The DA-500LKA is a 24V relay board. It is designed to accept the 24VDC from the Mier DA500CP than trigger its own dry contact output relays. This is useful if the DA500CP is being used in an application where the users require both the 24V output to be active, or requires the whistle to be in the “ON” position while using another accessory which requires a dry contact, Normally Open relay.