Relay Click 3 is a dual relay Click board, featuring two single-pole double-throw relays which can be operated by output pins of the host microcontroller (MCU). It offers an elegant and easy solution for controlling a wide range of power applications. Two SRD-5VDC-SL-C relays used on this Click board™ are hermetically sealed relays which require 5V across their coils while consuming about 0.4W while active. They allow up to 28VDC across the connected load, while conducting up to 7A. Relay 3 Click can be used in various PLC-based systems, as a remote ON/OFF switch, and other similar applications.

Relay Click 3 is supported by a mikroSDK compliant library, which includes functions that simplify software development.

Besides two relays, the Click board™ contains all the necessary electronic components for driving the relay coils. Two LED indicators provide visual feedback about the activity of the relays. Although it can withstand up to 7A through its contacts, the life expectancy of the relay drops proportionally with the current through the contacts. Therefore, Relay 3 Click is a good solution for power or signal switching in a range of low to medium power applications, including domestic appliances, office machines, audio equipment, etc.

Relay Click features two SRD-05VDC-SL-C small-size relays, from Songle Relays. These are reliable relays in a sealed plastic housing, offering good isolation. Despite its size, the SRD-05VDC-SL-C relay is able to withstand up to 7A and 220V AC/28V DC. It can endure up to 105 operations while loaded, and even up to 107 with no load applied. This relay is of a single-pole-double-throw type: when the coil is energized, it will attract the internal switching elements and close one of the contacts, while opening the other contact at the same time. Normally Closed contacts are usually labeled with NC, while Normally Open contacts are labeled as NO.

These relays are designed so that their coils can be easily activated by relatively low currents and voltages. The SRD-05VDC-SL-C relay can be operated with 5V, making it a good choice for activating it by an MCU pin. However, to provide sufficient current for the activation, an additional MOSFET has to be used. Gates of two MOSFETS (one for each relay) are controlled by the MCU pins, therefore are routed to the mikroBUS™. The gates are routed to RST and CS pins of the mikroBUS™ and are labeled as RE1 and RE2, respectively.

There are two LEDs (yellow) which are used to indicate the activity state of the relay. When the current flows through the MOSFET, the coil will be energized, and the relay will be activated. This current also flows through these LEDs, indicating that the relay is active. The LEDs are labeled according to the relay they are connected to: REL1 for the Relay 1, and REL2 for the Relay 2.

A Schottky diode is connected across the relay coil, preventing the back-EMF which can be generated because of the inert nature of the coil. The back EMF can have an adverse effect on the circuit and can potentially damage the control circuit. The diode is connected in the inverse direction, allowing the back-EMF to discharge through the relay coil, instead.

Each relay is equipped with the 3-pole screw terminal, rated for up to 6A. Therefore, the maximum current through the connected load should not exceed this value. However, as already mentioned above, high current negatively affects the life expectance of the relay itself, so switching large currents should be avoided. The middle pole of the screw terminal is connected to the common terminal of the relay (COM) while two other poles are the NC and NO contacts of the relay. Having both NC and NO contacts is useful, expanding the implementation possibilities of Relay 3 Click.

• Interface: GPIO
• Compatibility: mikroBUS™
• Dimensions: 57.15 x 25.4mm
• Input Voltage: 5V
• Operational Current Range (28V DC / 220V AC): Min. 0A, Max. 6A
• Operational Voltage Range (AC): Min. 0V, Max. 220V
• Operational Voltage Range (DC): Min. 0V, Max. 28V
• Mechanical Endurance: Min. 0 operations, Max. 100,000 operations

• Schematic
• SRD-05VDC-SL-C Datasheet
• mikroSDK
• GitHub