Adjustable Low Voltage Indicator

[nathanpc]

I was thinking about building a low-voltage indicator to integrate into my power supply project to check if the input voltage is enough to power the voltage regulators I'll be using, because I'm using a LM317 too, I wanted my low-voltage indicator to be adjustable, so I could adjust depending on the minimum input I need for the desired LM317 output. To start I searched on Google for a simple circuit with a great explanation (since part of the goal with the power supply project is to learn more), sadly I couldn't get any good results.

Could anyone provide a good source of information about adjustable low-voltage indicators?

[mariush]

There are ready made chips that monitor a voltage and output something on a pin when the voltage goes below or above a threshold.
Just search for "voltage monitor" or "voltage supervisor" on digikey.com or other specialized stores

For example:
http://www.digikey.com/product-detail/en/MC33161PG/MC33161PGOS-ND/919013
http://www.digikey.com/product-detail/en/MB3771PF-G-BND-JNE1/865-1021-ND/2039278

etc etc

I don't know something "analogue" aka made with just some passives, maybe you could do something with some zener diodes and an npn/pnp transistor but I can't be bothered to think of something.

If you want something cheaper, you could always take the "overkill" approach and use a 0.4$ pic16f chip or an attiny , use the adc function in it to read the voltage and do something if the voltage is too low.

But the microcontrollers expect to be powered from 2.5-5v so you'd need a reliable supply for this chip and you'd have to use a voltage divider if your voltage may be above 5v. The monitor chips above are more expensive but they work at up to 40v.
 

[ResR]

Maybe this helps, just needs to be regulated to detect voltage you need and you can left out some led's, the chip still works.
http://www.electronicecircuits.com/electronic-circuits/lm3914-12v-battery-monitor-circuit

[nathanpc]

Interesting... I'll order some MC33161 and LM3914, play around with them, do some tests, and choose the best one for my project. Thanks very much for the suggestions!

[Kevin.D]

Why would you want to do this anyway.?  dont you know what your input voltage is ,isn't it a fixed value.?
Use a comparator and bias the - side at (lets say 3V below the reg input voltage if the reg dropout was 2.5 V )by using  diode drops or a zener and resistor to get the required fixed voltage drop below input voltage ,then tie the compare + side to the ouput side  .So the comparator goes high when they are equil ,then use that output to drive your "warning approaching  dropout led" directly. I am assuming your input will  always have some minimum voltage available to drive your led and comparator.
You could do it simpler with a single transistor biased across the input and output so it is normally on and driving a led ,but then it will switch OFF when the voltage difference drops below a certain threshold you have set with a biasing zener,or diode in the base  .So this would be opposite than with the comparator ,here the LED being on when ok then switching off when approaching dropout voltage.

Quote :- "so I could adjust depending on the minimum input I need for the desired LM317 output"
The minimum input you will need will always be your required output + dropout voltage of the regulator which is constant . (~2.5V for lm317) so you dont need to adjust the warning reference.

[nathanpc]

My input voltage isn't fixed. I want to be able to power it from a 9V battery or a 12V wall adapter (or anything with more than 6.5V and less than 15V). I'll try to build the circuit you've suggested, thanks very much for the tips.

[w2aew]

You may also want to checkout one of my favorite little chips, the LM10.  The data sheet shows an application do a low voltage detector.

[nathanpc]

Just had a glance at the datasheet's examples and I liked their low voltage detector circuit. I'll definitely buy some LM10s too. Thanks very much!