Low volt indicator for LDO regulator

Looks like the 7665 is way too sluggish for that in terms of reaction speed. The NCP series chips could do that. However, keep in mind an intricacy of such chips: They do not have a separate supply voltage. Input is their supply. This means that at very low input amplitudes (well under 1V, usually) they "let go" because there is nothing left to coax the output FET into a conductive state. So you'd see a false alarm for very low audio levels unless you pre-bias the chip. For audio clipping indication I'd use a comparator.

Yes, I didn't look carefully enough. So long as the input is high enough it looks like it will work fine. My apologies!

Generally the output of an LDO tracks input with a constant delta once you reach drop-out. ...Jim Thompson

Okay, I'm kind of ignoring the numbers the OP used for dropout, because I think he added a margin to the actual dropout.

Most of the LDOs I've used have dropout voltages of a few tens or hundreds of mV at moderate currents (battery power). Even old bipolar ones like the LP2950, so 1V dropout, or "saturation" voltage)

But if you postulate an old semi-low-dropout like the LM1117 (>1V saturation voltage), no it won't work properly as shown.

Best regards, Spehro Pefhany

The OP's "LDO" died at a 1V delta... he said ;-) ...Jim Thompson

_Off_ perpetually, assuming the LDO in/out delta is always > ~.7

Ed

Yep, I stand corrected. ...Jim Thompson

Well, I meant to reply to Bitrex, so I was in error. You just agreed with his statement, after already pointing out that there's a problem with the circuit. In any case, the on/off thing is like a typo, very easy to type (or think) off when you mean on or vice versa. You pointed out the important factor: the regulator chip in/out delta.

Ed

It's an interesting problem. The proper way would use something like the old National "Miser" circuit to flash the LED at low duty cycle to conserve battery power. ...Jim Thompson

I don't like using battery power to indicate the battery is low. :-( I prefer auto power off and "negative indication", something like a led that goes out, or the gizmo being powered stops working. 'Course that's not always a good idea - and that does make it an interesting problem. :-)

Ed