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Kieren Johnstone wrote...

>I want to create a "low battery" warning LED, to warn me when the
>voltage drops below say 4V (from 4.5 supplied from 3xAA)... I
>told my brain to work something out, the best it came up with
>was putting some very high resistance resistor feeding the +ve
>supply to the base of a transistor, then the collector-emitter
>current would die (supposedly) so I could do that inverter thingy :)
>Then I told my brain to construct an example, it built what I just
>described (minus an inversion), all that arose was an LED slightly
>dimmer than the "power on" indicator LED.  Anyone wanna help me
>solve this? :)

For a more or less practical (depending on what precision you're
looking for) low-battery indicator circuit, try the attached.

R1 will probably have to be adjusted to get the right trip
voltage, since transistors differ in the base-to-emitter voltage
needed to get them to conduct (typically this is about 0.6 volts,
but can be as low as 0.45 volts or as high as 0.75 volts
depending on the transistor).  Make R1 larger to increase the
trip voltage, smaller to reduce it.

Resistor R5 can be omitted if desired; it just gives the circuit
a slight amount of hysteresis for a "snap-action" response.

Be aware that with a cheap-and-dirty circuit like this, the trip
voltage is going to change a bit with temperature, to the tune of
roughly -0.3% per degree celsius.  This may or may not be
acceptable depending on your needs.

Hope this helps a bit...

DD

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