Was looking for ideas on the simplest way to indicate that a 12V Lead Acid battery has reached a low level, somewhere between 12.2V -12.4V. In other words, a low voltage dectect circuit. The one real stipulation is that it must run off the battery itself.

Perhaps using a 9V zener to power a micropower comparator and a reference diode. A voltage devider off the battery would be the other comparator input.

The output would drive either an LED or small peizo.


Your thoughts!?

 

Use an LM10 IC that has an adjustable low voltage reference, an opamp and a comparator inside its 8-pins case.

 

Another option is to wire a 10K potentiometer across the battery with the pot arm going to the + of a comparator.
The - of the comparator would have a ~6.0 volt Zener diode wired to the battery -.
The open collector of the comparator would be attached to the - of an LED in series with a 1K current limiting resistor that is wired to the + of the battery.
The arm should be near the mid point of the pot and the + end of the potentiometer can be attached to several diodes in series connected to the battery + to drop down the voltage and simulate the low voltage to be tested for.
Adjust the pot arm to turn the LED on at the desired low voltage point.

To be sure you don't accidentally short out the pot and burn it out you can add a 1k resistor between the pot and the battery connections at both ends of the pot.

Danny

 

The 7809 is a power hog compared to the TLV3701, and you'll need to blink the LED. What I'm saying is that you have a mix of brilliant and half baked here...but that's why you asked, isn't it?

 

The 7809 is a power hog compared to the TLV3701, and you'll need to blink the LED. What I'm saying is that you have a mix of brilliant and half baked here...but that's why you asked, isn't it?

Half Baked is correct! I know that the 7809 is way too wasteful for the circuit
I plan. I am looking for a LDO VReg to replace this.

On the other hand, AudioGuru's suggestion of using the LM10 may deserve some merit. The datasheet I downloaded did not have much application information on using the device for this sort of application, though it did have a couple of possible circuits...the did not explain their use well.

 

Here's a 20 microamp voltage monitor for 38 cents if you want to stretch your designing abilities.

 

Here's a 20 microamp voltage monitor for 38 cents if you want to stretch your designing abilities.

Stretchhhhhhhhhhhhhhhhhhhhhhhhhhhh!

I am clueless as to how this could work.

 

Pick a trip voltage knowing that Vcc must never be above 5.5 volts. Feed the battery voltage through a resistive divider to the CAT809 or 810 chip. Attach its reset pin to the TLV3701 and set the TLV as a latch with positive feedback. Then attach an LED to the TLV or use another TLV or equivalent to make a low duty cycle oscillator to blink a LED.

The CAT chips come with "foul" being declared as high or low, your choice.

 

The circuit shown in post #1 can't work. One comparator input is looking at a fixed reference voltage from a LM385. The other input is looking at a potion of the 7805 regulator output which is also a fixed voltage, No where is the actual battery voltage monitored.
Look here:

http://www.reconnsworld.com/power_12vbattmon.html

 

Google: LM431 voltage monitor circuit
You'll find simple sircuits like this:http://www.reuk.co.uk/TL431-Battery-Voltage-Monitor.htm

Ken

 

I'm working on a similar problem myself. I was planning to use one of these:

http://ww1.microchip.com/downloads/en/DeviceDoc/21434g.pdf

which according to the datasheet only draws 1 uA. Possibly something for you to look at.

 

Here is my contribution:

The 12V zener I tested is 1N5927. U1 is TLC555.
The LED flashes when the battery voltage drops below 12.1V
Current draw is about 200uA.

 

That 21434 is like the CAT 809, only better. Higher voltage limit, lower current waste, 2 way active output.
63 cents. not a bad price.

 

That 21434 is like the CAT 809, only better. Higher voltage limit, lower current waste, 2 way active output.
63 cents. not a bad price.

I have a few of the 2.7 and 2.1 volt units lying around from my 2 week long interest in BEAM robotics.

 

The circuit shown in post #1 can't work. One comparator input is looking at a fixed reference voltage from a LM385. The other input is looking at a potion of the 7805 regulator output which is also a fixed voltage, No where is the actual battery voltage monitored.
Look here:

http://www.reconnsworld.com/power_12vbattmon.html

Your absolutely right...
My bad...

 

That 21434 is like the CAT 809, only better. Higher voltage limit, lower current waste, 2 way active output.
63 cents. not a bad price.

Who makes this 21434?

 

Who makes this 21434?

Microchip

Mouser sells them. This is the 2.7 volt threshold version. You can use a resistor divider to use it with any battery voltage that you want.

http://www.mouser.com/Search/Produc...virtualkey57940000virtualkey579-TC54VC2702EZB

 

Here is my contribution:

The 12V zener I tested is 1N5927. U1 is TLC555.
The LED flashes when the battery voltage drops below 12.1V
Current draw is about 200uA.

The 555 is there solely to pulse the LED I assume.

Q: can you change the zener voltage level, like 12V, to 12.6V by adding a diode in series with it???

 

Definitely. I also use a pair of resistors as a voltage divider between C1 and Q1 and ground to get a fine adjustment. A tenth or two (volts) are negotiable right next to the base of Q1.

This is a cool circuit. I copied it in my notebook because of the cmos 555 final stage.

 

Actually I have been surprised on the circuit ideas here as they have really not been anything like the circuits i've seen on the internet.

And as simple as I wanted it I am thinking it would be nice to have two LED's, a yellow LED for a warning and the Red for the "Unplug me and charge me" indicator. I'm not sure a 555 can do this, perhaps a comparator. I do like the Zener Regulator option.

Another Question... If the Vin(supply Voltage) of an Op-Amp/Comprator is 5.5V, can the + or - inputs be higher than 5.5V?
I did not see specs for them so I am guessing not. A simple voltage devider, however, could reduce the voltage in these pins to stay under 5.5V.