Hello,

I'd like your expert advice on how to have a "negative voltage" trigger something. Example will be: a circuit that will be triggered by -5v but not by +5v.

I'm looking for a simple and cheap way to achieve this without use of a rectifier or a microprocessor.
Is there a simple and cheap logic gate or comparator or anything this newbie is unaware of that will do this?

Thank you for your time for this newbie.

 

Use a Schmitt Trigger.

If you search this forum for that subject, you will find it in plenty of threads.

You could use a voltage compatitor such as the LM311.

__________________
Len

 

Are there schmitt triggers that will handle negative voltage..?
Could you please give me a url?

 

If the input is always +5V or -5V and nothing inbetween then a diode from the input to output and a resistor from the output to ground will do what you want. With 5V in the output will be 5V out, -5V in will give 0V out. If you want 5V out for -5V in then an inverter after the diode and resistor is needed.

Mike.

 

No need to use such a fancy device. Here is a example of level shifting the -5V signal to 2.5V. For simplicity, I've left out hysteresis and under/over voltage protection:

Attached Thumbnails

 

__________________
Inside every little problem, is a big problem trying to get out.

 

This is a simple solution, but the IC would need to be CMOS. I expect that you would need a transistor to interface to a TTL IC.

__________________
Len

 

This is essentially what I had in mind when I wrote the original post. But I suggest you should use a 22k in lieu of the 30k in order to allow for component tolerances.

__________________
Len

 

Why cant you use a 'rectifier'

__________________
Eric " Good enough is Perfect "
I will NOT answer PM's requesting technical help, please use the Forum
PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

 

It depends on exactly what Espresso has in mind. He stated he wanted the trip point to be -5V and that's what those values will trigger at. At this point, we don't know if Espresso intends -5V to be the minimum or maximum trip point.
We need more details from Espresso before we can get into specifics. I do see where you are going with this. ie: maybe Espresso wants to interface ECL logic running from a -5V supply. One can only guess at this point.
Espresso: More details please. Tell us exactly what you are trying to do.

__________________
Inside every little problem, is a big problem trying to get out.

 

He will need a pot to set the trigger point to exactly -5V because of the component tolerances.

__________________
Len

 

What exactly are you trying to trigger?

A timer?

A 555 is normally triggered by a negitive edge, if it must go below 0V then use a diode on the trigger pin to clamp it at -0.6V.

What trigger threashold do you need?

__________________

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.