     
     // init RF24
     rf.begin();
+    rf.setAutoAck(true);
     rf.setRetries(15, 15);
     rf.enableDynamicPayloads();
     rf.openWritingPipe(address);

        // send CO2 measurement over RF24
        if (buffer[0] == 'P') {
            // construct buffer: [length]"CO_2"[ppm high byte][ppm low byte]
            uint8_t buf[8];
            buf[0] = 6;
            memcpy(&buf[1], "CO_2", 4);
            memcpy(&buf[5], &buffer[1], 2);
            // dump send buffer
            Serial.print("Send:");
            for (int i = 0; i < 6; i++) {
                Serial.print(buf[i], HEX);
                Serial.print(" ");
            }
            Serial.println();
            // send it
            rf.write(&buf, 7);
        }
    }


    bool data = (digitalRead(PIN_DATA) == HIGH);
    unsigned long ms = millis();
    bool ready = zg01_process(ms, data);

    // process data if ready
    if (ready) {
        // dump buffer to serial
