Hello everybody,I'm new to this site and this forum seems to be perfect to introduce my project and get some guidance from you guys. Special thanks to Giorgos!
My aim is to make a LED Phone Case like you can see in the pictures below. My final goal is to get a slow breathing of all of the almost 30 LEDs as a nice effekt. Constraints are regarding the size since the Phone Case shouldn't be too thick.
Parts I want to use:-The biggest problem was the power supply. It had to be small like a coin cell but at the same time have a big current supply to power all the LEDs. Thats why I ordered a smartphone battery for the BLU Dash 5.0, as it comes flat (4mm) and has up to 2000mAh and a current of up to 1A (that's a suggestion from reading a paper about smartphone batteries).
-For the LED's I thought of 1206 SMD LEDs.
-The circuit should be built with a 555 timer IC. However, I need to use a low power SMD 555 that works with 3.7V power supply that I get from the battery.
-SMD resistors and BJTs.
The circuit I want to use:
This is a standard 555 timer circuit that I want to modify for my project. But first I want to make sure that I understood the circuit correctly. Here is what I think how it works. Please correct me if I am wrong!
1. As initial voltage is low at pin 2, the trigger will turn on the output (pin 3) and current will flow. At first the current will flow into the capacity as its resistance is lower than the BJT's. However, a little later current is also flowing into the BJT and the current will increase as the capcity's resistance will increase gradually. This gives us a gradual breathing till the LED is bright.
1. As the voltage across the capacity is getting greater than 6 volts, the treshold will fire and turn off the output pin. Current is now flowing from the capacity into the 555 and into ground. This phase might be slightly shorter since only the capacity current has to flow into ground as to decharge the capacity.
3. As the voltage across the capcity is getting less than 3 volts, again it begins to turn output on and so on.
The modifications I want to make:
The 555 timer I have to use will have a lower output/sink current (50mA/10mA). Thus, I have to use BJTs to get enough current through my LEDs which need approx. 3.2V and 20mA. But here we come to my questions: Is it possible to power 28 LEDs with the battery and the 555 timer and how could I manage to do that. Below is my first draft but I need to get a better understanding of how to power so many LEDs and your guys' advice. How can I calculate the current properly e.g. what outvoltage will the ouput pin 3 give me?
Any advice or comment on my project? Any help is appreciated!
Thanks,
Kon