A LPF to stop the battery charge status LED flashing on TinyPICO board

[FRDM4236]

A LPF to stop the battery charge status LED flashing on the TinyPICO board

My PCB layout which will shrink the TinyPICO is done and will be sent out tomorrow.

During the design process I found that the orange LED on the TinyPICO board flashing when no battery is connected. It's a bit annoying, and I use a Low Pass Filter to stop it.
It works on the paper, I will test it on my prototype soon. If anyone has a different solution please kindly advise.



The goal is that before the cap is fully discharged it starts to be charged again. If the RC constant is large enough the voltage on the cap can stay high enough to keep the LED off.
The lower LED flashing rate, the higher RC time constant (Higher value of the RC).

 

[FRDM4236]

The LPF works. Finally I got my equipment unpacked and I could use my scope again. The flickering pulse from the charger is about 12 Hz,10% duty cycle, 10% low and 90% high. The 10% low turns on the orange LED.

The freq. is pretty low and required a large RC time constant to stop the flickering. The R cannot be too large because it's a current limit resistor for the LED, so the C must be large. If the R is 4.7K , the C must be greater than 100uf.

There is a problem with this small board. The largest capacitance in 0402 is 10uf, in 0603 is 47uf, so it needs a 0805 for 100uf. I may be able to squeeze in a 0603, but there is no way for a 0805. With a 47uf, I still could see very little flickering. Because the LED intensity is very low, I don't notice much unless I get very close and look for it.

I need to add a small resistor between the cap and LED to limit inrush current. When the power is initially applied, the cap is a short circuit before it's charged up, the LED gets 5V and generates a very bright short strobe. It's very noticeable with the large cap and the LED life will be shortened. The resistor must be in low value, but not too low. I will use one between 470 -1K.
I will try to install a 0603 47uf on the top layer, just a 0805 footprint on the bottom layer for an optional user installable 100uf.

Recently I have searched a lot for ESP32 tutorial info. online to get my foot wet. This flickering problem has been with many other boards, not just TinyPICO. This fix can be applied to other boards too. All bigger boards will have enough space for a 0805 cap, even the TinyPICO has plenty of space.

 

[defragster]

Nice - electronic solution to an electronic problem. DIM flicker easier to tolerate and ignore - bummer on the 5V inrush ... and another problem to resolve with another part.