How to build a voltage trigger?

Need to build a voltage trigger, for a project. This is new for me so i need some tips to get started. the point is to build it, not to buy one. The trigger we are looking for is like the one used in this instructable:

Appreciate any help I can get.

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framistan7 years ago
You could use a ZENER DIODE that will not conduct untill the ZENER voltage is reached... then an LED could light up.. OR if you wan to get more complex... you could use an integrated circuit called a COMPARATOR.  What this thing does is monitor TWO inputs..  If  ONE input rises higher than the OTHER input, then the output  UNGROUNDS.  (so you must have a PULL-UP resistor on the output.    Comparator IC's usually have FOUR such devices in one package and are very cheap.
Robootzz (author)  framistan7 years ago
 Yes!! that sounds exactly what i want. i have some knowledge about comparators and such. do you have some kind of schematic or something like that. so i know how to do it?


You don't even need a zener. 2 resistors or a pot to set the trip voltage and a comparator IC (or a cheap op-amp). Most comparator circuit, however, have "hysteresis" designed in. This means that once it trips, the trip point moves in the opposite direction so that noise doesn't cause the output to fluctuate wildly. Maxim have a good article on it over at

Hope this helps
lemonie7 years ago
I think this explains it much better than the link you're showing?

That's a rather nice one.
Robootzz (author) 7 years ago
 1, at a fixed voltage. like a 1381. so it may perform well in "solar powered robot".  so instead of using a transistor of some kind, i have to build something that works like that. its for a schoolproject, and the teacher says its to easy if i dont make that part myself. 
This page from the BEAM wiki has a number of  voltage-triggered solar engine circuits:

Re-design7 years ago
Answer these and you'll probably get your answer.

1.  What voltage to you want it to trigger at?  Fixed or adjustable?

2.  When triggered open circuit or close circuit?

3.  How much power is it going to switch?

4.  What's it going to control.

It may be that something as simple as a relay or a single transistor is all that you need.