Datasheet

Then, bulk capacitance! A large 22uF ceramic (hence, low ESR) capacitor across the

supply on every node is used to absorb voltage transients, especially those caused by wire

inductance. Then, that supply gets further filtered using an R/C circuit comprising R7, R9

and C1, the latter being yet another 22uF ceramic and the relatively high resistor values

ensure that the C1 reacts very slowly to any change in the supply voltage. One thing to

notice is that we’ve used to resistors and we let the capacitor “float” in the middle. Why?

Assuming fairly equal wires for 5V and GND, this setup would result in the supply rails for

all microcontrollers in the chain to always have the same Vcc/2 potential, even if their Vcc

voltage is different as result of wire resistance x high current. This makes it easier to

discern the 0’s from the 1’s between consecutive nodes and thus get a reliable

communication channel despite the supply noise. Otherwise, since the detection threshold

is relative to the supply rails, we would have smaller error margins.

That simple circuit took a lot of tweaking to get right, but the result is very satisfactory

noise-immunity characteristics.

Power Dissipation and Over-Temperature

Protection

Despite LEDs being relatively efficient light sources, they still convert the vast majority of

their consumed power into heat. Furthermore, our linear constant current circuit uses

resistance (across the FET) to limit the current, resulting in the extra power being

converted to heat. In total, at full steam (driving all 3 LEDs at 100% duty cycle) our little

circuit dissipates around 5W! Keeping it from over-heating in this condition is unfeasible.

We’ve allocated largish thermal places on the PCB to improve cooling efficiency, but really,

the intention is to not leave the LED full-on for more than a couple of seconds. Rather,

working continuously at lower brightness is perfectly fine as well as generating fast, bright

pulses periodically.