Phototransistor Interface Circuit Design A phototransistor is a photodiode with a built in current gain, which makes it far easier to use than a plain photodiode and slower than one. The interface is a load resistor and often an amplifier, and the choice of that resistor sets the sensitivity, the…

Thermal Via: Design Rules and Process Limits A power device does not lose its heat to the air. It conducts it into the copper pad beneath it, and from there into the planes and the other side of the board, where a heatsink or an airflow can remove it. On…

RS-422 Interface Circuit Design RS-422 is a balanced interface that carries a single signal in one direction over a pair of conductors. It is the simplex counterpart of the bidirectional bus that most people know, and it is still used where a one way link has to travel a long…

Smartphone PCB Design Feasibility and Routing Planning A handset board packs several radios, a display, cameras and a battery into a space smaller than a card. Smartphone pcb design is a planning problem before it is a drawing problem, because the routing capacity has to be proven before the layout…

Mixed Flowing Gas Testing for Electronic Assemblies Salt spray measures resistance to chloride. It says almost nothing about what happens in an office, a data centre or an industrial cabinet, where the aggressive species are sulfur compounds and nitrogen oxides at concentrations far too low to see. Mixed flowing gas…

LED Series Resistor Design for Strip and Indicator Boards A light emitting diode needs its current limited, and the cheapest way to do that is a series resistor. Getting the value right is arithmetic, but choosing where to put the resistor and how to rate it takes a little more…

Thermal Relief Design and Solder Wicking A pad that connects to a ground plane is connected to a great deal of copper, and that copper carries heat away from the joint during soldering. The result is a pad that does not reach the soldering temperature while its neighbours do, and…

Bare Board Test: Finding Opens and Shorts Before Assembly A bare board is tested before any component is placed on it, and the purpose is to catch the faults that fabrication can produce: a broken trace, a short between two nets, a missing connection to an inner layer. Finding them…

LIN Bus Transceiver Circuit Design A LIN bus is a low speed single wire network used inside a vehicle, mostly to connect switches, sensors and small actuators to a body controller. The transceiver is the interface between the protocol controller and the wire, and it has to survive the electrical…

Cellular Module Board Design for Connected Products A product that connects over a mobile network carries a radio transmitter and a power amplifier on the same board as its logic. Cellular module board design is mostly about the supply, the antenna and the certification, because the radio itself is inside…