In this work, we demonstrated highly efficient visible light photodetector fabricated using spray coated nano lead sulfide (PbS) thin film at 100 0C on 300 GSM paper with pencil drawn electrodes. The effect of variations in channel length on the photoconducting properties of the nano PbS thin films was investigated. The results revealed that the device with 1 mm channel length has better photosensitive and responsive properties compared with other channel length devices. A proto type photodetector device fabricated in the laboratory using optimized nano PbS thin film shows photosensitivity of 102 and fast rise / decay time of 14.7 / 6.3 ms at 20 V. The results demonstrate the potential of application of nano PbS thin films for photodetectors and optical switches, which have high sensitivity and fast response time applicable in optoelectronic device technology.