Solid State Relay / Switch

Most of the times you may require to control gadgets/equipments (like motors, bulbs, heaters) working on 230VAC (mains) supply, using electronic circuit or micro-controller.   Normally, relays are used for on/off control for the mains operated gardges.  Due to electromagnetic operation, the contacts move and makes noise during on/off the mains circuit.  The circuit used here can switch on/off the mains circuit silently and does not have any moving parts. 


The project consists of three sections for safe working and isolation from the mains supply.

1) Triac section consists of BT136 with heatsink and discrete components to control (on/off) the load on mains power supply side.

2) Reistor Network consists of 4 resistors in series to control using  any of 3.3VDC / 5VDC / 9VDC / 12VDC supply.

3) Isolation is obtained from IC MOC3021, which has an LED and DIAC concealed in 6 pin DIP package.

The full circuit diagram of Solid State Relay / Switch is shown below:

Solid State Relay-CIRCUIT DIAGRAM.png

The circuit is simple and almost self explanatory as per the three sections listed above.  Scrubbers (resistance and capacitors in series) are added wherever required for Triac and Diac.  Care and safety should be observed, while wiring the mains power lines.  Never touch the circuit (mains side), while power is on.

Working Principle:

The LED in the MOC3021 glows (or switched on), due to appropriate DC power supply (marked in the circuit). through resistance network.  Due to LED light falling on the Diac, inside the MOC3021, the Diac starts conducting and Triac (BT136) gets triggered.  This makes flow of current through the Triac and completes the AC mains circuit loop, which switches ON the connected load.

For control (switch on/off) more current or wattage, use BTA16 or BTA41 Triac.  In addition to same pin outs (as marked in the circuit) and high power control, the BTA Triacs have tab insulation also (so, more safe). 

Maximum allowed Current flow through BT136 is 4A, BTA16 is 16A and BTA41 is 41A.  So you may select appropriate Triac to suit your requirements.


1) Never touch the circuit, when mains power is ON.

2) Use 6 pin IC base for MOC3021 and test any leakage between the pins, before inserting the IC.

3) Use mica insulation between Triac and heat sink.