Hello, thanks for taking the time to read this.
I am a newb and could use a tip if someone's already figured this out or knows a better way.
I'd like to use an RC timer to switch a relay. I need hysteresis to prevent short cycling, but I can't have the hysteresis charging the cap back up. I ran the cap voltage through a buffer to protect it, which behaves perfectly at node 'a'. (besides the short cycling without hysteresis, the circuit does the right thing if node a is shorted to node c - straight comparison).
The comparator op-amp also behaves correctly at node b.

My problem is that is seems like the comparator input signal dons crazy-pants in the hysteresis pot (tho I made it a very large resistance to keep the op amps from 'fighting' each other). Erratic as heck, doesn't sink down all the way....

Any ideas smarter than mine?
Is there a clever way to put the hysteresis on the threshold pin instead?


-the cap on the 'threshold' voltage is because I had the 12v supply rail switched by the relay coil itself (latches in running mode) and i didn't want the threshold voltage going nutso when the comparison resulted in killing 12 v. Just for debugging, I've got the 12v supply permanent to eliminate that cause.
-I'm using LM324 quad op-amp for buffer and comparator, though I tried using a 311P comparator (buffered output) instead with same result.

thanks very much for any tips, sorry if there's already a well-known solution.

 

I read the material regarding op amps, comparators, and the schmitt trigger on this awesome website called allaboutcircuits, and found the answer.
I didn't understand the feedback, which is what I was seeing destabilizing the circuit (you should've heard that poor little relay .

By leading the input signal to the inverted input and the hysteresis arrangement to the non-inverting input, the amplifier stabilized due to positive feedback, and the high output controllably bumped the threshold on the non-inverting input (thereby immediately saving the comparison from noise).
Since the schmitt trigger now output a High when I needed a Low for the relay, i just inverted it at an unused channel of the 324 op amp.
Maybe not elegant, but functional.
I could even use the relay to kill the 12 v supply without causing another feedback issue (cause the relay is mechanically slow enough)! - I didn't like the drain on my truck battery.

The fixed circuit is attached.
- hope this helps some other newb like me.

P.S: read the website.