We propose to equip smartphone-based HMDs (SbHMDs) with an additional touch pad. SbHMDs are a low cost approach to allowing users to experience virtual reality (VR). Current SbHMDs, however, provide poor input functionality and sometimes external devices are necessary to enhance the VR experience. Our proposal uses frustrated total internal reflection (FTIR) to realize a touch pad on the external surfaces of the HMD case; no special devices are needed. As simple FTIR approaches do not suit SbHMDs due to the spatial relation between camera and light, we design an arrangement of acrylic plates and mirror suitable for smartphone's built-in camera and torch-light. It extends the input vocabulary SbHMDs to include touch location, gestures, and also pressure.