PM Values Always Reading 0

Hi, 

thankyou for this simple but elegant sketch. I having it running on  Wemos D1R2 using GPIO4 (pin D2) for the UART, the sketch runs, serial monitor shows an output, however the PM2.5 values are always zero.

If I attache to another pin the UART read stops as it should.

Any ideas?

thnx
billd 