I have an Arduino board with a pump attached to it, and for powering both, I'm using a 18650 battery cell with a DC-DC Boost Converter. I plan to leave it working even when I'm not home, and I know that if the battery over discharges, there will be trouble, so I would like to design an over-discharge protection board.
I can see from this video that it is not a big deal to build one, however, I don't have a battery alarm buzzer. Is there any way to build such a protection board without that part?
Or is there any simpler way to build such a circuit with the components that I have: a relay (JQC-3F(T73)), some diodes (N4007 type), capacitors(22, 47 and 100uF) a potentiometer, a bunch of resistances (from 1mΩ to 51 kΩ) and transistors (PNP-8550, NPN-8050 and IRF520).
I'm attaching the scheme given by the person that made the video: 
Answer
Since you have a 5 V relay (which appears to have a 2.2 V drop off voltage) you might be able to do this:
simulate this circuit – Schematic created using CircuitLab
This will only work if:
- the relay will latch on with a little bit less than 5 V (most relays will)
- the relay will switch off at around 3 V
You have to close sw1 to start the thing up. You could use a piece of wire instead of a switch.
The relay coil will keep itself powered as long as the battery has enough voltage. When the battery voltage drops too low, the relay will fall off and switch off everything.
Do note that this is still a "hack" solution, when powered the relay will draw current and thus slowly (in a couple of days perhaps) discharge the battery.
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