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"negative voltage" trigger

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espresso

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

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Pommie said:
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.
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.
 
kchriste said:
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:
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.
 
espresso said:
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.

Why cant you use a 'rectifier' :confused:
 
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.
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. :D
Espresso: More details please. Tell us exactly what you are trying to do.
 
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He will need a pot to set the trigger point to exactly -5V because of the component tolerances.
 
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?
 
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