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**Working Principle of the Intelligent Biochemical Incubator Circuit:**
The temperature control system in the biochemical incubator is based on a new temperature sensor IC1. A voltage comparator circuit is made up of resistors R1–R7, temperature-setting potentiometers RP1 and RP2, and voltage comparator IC2 (N1, N2). The control execution circuit includes transistors V1 and V2, relays K1 and K2, and diodes VD1 and VD2.
This incubator can be built using an old single or double-door refrigerator by utilizing its cooling function and adding a heating device such as an electric heating wire or a 150W iodine-tungsten lamp, along with an exhaust fan inside the unit to ensure even temperature distribution. RP1 sets the upper temperature limit, while RP2 sets the lower limit. Relay K1 controls the heating system via an intermediate relay, and relay K2 controls the refrigerator’s cooling system through another intermediate relay.
Pins 5 and 2 of IC2 are connected to the center taps of RP1 and RP2 respectively, while pins 6 and 3 of IC2 are connected to the output of IC1 through resistor R3. When the voltage at pin 2 minus that at pin 5 is approximately 0.01V, the corresponding temperature is 1°C. If the internal temperature of the incubator stays within the set range, the voltage at pin 2 will be higher than that at pin 5, and the voltages at pins 3 and 6 will match or be slightly lower than that at pin 2. In this case, pins 1 and 7 of IC2 will output a low level, turning off V1 and V2, and relays K1 and K2 will not activate, so neither the heating nor cooling circuits will operate.
When the internal temperature exceeds the upper limit, the voltage at pins 3 and 6 of IC2 becomes higher than at pins 2 and 5, causing pin 1 of IC2 to switch from low to high, which turns on V2. This activates relay K2, closing its normally open contact and enabling the refrigeration system. Conversely, if the temperature drops below the lower limit, the voltage at pins 3 and 6 of IC2 becomes lower than at pins 2 and 5, causing pin 7 of IC2 to go from low to high, which turns on V1. This activates relay K1, closing its normally open contact and starting the heating system.
For component selection, resistors R1–R5 should be 1/4W metal film resistors with 1% precision, while R6–R9 can be 1/4W carbon film resistors. RP1 and RP2 are high-precision wirewound potentiometers. Capacitors C should be monolithic capacitors. Diodes VD1 and VD2 are 1N4148 silicon switching diodes. Transistors V1 and V2 are S9013 or C8050 type NPN silicon transistors. IC1 is a temperature sensor like LM35DZ, LM36, or TMP36, and IC2 is an LM393 operational amplifier. Relays K1 and K2 are 12V DC relays.
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