The design of the proposed readout circuit provides benefits of detection speed, portability, low-cost and less human operational errors compared with the measurement by traditional instruments. Thus the added value is brought for biosensors and applied in home care. A novel readout circuit for a gas sensor based on an organic diode with vertical nanojunctions (VNJ) is proposed in this study. There are seven parts included in the readout system. First part is a preamplifier, second part is a peak-detect-and-hold circuit, third part is a divider, fourth part is the saturation detector, fifth part is the auto-reset circuit, sixth part is a logic gate and a buffer, seventh part is a micro-processor control unit (MCU). STM32 is the CPU of proposed MCU by ALIENTEK. The ADC of MCU is used to transform the output data of readout circuit. The designed circuit is accomplished by Taiwan Semiconductor Manufacturing Company (TSMC) 0.35 μm 2P4M 3.3 V mixed signal CMOS process, the area of chip is 0.74×0.75 mm2. Finally, the differences between experimental results with postsimulation results in 10 ppb ∼ 3 ppm of ammonia, the differences are within 7.24%. The sensing system is able to detect minimum ammonia concentration of 10 ppb, while the maximum one reaches around 3 ppm.