The circuit schematic diagram of the tester is shown in the figure. The whole circuit is divided into two parts: the detection circuit is composed of transistors VT1 and vt2; VT3 and vt4 form an alarm generator. The detection part is composed of Schmidt trigger. When needle electrodes a and B are inserted into food, the resistance RX of the food itself is connected to the circuit. Because the internal resistance of food is related to the freshness of food, the fresher the food, the greater the resistance. When the distance between electrodes a and B is 1cm and the insertion depth is 2cm, the resistance of fresh fish is 20K, and it is not suitable to eat if the resistance is below 10K. When R0 = 5.6k in the circuit, Rx ≤ 10K, VT1 turns on, its collector voltage decreases, vt2 turns off, and the collector voltage increases, driving the sounder composed of VT3 and vr4 to sound. Due to the use of Schmitt trigger, the circuit has high sensitivity. When Rx is lower than a certain resistance value, the circuit will give an alarm sound.