Low Voltage Circuit - not working

[kuksul08]

I built this circuit: http://www.reuk.co.uk/LM741-OpAmp-Voltage-Indicator.htm

My goal is to have it indicate when my battery pack reaches 4V (4.8V nominal). I followed the directions as best I could. My components vary slightly: I used a 2.5V zener diode, and 68 Ohm resistor for the LED.

When I apply power (I tried 4V, 6V, and 8V), the red LED lights up all the time. I cannot dial in the potentiometer to ever shut the LED off. I triple checked the wiring... does anyone have experience with this?

Thanks

 

[Mr Happy]

You will need to "debug" the circuit, and to do that you will need a voltmeter. Specifically you will need to measure the voltages at pin 2 and pin 3 of the op amp and see how the voltage at pin 3 varies when you adjust the pot.

Also be careful when using a smaller resistor value for the LED. If you make it too small you could overload the source/sink capability of the op amp. It isn't designed for driving high power loads.

 

[kuksul08]

You will need to "debug" the circuit, and to do that you will need a voltmeter. Specifically you will need to measure the voltages at pin 2 and pin 3 of the op amp and see how the voltage at pin 3 varies when you adjust the pot.

Also be careful when using a smaller resistor value for the LED. If you make it too small you could overload the source/sink capability of the op amp. It isn't designed for driving high power loads.

Okay, I will try to debug it and post the results here. I used a smaller resistor because my input voltage is 4 to 6 volts, and the original circuit is designed for 12 volts. Was that an okay change to make? I measured the current going into the circuit to be 28mA with a 4AA power supply (5.4V or so).

 

[Mr Happy]

You should be aiming at a 10 mA drive current for an LED typically. This will make it easily bright enough to see and will give it a long life.

If you have a nominal 6 V supply and the red LED has a Vf of 2 V you need to drop 4 V in the resistor. By Ohm's law, R = 4 V / 10 mA = 400 ohms. So your 68 ohm resistor is a bit on the low side.

 

[Mr Happy]

Incidentally this is a wonderful example of why using a breadboard is a nice way to test circuits before committing them to the soldering iron.

 

[kuksul08]

Okay, since I want it to turn the LED on around 4V, my source voltage is 4.11V (3AA's).

For the pot settings, the fully biased to the top would mean a closed circuit between Vin and position 3.

Pot setting= fully biased to top
V2= 1.300V
V3= 4.100V

Pot setting= 1/3 to bottom
V2=1.293
V3=3.201

Pot setting= 2/3 to bottom
V2=1.292
V3=0.928

Pot setting= fully at bottom
V2=1.292
V3=0.000

edit: Yes, a breadboard would be nice! I was basing my resistor value off the time when the LED would be ideally turning on, so (4V-2.2V)/.025A = 72R. hmm...

edit2: but yeah, my V2 doesnt look right.

 

[Mr Happy]

Well firstly, you said it was a 2.5 V Zener, so your reference voltage at pin 2 should be about 2.5 V. This is the first thing that looks wrong. Do you have the diode the right way round? I think you do, but I cannot see every detail of your construction.

Secondly, the LED should turn on/off when the voltage at pin 3 crosses the voltage at pin 2. From what you say it doesn't seem to be doing this. It may be because the 1.3 V at pin 2 is too close to the negative supply for the op amp to deal with, so to begin with I would try to figure out why the reference voltage is wrong.

 

[kuksul08]

Hmm, well I used the 'diode' setting on my multimeter (I've never used it before). In one direction it reads 0V, and in the other direction, it reads 1.34V.

I'm confused now :/

 

[Mr Happy]

Do you have any spare diodes? What I would do is make a test circuit with 4 V battery - 10k resistor - diode in series and try the diode both ways round. In each case measure the voltage across the diode and see what you get. One way round should give you the Zener voltage.

 

[kuksul08]

Well, I happen to be a genius though, and I only bought one diode...:duh2:

Thanks for the help (in addition to all the help in the past) Mr.Happy!

 

[SemiMan]

Hate to tell you this, but this circuit is never going to work with that op-amp. The 741 is not a low voltage op-amp. You may, may be able to get some level of functionality at 8V. Even with a rail-rail input/output op-amp I think it is going to be iffy.

The other poster was right, the 741 will likely not work well with the input 1.3V from the rail.

This is not a great circuit for low voltage detection.

Semiman

 

[kuksul08]

Hate to tell you this, but this circuit is never going to work with that op-amp. The 741 is not a low voltage op-amp. You may, may be able to get some level of functionality at 8V. Even with a rail-rail input/output op-amp I think it is going to be iffy.

The other poster was right, the 741 will likely not work well with the input 1.3V from the rail.

This is not a great circuit for low voltage detection.

Semiman

Great.. well thanks for the help lovecpf