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I am working on a LED (3 V/20 mA) torch circuit powered by 3.7 V/1000 mAh Lithium battery. Am planning to add discharge protection feature to this circuit. If battery voltage go down to 3.3 V the torch should cease operation. However I can not figure out how to do that. I have come across some circuits where they have used a 3.3 V zener diode and a transistor to turn the intended load on. However I couldn't validate the working of this circuit.

I tried simulations using circuit lab but voltage across zener is dropping after connecting to transistor base. Need help in figuring out this circuit. I have added a lamp with resistance of 25 ohms to simulate 7 LED's connecting in parallel in the below circuit. Any help will be much appreciated? enter image description here

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    \$\begingroup\$ The circuit you have there wont work, but try putting the zener diode and the resistor in series with the base of the transistor, and changing it to a 2.5V zener diode \$\endgroup\$
    – BeB00
    Mar 29, 2020 at 10:57
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    \$\begingroup\$ I would consider using a DW01 based module. The DW01 is a Li-Ion battery protection IC, it needs some MOSFETs to operate. You can get the DW01 including the MOSFETs on a ready-made module for cheap on Ebay etc. That will not cut off at 3.3 V though but at (if I remember correctly) at 2.7 V. If that is OK is your choice. \$\endgroup\$
    – Bimpelrekkie
    Mar 29, 2020 at 11:14
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    \$\begingroup\$ Also have a look at: electronics.stackexchange.com/questions/279696/… where Bruce's answer has a circuit that can be set to almost any voltage. \$\endgroup\$
    – Bimpelrekkie
    Mar 29, 2020 at 11:17

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BeB00 is right, your schematic will not work as it is simply keeping the transistor turned on, and the zener is not even doing anything there - it would be the same if you removed it, no change in circuit behavior. Here is what BeB00 has suggested to you (plus my own suggestions):
enter image description here

The way it works is that the battery voltage has to be above the base-emitter voltage of around 0.6V PLUS the zener voltage of around 2.7V in order for the base current to flow through R1, the zener diode and the base of the transistor, which in turn makes the current flow through the LED1, R2 and the collector to the emitter (ground or negative side of the battery).
The second LED (LED2) is an option if you can't find or get a 2.7V zener diode, and it has to be a blue or white LED which has around 2.7V voltage drop in this circuit (you would have to measure it once you place it in this circuit).
You could also use 5 standard diodes (like 1N4148, 1N4001, or any rectifier or signal diodes) instead of the zener or LED2, and you would have to connect all of them in series and in forward direction like the LED2.

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