Relay Interfacing:
A relays is an electrical switch that opens and closes under control of another electrical circuit. It is therefore connected to ouput pins of the microcontroller and used to turn on/off high-power devices such as motors, transformers, heaters, bulbs, antenna systems etc. These are almost always placed away from the board sensitive components. There are various types of relays but all of them operate in the same way. When a current flows through the coil, the relay is operated by an electromagnet to open or close one or many sets of contacts. Similar to optocouplers, there is no galvanic connection (electrical contact) between input and output circuits. Relays usually demand both higher voltage and current to start operation, but there are also miniature ones which can be activated by a low current directly obtained from a microcontroller pin.
The figure shows the solution specific to the 8051 microcontroller. A transistor is used here to activate relays because of its high current gain. This is not in accordance with “rules”, but is necessary in the event that logic one activation is applied since the output current is then very low (pin acts as an input).
In order to prevent the appearance of self-induction high voltage, caused by a sudden stop of current flow through the coil, an inverted polarized diode is connected in parallel to the coil. The purpose of this diode is to “cut off” the voltage peak.
I2c Interfacing:
