Hi everyone,
I need a simple circuit which runs a buzzer when voltage falls below a specific value. I need 2 ckts to monitor batteries of 6v and 12v.. Could somebody help me for a simple solution using zenners and transistors?

 

Hello,

Are the circuits to be on separate units?
You can use a comparator with a reference on one pin and the battery voltage on the other pin.
like the circuits halfway down this page of the eBook:
http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

Greetings,
Bertus

 

Hello,

I found a circuit on the net:
http://www.elecfree.com/electronic/low-voltage-battery-monitor-by-lm339/

Greetings,
Bertus

 

Yes, the circuits are seperate.. a 6v tester for a 6v battery.. And thanks for that above link.. i hope it should do the job.. But I have a doubt.. Can I monitor other voltages by replacing that zenner?? also what should I modify for a more precise value, say 6.2v??

 

Hello,

You can replace the zener with a 4.7 Volts zener.
This way you can adjust the voltage from 4.7 Volts and up.

Greetings,
Bertus

 

Well, I breadboarded this circuit

But the problem is, pin 6 is always high for any voltage no matter what value the 100k preset is set to. I used a 5v6 zenner diode. What could be the problem?? Also how can I confirm the value of a zenner? Would connecting it in parallel and in reverse biased with an RPS and voltmeter show the cutoff point??

 

What is the IC that you are using?
If it is an LM339, you have the wrong pins connected.
If it is an LM741, you are using the wrong IC; you should be using a comparator instead of an operational amplifier.

Another possible problem is low Zener current. Zeners need to have a certain range of current flowing through them in order to operate in their linear region; usually somewhere between 5mA and 30mA - but that varies, depending upon the particular Zener.

You can actually use an LED instead of a Zener. Not quite as accurate, but all you're looking for is something reasonably stable anyway.

 

Ohh Sorry that I didnt specify the IC .. I searched and found this online

So when i posted it, searched again but found another one similar but didnt look deep ..

The site claimed this circuit to work with 741.. So can you help me with a simple circuit for a low voltage buzzer, one for 6v and another for 12v..???

 

The above circuit really does work !! The problem was that the zenner is wrongly drawn.. I reversed it and used a 3v3 zenner as a 6v circuit and it works fine after some adjustment on the preset.

 

Hello,

If you want more accuracy you can use a TL431 in place of the zener.
The TL431 is a 2.5 Volts voltage reference. (see datasheet).

Greetings,
Bertus

 

Thanks for that info Bertus. I will use it for precision applications.