up everyone's signals optimally.

Fred.

You could use a comparator, an opamp or logic gates and add resistors to get hysteresis
All three options do not differ that much in cost and functionality

I do however doubt how you would be able to get more than a volt of noise on your hall sensor signal..

The fact that we have noise at all proves that it's over the cut off voltage for the inputs, which is often 2.5V or so. So yeah, this is a real issue :-)

Mind explaining the pros/cons of each method/IC type?

Fred.

Fred wrote:The fact that we have noise at all proves that it's over the cut off voltage for the inputs, which is often 2.5V or so. So yeah, this is a real issue

Mind explaining the pros/cons of each method/IC type?

Fred.

I see, I am wondering then how this noise is induced then.

The logic gate vs opamp vs comparator thing depends a bit on the requirements. Especially the speed and the voltage levels.

Logic gates are nice because they have push-pull outputs and are usally very fast. A schmitt trigger inverter contains 6 gates which might be useful for other stuff. Disadvantage is that modern logic gates cannot go up to 12 volts (74HCxx logic only goes to 7V absolute max)

Opamps can be slow, expensive, not have full output swing from rail-to-rail etc. They can be the best choice if you want a very high input impedance though.

Comparators are the most suited I think. As most comparators have an open drain output they can also be used to shift from 12V down to 5V for the microcontroller. The downside of the open drain might be speed and some extra resistors.

Without knowing the exact requirements I think the OEM solution from Abe's thread is a good starting point.

What about his choice of TL082 over the comparator? He justifies it with seeing higher-than-5V spikes of very narrow duration attenuated because the output doesn't swing before the spike goes away and therefore never swings.

Fred wrote:What about his choice of TL082 over the comparator? He justifies it with seeing higher-than-5V spikes of very narrow duration attenuated because the output doesn't swing before the spike goes away and therefore never swings.

Yes everything above the propagation delay (which isn't the right term for this..) will get filtered, but those frequencies should not reach the comparator/opamp anyway so I do not get the point.
As I said: we should get the requirements first and then we could design something. There are quite a few ways to accomplish this and then we can choose the circuit which suits the requirements best.