Circuit principle:The circuit consists of oxygen sensor os-12, DC amplifier IC1, a / D converter iC4, LCD f2100-34pi, voltage comparator IC2, positive and negative power converter IC. And so on. Os-12 is a galvanic cell type oxygen sensor. It can detect the oxygen in the air and output a signal of about 50 mV. It has a linear output in the range of 0-100% oxygen concentration. Its output input a / D converter can display the oxygen concentration accurately by digital. IC1 is a DC amplifier, which can amplify the output signal of the sensor by 8 times. The amplified signal is converted by iC4 (icl7107) a / D and then directly drives the LCD to display the oxygen concentration digitally. Icl7107 is a 3.5 bit monolithic A / D converter. When combined with LED display, the current consumption can be reduced to about 1mA. D1 is a constant current diode used as the reference voltage of a / D converter. Cr033 is a constant current MOSFET, if 330 is provided μ The temperature coefficient is close to zero. The reference voltage of voltage comparator IC2 is obtained from D1, which is used to detect the concentration of oxygen below 18%. If the oxygen concentration is low, it will give an alarm. IC3 is a converter from positive power supply to negative power supply, and its leakage current is very small. The power supply uses four 450mA NiCd batteries, which can be used continuously for 100h. The circuit can monitor the oxygen concentration in underground tunnels and the bottom of caves to prevent anoxic accidents.