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[SOLVED] Simple Low-voltage Circuit switch for 4.8V pack powering LEDs?

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somegeek

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I built my kids a couple of 'control boxes', least that's what they call them. :) They have 4.8V 700mAh NiCd battery packs in them. The LEDs are wired with 100Ω resistors. I'd like to put a low voltage light on the box, a type of simple circuit that would shut power to the main circuit and switch to a 2nd circuit(with a single red LED for example) when the voltage drops below 4.4V or 4.0V? Some kind of feedback for them would be cool indicating it's time to charge. They're charging them with a trickle charger(4.8V@50mAh - old Futaba Rx charger). This way they can charge when they need and if they leave it plugged in past full, the pack won't be damaged.

Appreciate any input.

somegeek
 

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an op-amp whose input connected to a ref voltage provided by a zener and the voltage to be compared can do the function you require.
 

picgak said:
an op-amp whose input connected to a ref voltage provided by a zener and the voltage to be compared can do the function you require.
Click to expand...

Thanks for the reply, picgak. I worked on this a bit tonight using the circuitlab.com simulator but came up short. Any direction you can provide here? I have a 3.9V zener diode in there and when I set input voltage to 3.9 my output voltage is 6.3V. 3.8V takes the output down to 3.925V. 4.8V input puts my output at 88V.

circuit.png

Looking for a nudge in the right direction please. :)

somegeek
 

remove the feed back resistor R20 the circuit will work
 

picgak, I removed Rf however the OpAmp output is negative regardless if I have an input of 3V or 5V to this circuit.

**broken link removed**

Thoughts?
 

Yes there is a problem the resistor connected to R13 should be connected to +VCC instead of the varying voltage. you can increase the value of R13 also.
I tried your circuitlab link but unable to open
 

Thanks for the input, picgak. I went ahead and posted on another board and have an active thread there.

somegeek
 

Congratulations on your success
 

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