Food spoilage meter circuit

Food spoilage meter uses four transistors, some resistors and capacitors, and a speaker to make a food spoilage meter. When using it, just insert two needle-shaped electrodes into food such as fresh fish or meat. If the horn does not sound, it means the food is edible; if the horn sounds an alarm, it means the food has spoiled and is inedible. Note: This measuring instrument has a scientific basis in principle. It only quantifies the "experience value". However, it can still be used as an auxiliary device. Working Principle: 　　The circuit schematic diagram of the measuring instrument is shown in Figure 1.

The entire circuit is divided into two parts: transistors VT1 and VT2 form the detection circuit; VT3 and VT4 form the alarm generator. The detection part consists of a Schmitt trigger. When needle electrodes A and B are inserted into the food, the resistance Rx of the food itself is connected to the circuit. Since the internal resistance of the food is related to the freshness of the food, the fresher the food, the greater the resistance. When When the distance between electrodes A and B is 1cm and the insertion depth is 2cm, the measured resistance of fresh fish is 20K. If the resistance is below 10K, it is not suitable for consumption. When R0=5.6K in the circuit, Rx≤10K, VT1 is turned on, its collector voltage drops, VT2 is cut off, and the collector voltage rises, driving the sound generator composed of VT3 and VR4 to produce sound. Due to the use of Schmitt trigger, the circuit sensitivity is high, and the circuit will sound an alarm when Rx is lower than a certain resistance value. The component selection and production component list is shown in the table below. No. Name Model Quantity R0 Resistance is adjusted according to the food to be tested 1 R1, R5 Resistor 2.2K 1/8W 2 R2 Resistor 120Ω 1/8W 1 R3 Resistor 5.6K 1/8W 1 R4 Resistor 8.2K 1/8W 1 R6 Resistor 30K 1 /8W 1 R7 resistor 1K 1/8W 1 C polyester capacitor 0.047u 1 VT1-VT3 transistor 9014 3 VT4 transistor 9015 1 S switch 1 Use an 8 ohm small speaker; the needle electrode can be made of stainless steel wire or electric furnace wire. The length is 2.5cm, and it is soldered directly to the circuit board. It cannot be replaced by metals such as copper and zinc. Since different foods have different resistances, the resistance of R0 needs to be changed when measuring different foods. When R0=5.6K, it can be used to determine the quality of fresh fish; when R0=120K and Rx≤200K, the horn sounds and it can be used to determine the quality of pork; other various foods can be determined according to experiments. If a variable resistor is used instead of R0, it will be more convenient to mark different food words on the dial.