What did I do wrong: I connected the input to 2 and 6 pins (and the 8 and 4 pins to +12V and 1 to ground, of course) and the relay to the 3 output and ground but nothing happened. The relay didn't click whether the input was 8V or 12V. At one point I must have shorted another pin because the 555 became very hot.
http://forum.allaboutcircuits.com/images/icons/icon5.gif

Bill may not have read your requirements. The switching thresholds on the circuit he posted are vcc/3 and 2*vcc/3. That comes out to 4V and 8V in your case. Not exactly what you are looking for. Some resistive level shifting might solve your problem. Maybe Bill can design that for you.

 

Check post #11.

I am looking at the diagram and at a LM393 but I am not 100% sure what the pins are in your diagram.
Thanks

 

Hello,

Take a look at this page of the eBook:
http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

Look for the positive feedback resistor.

with the signals belonging to the circuit:

Bertus

 

I am looking at the diagram and at a LM393 but I am not 100% sure what the pins are in your diagram.
Thanks

It's time to learn about datasheets.

 

It's time to learn about datasheets.

OK, can you check if my diagram is right? Please bear in mind that I am a true novice.
Thanks

 

Hello,

What did you note at the output of the comparator?
Is that a 1 or a 7, the 1 is correct see this picture taken from the datasheet:

Bertus

 

OK, can you check if my diagram is right? Please bear in mind that I am a true novice.
Thanks

I realize you're a novice,but I still don't see how you could get pins 2 and 3 swapped, and label the output as pin 7, which is clearly on the other comparator.
You MUST connect the inputs (5 and 6) of the unused comparator to ground.

EDIT: I meant no offense. I suppose I make too many assumptions about beginners.

 

Here is my contribution, simulated but not tested.
Take output from output of U1 to drive relay driver.

 

I'll bet that if you simply used a 20 Ohm to 33 Ohm resistor in series with the relay's coil, you would get the hysteresis that you need.

Increasing the effective total resistance of the relay coil will decrease the current through the coil for a given voltage; which increases the voltage required to actuate the relay.
Relays typically must actuate with 70% of the rated voltage applied; in your case that would be 12v*70%=8.4v. Adding 20 Ohms in series with the coil will increase the actuation voltage by roughly 28%.

 

Wookie, you're a genius. Never did think of such and easy solution.

 

I 'm sorry for not having caught this earlier. This thread is a camouflaged automotive modification, and has already been suppressed once here: http://forum.allaboutcircuits.com/showthread.php?t=66915

The OP not only disobeyed the initial warnings of the moderating team, but lied in this posts about the purpose of the project in this post.

You are reminded that automotive circuitry isn't as simple to design as any other hobby circuit. An automobile is a harsh environment for electronics and requires special design techniques.
AAC does not provide design aid for automotive circuits, as described in the ToS, Section 6.

eugen39, don't try to mislead the forum users again, or measures will be taken against you.

You are free to use the AAC forum as you see fit, as long that you stay within the ToS limits.

Have a nice day,
Georacer