Need a simple small low voltage cutoff circuit

[budm]

Re: Need a simple small low voltage cutoff circuit

You also use this one, the LED will be on if it is above the low batteries point, goes off when it is too low.
http://www.reuk.co.uk/TL431-Battery-Voltage-Monitor.htm

[flinx]

Re: Need a simple small low voltage cutoff circuit

I want to make another one of these. Which resistor controls the point at which the alarm goes off?

this one is for a12v lead acid battery and I want it to go off at 12.1 volts.

[Th3_uN1Qu3]

Re: Need a simple small low voltage cutoff circuit

Use this calculator: http://www.raltron.com/cust/tools/voltage_divider.asp

Fill in 12.1v for input voltage, 2.5v for output voltage (2.5v is the voltage provided by the TL431 reference), and a value for one of the resistors and it will calculate the other resistor. Note that it does not know about K so you would enter 10K Ohms as 10000.

[flinx]

Re: Need a simple small low voltage cutoff circuit

I thought I had some extra LM393, but I did not, I had to order some.

I built it tonight and it worked the first time. the LED lights at almost exactly 12.1v.

Front


Back


I need to get an audible alarm for it.

[NCSailor]

Re: Need a simple small low voltage cutoff circuit

The overall low voltage protection might be a good first step but it is not enough to ensure safety.
Each LiPo cell MUST have both the individual low voltage cutout and possibly more important, a smart charger that will not try to charge an individual cell that is below 3V at the normal charge rate; while also ensuring that each individual cell is never charged at higher than 4.2V.
Your system only protects you while all three cells are at the same voltage. This may be OK for new cells but is an unsafe assumption going forwards.
One safe way around this is to use only cells that have individual protection circuits built in.