The problem, I want something to turn on when a drop in voltage is detected.

I know this can be done with a relay, but its not very versatile, unresponsive and slow.

I'm looking for the opposite of a transistor I guess. Something that will detect a slight drop in current, then allow more current through another circuit. What is the simplest way to accomplish this?

 

A little more on the intention/circuit/devices are required, in some cases a simple comparitor will do it, LM311 etc.
Max.

 

Your problem is not descried very clearly as how is the voltage ?V and current ?mA ?

 

Anti-transistors haven't been invented. We just wire up the regular kind to work the logic backwards.

 

Need more specifics:

Rate of change of voltage?
Magnitude of the voltage before and after the drop?
What happens when the voltage goes back up?

 

The specifics are not known,I was more after a few examples of how this is accomplished so that I can research it and test myself. Voltage range is about 9v DC but I'm unsure of the actual trigger voltage drop etc.

Anti-transistors haven't been invented. We just wire up the regular kind to work the logic backwards.

How are they wired up?

A little more on the intention/circuit/devices are required, in some cases a simple comparitor will do it, LM311 etc.
Max.

Is there a simple example of this?

 

Google is your friend.

 

I have used the LM311 to detect a change in a 240vdc mobile application.
The output will turn on/off as the set point is exceeded or below.
Max.

 

Either use a PNP transistor or still use a NPN transistor but flip over your voltage divider.

You could also take a look at schmitt triggers or op-amps as comparators if you want to learn something a little more complex.