Working with Relays: an Introduction and Example

The objectives of this tutorial are to familiarize members with relays; produce an example solderless breadboard circuit using a relay and a DIP switch; and provide an example using a PCB board, DIP switch, and Darlington array to control the relays semiautonomously.

 

The first part of the discussion is a discussion about why use a relay?  An indication that you may need relays in your design are your applications include the need for voltages/amperages that your microcontroller organically cannot provide by itself.  Examples include high voltage motors for propulsion, large LED arrays, solenoids, or objects you want to either fail open or closed such as heaters or cooling applications.  

 

Relays provide the option to have devices either connected as normally open or normally closed.  This means that when power is connected to your circuit by default the gate is either closed or open.  Where the 'normally' indicates whether the relay state is tripped as true.  The device will be in its normally closed state if the power to the relay still persists if a signal is lost to the relay.

 

The example circuit that we'll breadboard includes the following parts:

 

(1) 3 Pole terminal block

(2) 1500 Ohm resistors 1/4 W, 5%

(2) LED,RED,635nm,T-1(3mm)

(1) 18 pin IC dual wipe socket

(1) 8 position DIP switch

(1) RELAY,12VDC@15A,SPDT

(1) small signal diode switching

(2) solderless breadboards

(1) 12VDC regulated power supply

(1) 470 micro-Farad capacitor 25V

Various connection wires

 

 The circuit works by providing an indicator light for power first, then the user controls the DIP switch position 1 manually to actuate the relay.  This also turns on the LED indicator light and powers the signal diode in parallel with the relay.  Note the hazards in this design are the orientation of the IC, switch, capacitor, relay, LEDs, and diode. 

 

 

The relay itself in this configuration is not connected to a device or an external power source.  This is due to the very difficult fact that this type of relay does not fit on a breadboard with the exception of in the IC slot.  This also means that you will have to customize your protoboard to account for this at the next step in design.  The future design will also integrate the Darlington array on the IC slot.  As a preventative measure it shouldn't be included on the board in the first go around.

 

To be continued...