**how to choose resistor for LED.**1) Check the voltage drop on led (

**Vd**)

2) Find out what will be the operating voltage (

**Vs**)

3) Decide how much we need current (it is selected on the power supply capacity, or the maximum output current of IC pin,and LED Imax)

**I****R=(Vs-Vd)/I**remarks

If the calculated resistance value non-standard should be taken nearest appropriate. (After you select you should recalculate current)

When LED work or heating it voltage drop decreases slightly sometimes it is necessary to take into account.

In my design, I take current value approximately equal to half of the maximum as LED is located very close to the photodetector, and no need for high brightness (less current will increase reliability).

In case of phototransistor is also possible to perform calculations on the basis of the manual but I do not know what model I have and what it features and is unknown to me consecrated created by DMM LED. So I just found right in experimental way.

I made HEX for you (speed 9600 standart settings) i test it with my software it work OK.

https://www.dropbox.com/s/qyisyfoqxdlw4gz/bu%2086%20uart.hex?dl=0circuit in attachments

**value of R3 must be chosen experimentally**To do this, run the COM Port terminal and view the data packets

end of package must always be the same is indicator of a well-functioning cable

01 11 be a1 fe fc 7c 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe fc e4 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe fc da 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe fc a0 01 00 be bf be be 04

** 00 86 86 86 86 80 **01 11 be a1 fe fe fc 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe a8 be 01 00 be bf be be 04

** 00 86 86 86 86 80 **01 11 be a1 fe 7e fc 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe 7e fe 01 00 be bf be be 04

** 00 86 86 86 86 80 **01 11 be a1 fe e4 a8 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe e4 7e 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe e4 7c 01 00 be bf be be 04

**00 86 86 86 86 80 **possible terminal will display data like this

fc a0 01 00 be bf be be 04

** 00 86 86 86 86 80 **01 11 be a1 fe fe fc 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe a8 be 01 00 be bf be be 04

** 00 86 86 86 86 80 **01 11 be a1 fe 7e fc 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe 7e fe 01 00 be bf be be 04

** 00 86 86 86 86 80 **01 11 be a1 fe e4 a8 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe e4 7e 01 00 be bf be be 04

**00 86 86 86 86 80 **01 11 be a1 fe e4 7c 01 00 be bf be be 04

**00 86 86 86 86 80 **so it can be difficult to understand if everything is okay