The purpose of this project is to replace the defective control board with a new microwave control board, regardless of brand and capability. The microwave was dumped as electronic waste due to damage to the microwave control panel. Magnetrons, high-voltage transformers, high-voltage diodes, drive mechanisms and other components can be seen everywhere in the market except for control boards. The block diagram of the microwave oven is shown in Figure 1
Circuit and Operation
The circuit diagram of the microwave oven is shown in Figure 2. It consists of 5v voltage regulator 7805 (IC1), microcontroller AT89C52 (IC2), 4 12v relays (rl1 through RL4), 5 pnp transistors (BC557), 14 tactile switches, 1 16 × 2-wire liquid crystal display (LCD1 ) and some discrete components.
The circuit uses 230v AC power supply. Transformer x1 steps down the voltage to 12 volts. Diodes d1 and d2 act as rectifier diodes. Capacitor c1 is connected as a filter. The rectified, filtered and stabilized DC voltage is sent to the power section of the circuit, and 5V is sent to the control circuit through the 7805.
The At89c52 microcontroller adopts 14 control switches, 16 × 2 LCD and 4 relays, and the clock frequency is 11.0592 MHz. Use zener diode zd1 to connect 12v relay to 5v microcontroller through pnp transistor through zd4. When the door switch is open, diode d5 prevents the 12v power supply to the microcontroller.
The relays RL1, RL2, rl3 and rl4 are respectively connected to the microcontroller through four relay driving transistors T1, T2, t3 and t4. Transistor t5 is used to drive the piezoelectric buzzer. The Pnp transistor is used as the active low output of the controller. The LCD is connected to port P2. You can use vr1 to adjust the contrast of lcd1. D4 to d7 are the protection diodes for the relay drive transistors. The crystal frequency of the At89c52 is 11.0592 MHz.
Software
The software is written in C language and written with Kiel μvision 4 software. The logic behind the software is briefly described for ease of understanding and editing. In the main part of the software, the microcontroller continuously scans the status of seven input switches, including vegetable (VEG), meat, rice, freezing (FRZ), baking, sublimation and defrosting (DFR). If these switches are found to be deleted by the program, the software jumps to the corresponding subroutine. In the automatic subroutine, each branch sets the total cooking time and on/off delay time for the magnetron and grill heaters. The total delay time and on/off delay time for each cooking method are shown in the table.
During the defrost process, the program executes the defrost subroutine. The defrosting technology provided by microwave ovens is used to cook or heat refrigerated food. Normally cooked refrigerated food results in the outer parts of the oven burning quickly, resulting in food spoilage.
To compensate in defrost mode, the system initially only switches the magnetron on and off for a few seconds. It slowly increases over time and slowly decreases over time. This method helps to evenly penetrate the heat into the food.
The BBQ option only offers meat. The mixed option only offers rice, meat and cakes. On decompression, the software executes a manual mode subroutine. In manual mode, the total cooking time can be selected by pressing the up or down button. The time increases or decreases by one second with each press.
Hold up or down to increase or decrease the time by 10 seconds. After setting the time, any cooking mode (Microvegetable or Combi) can be selected. The switching delay is selected according to the type of cooking according to the preset sequence. Accurate delay time can be achieved by enabling Timer0 of the microcontroller (IC2).
Unlike traditional ovens, dual protection provides door opening while cooking, in software and hardware. When the door is opened, the coil is disconnected from the ground of the magnetron relay. At the same time, the software detects this condition and turns off the magnetron.
Lcd1 displays all parameters. It is connected in 4-wire method or 4-bit mode of operation. The corresponding software is easy to understand.
ICs used by Topview programmers for programming at EFY Labs.
Build and Test
The PCB layout of the microwave oven is shown in Figure 3, and its component layout is shown in Figure 4.
Carefully place all components on the PCB and solder. Load the software program (hex file) onto the microcontroller chip. Use integrated circuits on PCBs. Solder the tact switches on a separate PCB and connect to the main PCB, according to the circuit diagram. Once the circuit is assembled, package it in a suitable box. Fix the switches and relays on the front and rear sides of the cabinet. Use a 2-wire cable to connect the relay coil on the PCB. Connect the grill heater, high voltage transformer, stirring motor and external high current transmission lines through relay contacts.
Connect the wires to CON1, CON2, relay, con3 and CON4 as shown in Figure 2, except for the high voltage transformer and heater connections. Replaced the high voltage transformer and grill heater with two 100w lamps. Turn on the oven and make sure all controls are working properly.
Check the minimum time duration and make sure the lights are on/off in the order shown in the table. If the working time and duration of all cooking methods are satisfactory, switch off the unit and connect the high voltage transformer and heater. Cover and prepare any food of your choice.
Operation Guide
Auto Mode
- Turn on the power and the display will show the standby state.
- Choose an option (frozen, defrosted, baked, vegetable, rice or meat).
For example, meat:
(a) Press the meat. The buzzer will beep. (b) Press Micro. (You can choose Micro, Grill or Combi). ©Press the start key. The display will show Meat/Auto/Micro. The time will be shortened from 15:00. When the time is up, the buzzer will beep. After the beep, the display will show that cooking is complete.
3. You can choose any one of the options: Freeze, Thaw, Roast, Vegetable or Rice and repeat the above steps (a, b and c).
Manual Mode
1. Turn on the power. The display will show the standby state. 2. Press the up key. The display will show the set time. 3. Press the up/down keys to increase/decrease the time. 4. Press and hold Up/Down to increase the counting speed.
For example, to select Veg For 3.00 minutes,
(a) News Vegetarian. The buzzer will beep. (b) Press Micro. (You can choose Micro, Grill or Combi)© Press the start button. The display will show Manual/Vegetable/Micro. The time will decrease from 3:00. At the completion time, the buzzer will beep. After the beep, the display will show that cooking is complete.
5. You can choose any option: Freeze, Thaw, Bake, Rice or Meat and repeat the above steps (a, b and c). - Mr. Bai Jilong has been engaged in the electronics industry for 15 years. He has developed more than 100 products so far, and most of them have been mass-produced.
- It took 5 years since 2018 to record thousands of practical-level electronic engineer series courses, from components to core modules to complete products - Lao Bai's original intention is "May the world's engineers not take detours"
- Among them, there are courses that explain MOS tubes and IGBs in detail