Threshold Circuit - Warning - I know practically nothing about building circuits

[Zook104]

Hi

First off this is my first post in this forum so I apologies if I have posted this in the wrong place or if this thread is unsuitable for this forum.

Though I have a lot of background knowledge about science and how components of a circuit may work I have very little knowledge about actual circuit building, so once more I must apologies if I do not understand any terms used.

I require a circuit that when triggers some form of motor or LED when an input voltage falls below some threshold. That is when this voltage drops the LED turns on. I am attempting to teach myself some amount of electronics but this is unfortunately is moving too slow as I want to use this idea in an up and coming project.

Any and all help would be appreciated.
Thank you

 

[BobK]

This can be accomplished with a comparator. It compares two voltages its output depends on which is higher than the other. You feed it a reference voltage (the threshold) and the voltage to be compared. If you have a regulated supply, the reference can be as simple as a voltage divider (two resistors). For battery operation, a separate voltage reference should be used.

Bob

 

[Arouse1973]

Bobs right on the mark, but if your input is not rock solid then your LED could flicker on and off. Why not try a comparator with a bit of positive feedback. This gives a bit of a lazy circuit who's thresholds change when the device switches. This is a Schmitt trigger and you are applying a bit of hysteresis. Google Schmitt trigger op-amp and you will see what I mean.
Adam

 

[Zook104]

Thanks Guys

I will look into these right away!

 

[KrisBlueNZ]

The 555 timer contains two comparators with fixed threshold voltages and can be connected to operate as a Schmitt trigger. It can also drive LEDs and small motors directly. Google 555 Schmitt.