It can be infuriating when trying to use a Power Bank battery pack to run small things that don't use much power. I have four phone charger power banks and they all need 40-50mA of load to keep them running, otherwise they turn off. The turn off times range from 5, 30 and 40 seconds. The 5 second one is the oldest.
The other day it occurred to me that perhaps I could make a small PCB that just incorporated an appropriate load resistor to keep the battery outputting power. Luckily I had a quick Google search to see if there where some other projects out there and sure enough there where a few. Some people used MCU's like a ATiny85, others used a 555 timer. I decided to test out a 555 timer as I have a few DIP and SMD parts in stock. I bread boarded up a circuit and yesterday afternoon I had a play with testing my three 30-40 second Power Banks:
Initially I tried using a plain 555 timer IC to drive a load resistor, however I wasn't happy with such a high turn on, so later I tried a N-MOSFET to invert the waveform. It never turned off properly. I might have been able to get around that, but I thought a PNP transistor might be an easier to work with and it was. This is the circuit I ended up using:
It took most of the afternoon to work all this out and to trial/tweak the capacitor and resistor values to get to a low current, but still keep the Power Banks alive. Interestingly, they all needed a pulse length of at least 420mSec at 40mA when using a pulse rate/period of 9 seconds. The shorter the pulse rate, the short the keep alive pulse could be - up to a point.
It didn't take long to design up a circuit for it in KiCAD and layout a PCB. To clean up the ripple noise a bit I added a two order Pi filter as well and this is what it will look like:
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