Littlefuse's [Phase Control Using Thyristors](http://www.littelfuse.com/~/media/electronics/application_notes/switching_thyristors/littelfuse_thyristor_phase_control_using_thyristors_application_note.pdf.pdf) is an excellent reference on the subject of both thyristors and triacs.

[![enter image description here][1]][1]

*Littlefuse's application note suggests two 100 Ω with a filtering capacitor.*

You can omit C1 but may find the circuit more prone to triggering by noise. For 230 V operation double the resistor values.

You also need to calculate and include Rin. Typically you should aim for 10 mA through the infrared opto-LED.

[![enter image description here][2]][2]

*Figure 2. The LED loadline chart. Source: [Loadline resistance calculation](http://lednique.com/wp/wp-admin/post.php?post=544&action=edit). ([Disclaimer](https://electronics.stackexchange.com/users/73158/transistor).)*

For 10 mA in the infrared opto-LED, slide across from 10 on the *If* axis to the IR LED curve and pick the closed loadline: 390 Ω in this case. That's your Rin value.


Other links:

- [Thyristor Theory and Design Considerations Handbook](http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF) by ON Semiconductor.

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Update: I forgot to mention, triacs don't have a base. They have a "gate". "Base" is reserved for transistors.


  [1]: https://i.sstatic.net/xId0F.png
  [2]: https://i.sstatic.net/PFLto.png