# Using Schmitt trigger for battery life indicator

Hi,

I am doing a low battery indicator for a 6V battery using Schmitt trigger.
I want the led to switch on when the voltage is below 5V and switch off above that… How can I get this done?

I have this circuit diagram attached to show how I have built the circuit but there is this problem where the led would still light up even when the input to the Schmitt trigger is disconnected.

Anyone can help?

How to use this forum

First, resize your image down to about 100 Kb, edit your post and swap them over, thanks.

3 Mb for a circuit is too much.

Using the input trigger voltage of an inverter is pretty chancy, methinks.

I would use a 5.1V zener and a comparator chip like an LM339.

Just a thought...

[quote author=Nick Gammon link=topic=150943.msg1133206#msg1133206 date=1361857602] How to use this forum

First, resize your image down to about 100 Kb, edit your post and swap them over, thanks.

3 Mb for a circuit is too much.

[/quote]

Indeed - we don't need to see every RGB pixel on your screen!

Back to the question - no a schmitt trigger like an HC14 is not suitable for this application, it has very little precision at all (and a very large hysteresis - you need some hysteresis but perhaps 1%, not 50!).

A comparator as suggested, but the 5.1V zener isn't going to be useful if the circuit is running at 5V.

The normal method is to set up a wheatstone-bridge arrangement, one arm of which uses two resistors to sample the battery voltage (typically approx 50% of it). The other arm would use a zener and a resistor to form the voltage reference (say 2.7V). You choose the actual resistors in the divider arm to make the bridge balance when the battery voltage is at the value you want to sense.

A comparator then takes its inputs across the bridge. Adding a little hysteresis is advised to stop rapid fluctuations due to noise.

http://bit.ly/IVZaeV

// Per.

MaJiG:
Using the input trigger voltage of an inverter is pretty chancy, methinks.

I would use a 5.1V zener and a comparator chip like an LM339.

Just a thought…

Can I use the circuit as attached for a 6V source?

And how do I determine the zener diode value according to the voltage source?

comparator_ckts.pdf (15.8 KB)