I need a sort of a reverse relay. It needs to block the current (about 500ma 5.5vdc) flowing in a circut as long as there is 2ma 1vdc signal present. It would be best there was no power consumption other than the signal when the circut is blocked. I can't search for an answer because I don't know what the parts.
Thank You

 

Try "JFET" as a search term. Don't know if you'll find one for your combination of needs, but it's worth a try.

 

k37doy,

This would be accurate, but may be more than you need. U1 is a comparator, not an opamp.

 

k37doy,

This would be accurate, but may be more than you need. U1 is a comparator, not an opamp.

two small problems.

1 - many comparators have open-collector outputs, which require a pull-up resistor in order to actually provide an output signal.
2 - mosfets typically require 10v to fully turn on.

so, in order to make your circuit work, it will require a pullup resistor on the output to a supply of 10v or 12v, of 1k or 1.2k ohms for a 10ma current.

 

1. Push pull comparators are easily available (ie, mouser/digikey).
2. Logic level mosfets (Vgs fully on at 4.5V) are easily available.

Just pick more capable parts.