I have to use an Op-Amp (LM 741) to create a low battery indicator of an inverting amplifier when teh battery voltage for a 1.5AA battery has dropped below 1.2V. I cannot use zener diode and I can only use one LED. Any ideas on where I should begin or any equations that I could use?

 

What power supply voltages can you use?
What do you mean by "of an inverting amplifier"?

 

either 1.5V or 9V for power supply voltages
To build a low battery indicator that "indicates" by lighting an LED connected to the output of an inverting amplifier.

 

like this
http://www.google.co.uk/imgres?q=lo...w=219&start=0&ndsp=20&ved=1t:429,r:0,s:0,i:69

replace the zener with an led for the reference source, it will give a fixed voltage of 1.8volts

 

You have to be aware of the input common mode range, and output voltage range, and design around those.

 

With the single-polarity supply, some lousy old 741 opamps do not work with an input voltage less than 3V. Add a negative supply or use a modern opamp that works with its input voltages down to zero volts.

 

Can you assume the 9V battery is always 9V, and does not drop in voltage with use?

 

you have to measured and calculated data so yes

 

The 9V batteries at The Dollar Store are actually only a few volts and are leaking all over the place.

9V batteries made by (made for) Energizer and Duracell are guaranteed to still work fine 10 years from now. But they cost 3 or 4 times as much as the garbage at the Dollar Store.

 

you have to measured and calculated data so yes

In order to set the 1.2V threshold, you must have a reference. The 9V supply, if not a battery, will presumably be stable, and so can be used as a reference.

 

My turn on for the led is 1.73v and my low battery input voltage is 1.037v and my other battery source is9.43v.. I placed my led at non inverting input terminal and I tried to get my Vn to equal 1.73v which would just barely turn on my led.. I also made my low battery make a negative voltage so the difference would give me 1.73 v.. is my idea and thoought process wrong