You will want to use 9V or 5V relays for most Boss pedals. The 5V are easy to obtain where the 9V may be a special order. You will also need to decide whether to us use the latching or non-latching type depending on your application.Latching relays use much less power so they are better suited to battery powered circuits. Latching relays usually require more complex circuitry to implement compared to a non-latching type. To further complicate the design latching relays come in single coil and double coil types. Check out the GEOFEX website for lots of information about relays and relay switching circuits.