â–¡ Liu Yaosheng's homemade adjustable thermostat and timing electric oven is a versatile device that can function as both a drying apparatus and a kitchen appliance. The oven features a thermocouple temperature sensor, an adjustable thermostat, and a timing circuit for control. This design allows users to regulate temperature and time precisely, making it suitable for various applications.
The power supply section of the circuit includes a transformer, a bridge rectifier, an integrated voltage regulator (7812), and capacitors, providing a stable 12V DC output for the rest of the system. A current-limiting resistor and a lamp are also included to protect the circuit from overloads.
The thermocouple used in this project is a shell-mounted armored type, known for its durability and accuracy. Due to the low sensitivity of the thermocouple, the generated signal is small and requires amplification. An operational amplifier is used to boost the signal, followed by a low-pass filter to reduce noise and improve stability.
The comparator circuit uses an open-loop configuration to compare the actual temperature with the set point. When the temperature exceeds the threshold, the comparator triggers the control circuit, which then turns off the heater. A 555 timer is used to manage the timing function, ensuring precise operation.
When the infrared light between the two sensors is blocked, the capacitor charges, eventually triggering the alarm through the 555 timer. This mechanism ensures that the system responds accurately to changes in the environment. During testing, adjustments can be made using a potentiometer to fine-tune the sensitivity of the system.
During installation, the placement of the infrared sensors is crucial. They should be aligned directly with the grain drop pipe, and their positioning may vary depending on the setup. Other components are mounted on a small PCB board, with standard resistors and a 555 IC used for reliability.
In addition to the oven, the article also discusses a computer voice karaoke machine developed for campus use. This device allows users to send personalized messages and songs via internal phone networks. It utilizes a modified telephone and a simple circuit board to provide an easy-to-use interface for sending greetings and music.
The author highlights the importance of practical electronics in educational settings, emphasizing how such projects enhance learning and creativity. The karaoke machine, for instance, offers a fun and effective way to communicate with friends and family.
Regarding the temperature control system, the original design had some issues with frequent switching due to a lack of hysteresis. To address this, a feedback resistor was added to create a hysteresis effect, reducing unnecessary cycling of the heater. This improvement makes the system more stable and extends the lifespan of the components.
Overall, the project demonstrates a well-thought-out approach to building an adjustable electric oven with temperature and timing controls. While there were initial design flaws, these were successfully resolved through modifications, resulting in a functional and reliable device.
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