Basically, a relay is like a switch for controlling high voltage devices like fan, light etc. They are available in many shapes, sizes and power ratings. It works on the principle of electromagnetic induction and it is easy to use with a microcontroller with the help of some active devices. The relay consists of five terminals. Two input pins and three output terminals.
Since an electrical relay has two stable states, either energised or latched or de-energised or unlatched, it is coming under the category of binary actuators whereas an actuator is a device which converts an electric signal to a corresponding physical quantity such as sound, force etc. Based on the number of stable states the output of a relay have, the relays are classified as binary and continuous relays.
Relays come with single or multiple contacts within a single package. The larger power relays are used for mains voltage or high current switching applications. There are electro-mechanical as well as solid state relays. Electro-mechanical relays are electromagnetic devices. They convert a magnetic flux into a pulling mechanical force which operates the electrical contacts in the relay. This flux may be generated by the application of a low voltage(AC or DC) electrical control signal across the relay terminals. A common such relay has an energizing coil called ‘primary circuit’ wound around a permeable iron core.
Even though electro-mechanical relays are cheap and can function in low power, it has several disadvantages. It is a mechanical device having moving parts which considerably reduces the switching speed. Besides, the other common drawbacks of mechanical devices such as wear and tear as well as noise also affect these devices. Solid state relays which are electronic relays without any mechanical contacts were invented to overcome this disadvantage.
OpenLab uses an SPDT relay which is a solid state relay. This chapter gives a detailed description of how an SPDT relay has been interfaced with the OpenLab.
- NO – Normally open
- COM – Common terminal
- NC – Normally closed
The relay is activated (energizing coils) using a micro-controller with the help of a transistor for switching and a freewheeling diode for protection.
Relay and its peripheral components are assembled on motor-board. Default port which is provided near the motor relay board is port C. Relay can be activated by any one of the 0-3 pins of the port.
The motor relay board can be activated by turning on the switch on the top left side.Then connect the motor relay board to port C using FRC cable.Bit selection pin is used for the custom selection of pins. Test pins can be used for debugging and testing purposes.
Using the Bit Selection Pins
Bit selection pins are used to connect the specified port pins with the motor relay board pins. The first 4 port pins are connected with relay inputs. Remaining pins are connected to motor drivers.
We need to use jumper caps to establish the connection between the port pin and motor relay board pins.
Connecting a Device with OpenLab
- Activate the motor relay board by turning on the switch on the top left side
- Connect the motor relay board to port C using the FRC cable
- Short the bit selection header you want (First four bits 0-3)
- In the below example, we should control 230 V lamb using the motor relay board
- Wire the circuit like the above diagram
- Burn the program into the controller