parts:
U1 LM741 op amp IC
R1 100k ohm potentiometer
R2, R3 10k ohm resistor
R4 330 ohm resistor
LED1 light emitting diode

all resistors are 5 or 10 percent tolerance, 1/4-watt
R1 controls the trip-point of the circuit. adjust it accordingly. to reverse the logic (have the led light up when the battery has at least X amount of power,) connect the led to ground through R4.

Quote

(andrew @ June 26 2003,21:28):


parts:
U1 LM741 op amp IC
R1 100k ohm potentiometer
R2, R3 10k ohm resistor
R4 330 ohm resistor
LED1 light emitting diode

all resistors are 5 or 10 percent tolerance, 1/4-watt
R1 controls the trip-point of the circuit. adjust it accordingly. to reverse the logic (have the led light up when the battery has at least X amount of power,) connect the led to ground through R4.

OOPS! Nice idea but you have two variables feeding your op amp. Make one fixed by replacing the lower resistor of the voltage divider with a zener or programmable zener TL431 set to 6 volts or so and the squircuit will work fine. Zener diodes need a minimum current and their zener voltage may vary with supply voltage and temperature. The TL431 or equiv are easily "programmed" with two resistors. Data available from Texas Instruments and many other places on the web. If you want to add hysterisis, add a 470K resistor between pin 6 and pin three. You may need to experiment with the exact value to get the desired offset (try a 1 meg pot first).

Sorry can I know why we need to use a zene diode and not the resistor R2?

A zener is a weirdo kind of diode that can be a poor man's voltage regulator. You put it in reverse-biased. Above its voltage rating, it conducts.

See, what Peter's saying is that both + and - of the opamp are on the (variable because it's a battery) 12V, so readings won't be accurate. You need a steady reference (-, pin 2).

He suggests a zener instead of R2, which will hold pin 2 at 6V, even as the battery runs down. So by referencing this lower fixed voltage (6V) to the battery coming down from 12V, we know when its discharging. Circuit will fail below 6V tho, which is OK.

Only thing is, Peter suggests hysteresis (a good idea, to stabilize) on pin 3, whereas it should instead go a 10K from pin 6 to pin 2. No biggie.

I am grateful to Peter for this idea, because my Vcc is 5V, but I must measure a signal of 6-8V. So I will put a 6V zener where he suggests, and a resistor divider on pin 3 instead of the pot, to make a difference opamp. Hopefully it will then take the difference between6V, and 6-8 on pin 3, which theoretically will be 0-2V. Only thing I'm worried about is temperature drift, as mine will be an outdoor device.

Wish Peter or anyone could comment?

Perhaps I misunderstood the comments on this, but if you use a 6V zener diode instead of the voltage divider, then you will insure there is 6V at this pin only if the battery voltage does not go below 6V. A zener will not give you more than what it has available. If the battery voltage goes down to 5V, for example, then you will still have 5V at this pin. Thus you still have two variables and if you adjust the set point with 12V, then the set point will be different when the voltage drops to 5V, or anything below the zener forward voltage, for that matter. However, since you are only trimming a set point, you can make the adjustment with the pot while using a variable supply and still accomplish a set point at a particular voltage drop.
MP

Quote

(andrew @ June 27 2003,02:28):


parts:
U1 LM741 op amp IC
R1 100k ohm potentiometer
R2, R3 10k ohm resistor
R4 330 ohm resistor
LED1 light emitting diode

all resistors are 5 or 10 percent tolerance, 1/4-watt
R1 controls the trip-point of the circuit. adjust it accordingly. to reverse the logic (have the led light up when the battery has at least X amount of power,) connect the led to ground through R4.

This circuit doesn't work.

Try this one. Zener diode for 12V battery is 11V, threshold is 11,7V. LD1 light when voltage drops under threshold voltage, threshold voltage is zener voltage + 0,7V
For another threshold voltage use another zener diode.

hi!
can you tell me values for 6 volt?