NodeMCU init.lua, from https://www.ibm.com/developerworks/cn/iot/library/iot-mqtt-nodemcu-monitor-air-quality/index.html

nodemcu_init.lua

-- 1.Setup the device ID and access credential.See later.
 
-- PIN assignment
-- D5 is the driver PIN for the dust detector
dpin = 5
-- D0 is the LED on the NodeMCU board
lpin = 0
gpio.mode(dpin, gpio.INT)
-- The current pulse params
rising_ts = 0
falling_ts = 0
-- aggregated timing vars
high_time = 0
low_time = 0
-- determine if the INT is legit
prev_level = -1
 
-- setup Wifi
wifi.setmode(wifi.STATION)
wifi.sta.config("SSID","password")
wifi.eventmon.register(wifi.eventmon.STA_GOT_IP, connected)
 
-- This is the main application loop
-- It starts after the network is up
function connected (e) 
    -- 2.Connect to the MQTT service.See later.
-- Timer to take a measurement and send data 
-- every 10 minutes.
    tmr.alarm(1, 600000, tmr.ALARM_AUTO, function()
        -- Trigger the “dpin_cb” function (defined below)
        -- when a pulse comes in.
        -- dpin_cb will turn off the trigger after 30s
        gpio.trig(dpin, "both", dpin_cb)
    end)
end
 
-- Define the INT callback function
function dpin_cb (level, when)
    -- current_level = gpio.read(dpin)
    if prev_level == level then
        -- there is no change. ignore
        return
    else
        prev_level = level
    end
if level == 1 then
        rising_ts = tmr.now()
        print ("raising edge : " .. rising_ts)
        -- turn on the red LED
        gpio.write(lpin, gpio.LOW)
    else
        falling_ts = tmr.now()
        print ("falling edge : " .. falling_ts)
        -- turn off the red LED
        gpio.write(lpin, gpio.HIGH)
    end
    -- Start aggregated timer after a complete pulse is detected
    if falling_ts > 0 and rising_ts > 0 then
        if falling_ts > rising_ts then
            high_time = high_time + falling_ts - rising_ts
        else
            low_time = low_time + rising_ts - falling_ts
        end
    end
    -- Sampling period is 30*1,000,000 macroseconds
    total_time = high_time + low_time
    if total_time > 30000000 then
        lpo = low_time / total_time
        -- remove the INT and reset timers
        gpio.trig(dpin, "none")
        rising_ts = 0
        falling_ts = 0
        high_time = 0
        low_time = 0
        -- turn off the red LED
        gpio.write(lpin, gpio.HIGH)
        -- Very rough estimate.More calibration needed
        pm25 = lpo * 100.0 * 1.5
        -- 3.Send data to the MQTT server.See later.
    end
end