Overview
The project is a small Ethernet remote device which consists of multi-function I/O ports, employs a simple ON/OFF application and provides fast communication.
Explanation
The circuit can consume about 200mA at 3.3V since Ethernet requires high currents when used with long cables. A relay gets connected to connector CONN3 for the possibility of connecting a small 6V relay on the output. This provides protection for the whole circuit from very high voltages that can be induced by the coil of a relay. A resistor can be added in parallel to the relay if the relay used has a large coil. The voltages will be prevented to rise too fast before the diode cuts them since the diodes have a finite response time.
A small speed increase over the 8MHz from the ATmega88 internal clock is given by the ENC28J60 that can supply a 12.5MHz clock signal. During the assembly, the LM2937-33 voltage regulator is soldered to the board along with the required capacitors. The green LED on the Magjack and the Hub/Switch link LED indicate a working link negotiation since the Ethernet has a pulse based link negotiation protocol.