Pick-by-light LED node

Bottom side

After the evaluation with the IAR/ Luminary Micro LM3S102 board, we decided to have a single processor per pick display. The drive capability of the output pins was not as high as the AT8515 in previous designs, which is why there are transistors per segment (NPN) and a transistor (PNP) per display. The board was made as low-cost as possible; two sided, no solder mask, no silk screen, etc. Besides the time, the budget for respins was limited.

Bottom side of Luminary Micro LM3S101 board for LED pick-by-light

Unfortunately, the Altium PCB layout package had the wrong pinouts for the transistors. When powering up the board, the power supply immediately went into current limit. As can be seen from the assembly, we no longer had much in the way of solder paste or heat guns as that had been packed for another continent. We were a pin short for the RS-485 buffer control using the standard JTAG controller, however, if the debug was changed to SWD we had a spare pin, but someone would need to buy another probe and a matching toolchain.

Display side

The displays would be multi-dropped on a RS-485 network with 31 or 63 drops per string (depending on the buffers). The push-button was to acknowledge a pick, the LED out the side was for viewing from the end of the aisle, and the three 7-segment displays would allow up to 999 items to be picked. There would be 16,000 of these displays in an average warehouse.

Top of Luminary Micro LM3S101 board for LED pick-by-light

The serial interface messaging began on the evaluation platform over RS-232 and tested against a Linux PC, but was modified to work with RS-485. (Tested initially against the AVR32 evaluation board; see serial.c (22kB)). We handed over the logistics project to Duncan Drennan for completion in March, 2008, before leaving South Africa. (The pharmaceutical warehouse director’s son, Duncan, was the electronics engineer who discovered the Altium packaging problem). See the finished item here.

Circuit diagram of Luminary Micro LM3S101 board for LED pick-by-light

Circuit Diagram

The circuit diagrams were originally entered in OrCAD; Altium was used for this node version.

The date on the circuit diagram was 17th December, 2007. The layout, annual shutdown for the circuit board manufacturer, and ordering components would have pushed the assembly to end of January, 2008. Our container had already left South Africa.