Inventory system and methods of using the same

A modular shelving unit comprising:

• a shelf which is placeable on a rack, the shelf comprising a platform disposed on a weight sensor, wherein the platform is configured to support X holders, wherein X is a first number and each of the X holders is configured to store products, wherein each shelf further comprises:
• Y optical sensors disposed below Y holders when the Y holders are disposed on the platform, wherein Y is a second number that is one less than X, the Y optical sensors being in proximity of the Y holders, respectively, wherein each holder includes an opening which is aligned with one of the Y optical sensors when the holder is disposed on the shelf, and
• a processing circuit coupled to the Y optical sensors and the weight sensor, the processing circuit configured to:
• (i) receive either a first output from the weight sensor and a second output from one or more of the Y optical sensors or the first output from the weight sensor without the second output;
• (ii) convert the respective outputs into a digital signal that comprises:
• (a) an identifier of the weight sensor and an identifier of the one or more optical sensors when both the first output and the second output are received by the processing circuit;
• or
• (b) when the second output is not received, an identifier of the weight sensor and an indication of a weight or weight change associated with the first output from the weight sensor;
• and
• (iii) transmit the digital signal to a terminal.