After the POR, PWRT provides an additional time delay for the normal operation of the software. The PWRT provides a fixed 72-ms nominal time-out on power-up only from the POR. The chip is kept in reset as long as the PWRT is active. The PWRT’s time delay allows Vdd to  rise  to  an acceptable level.

The Ost provides a 1024 oscillator cycle delay after the PWRT delay is over. This ensures that the  crystal oscillator  or  resonator  has  started  and stabilized.

BOR may be used to meet the  start-up  conditions. If Vdd falls to 5 4.0 V (3.8 – 4.2 V),  the  brown-out situation will reset the chip. Once Vdd rises above BOR.parameter6 Vdd, the PWRT will execute a 72-ms time  delay [15].

The PIC microcontroller reads the  information  from the RH measurement circuit via an A/D converter and START and UP buttons. Then, determined and measured RH values are compared by the PIC micro- controller. According to the  results  of the  comparison, it drives or stops the nebulizer and DC fan. In   addition, a three-digit common cathode display is driven by this element and displays the RH of the incubator environ- ment. The PIC 16C74A microcontroller has a 192 Byte RAM, 4KByte EPROM memory, 33 input and output pins, eight channels-8 Bit ADC, three timers, two CCP modules and 12 interrupt sources [15]. Once the PIC is time used for the first time in the circuit design, the proper hardware must be designed and the software needs to be written. The 213 byte memory is used in this application. Microcontrollers require some simple peripheral circuits when they are used for circuit design (figure  5).

The power supply supplies the required energy for both the microcontroller and associated circuits. The oscil- lator provides the clock signal for the  timer  circuit of the microcontroller. A 4-MHz crystal is used because of the required processing  speed.

There are three buttons on  the  control unit.  The RESET button restarts the system. The UP button enters the set values for the RH desired to reach in the

incubator environment. The START button starts the measurement and control processes. The humidifying process start with the entering of the determined value via the UP button. The determined values are shown at this time on the  three-digit,  seven-segment  display. Then, if the START button is pressed, the humidifying processes start and  the  measured  values  of  the incubator environment are displayed.  If  the  humidity set point needs to be reduced, it is first increased to a maximum value by pressing  the  UP  button.  Then,  the set point value will be back to zero. After that, it can be increased  to  the  required  set  point  value.

The A/D converter starts or stops the data converting according to data from the RA0 port of the PIC. The measured values from the RH measurement circuit are converted to 8-bit digital values by a A/D converter circuit (figure 6). After the determined values are entered, and the control processes are  started via related buttons, the determined and  measured values are compared in the control unit. If the determined values are greater than measured values, the ultrasonic nebulizer and circulation fan are activated by the microcontroller. If it is not, they are stopped by the microcontroller, i.e. on/off control has been used to control the humidifying processes of the incubator chamber. The nebulizer and circulation fan have been driven through the opto-isolator circuit  (figure  7). Since the density of the water particles produced in the nebulizer has been adjusted to a low level using the potentiometer P3 as explained above, the RH of the incubator environment is controlled with small swings around the determined values. In  other  words, the air of the incubator environment is humidified around the present values with same fluctuations.