simulate this circuit – Schematic created using CircuitLab
I am working on a battery charger with indicator. I am using a LM358 to compare the voltage of my battery with a reference voltage of 8V from 7808. My input supply is 12V. The battery is connected to pin 2 (inverting input) and 7808 output is connected to pin 3 (non inverting input) of LM358. The Comparator is connected to 12V VCC through a 4.7k Resistor. Indicator LED is connected on pin 1 of LM358.
The expected output is that once the battery crosses 8V, the output of the comparator must swing low and LED will turn OFF. Initially, the output is high and LED glows as expected. But when the inverting pin crosses 8V, the output doesn't swing (High to low). However, when I touch any metal part to any component (lead of the component), the output swings low (from High) and LED goes OFF. But this doesn't happen automatically, every time I touch a metal I see the swing.I want this to happen automatically.
And if the battery is above 8V, and when I connect it for charging, the LED does not glow; which is correct. It is just the transition from High to Low I need. Can anyone help/suggest me what am I missing here?
Note: LM358 is used in open loop configuration and no feedback is given.
P.S.: I have inserted the schematic. Sorry if it is not clear, I am new to the interface.
Thanks in advance.
Best Answer
The current driving the LED has to come through R2 (1 kohm). That's fed by the LM358 from its power pin, which means it has to come through R3 (4.7 kohm). That's part of the problem. 5.7 kohm in series with a LED would probably still light it OK, but I suspect that when the LED is on the 358 browns out and stops working until the supply voltage recovers. If you really want R3 in there, try 4.7 ohm instead of 4.7 kohm.