An accessory device removably mounted to the headstock of a string instrument including an arm assembly for, actuating the keys of a keyboard instrument. The functional element is configured to protect the integrated circuit from transient voltage. The two temperature sensors may be convectively coupled to the heater by a fluid ambient. The radius track provides a substantial surface area sufficient to absorb all of the recoil force generated when the shotgun is discharged. The receiving is responsive to the sending.