Low Battery Cut-off with Indicator Circuit for Cell Phone/Tablet Battery Pack Chargers

The Design

The circuit functioning of the proposed cell phone/tablet low battery indicator with cut off can be understood as follows:

Assuming the connected battery pack to be fully charged, the potential at pin#2 is allowed to be at a higher level than at pin#3 by setting the 10k preset appropriately.

The above condition ensures a zero or logic low at the output pin#6 of the IC.

The above low output enables the connected transistor TIP127 to conduct and charge the cell phone or the tablet at its collector. 

As the battery pack drains below the mentioned 7.2V mark, pin#2 voltage becomes lower than pin#3 which instantly makes the output of the IC high, switching off the transistor and the load.

The situation is indicated by the red LED which just lights up due to the low battery conditions.

At the above threshold, the output might oscillate for some time due to battery voltage trying to restore at the previous mark as soon as the load cuts off.

Although not necessary, the above outcome can be avoided by adding a resistor network: one across pin#3 and zener cathode, and another across pin#6 and pin#3, the values can be anywhere between 10K and 100K.

How to Set up the Circuit:

It's simple, apply the desired low voltage threshold to the circuit and adjust the preset until the LED just illuminates brightly.

As mentioned earlier, the possible oscillation at the thresholds can be prevented by adding some hysteresis to the above design, it may be done by the following two methods, the second option appears to be more logical and clean.

While setting the preset, make sure the feedback link stays (pin6 to pin3) disconnected, you can connect  it back once the preset adjustment is complete.

Note:  Please add a 3V zener diode in series with pin#6 of the opamp, (anode to pin#6) to counter offset problem. 

 

Feedback from Mr. Robert

Hi, assembled everything but it does not seem to work. I disconnect

1n4148 diode between pin6 and pin3. Connect 12v voltage and trim to LED lights up. Then connect the diode between pin6 and pin3 again, but I always got 12v out on load,it doesnt matter what I do I get 12v out everytime and it does not cut off .I thought it would cut off when led is on. What can I have done wrong?

Solution:

It could be due to leakage voltage from pin#6 of the opamp which is not allowing the transistor to switch OFF....connect a 1K resistor across the base and emitter of the TIP127, or you can also try a 3V zener in series with the base of the transistor, the anode will go towards the base and the cathode towards the resistor.