Detector Base, 6-inch. The D250A is a two-wire, 12 VDC or 24 VDC detector base without an alarm LED. It accepts Bosch D254 and D255 heat detector heads. Use the detector base with either of these heat detector heads in circuits where specifications call for fixed-temperature heat detectors. You can also use this detector base with the Bosch D256A Rate-of-Rise and Fixed-temperature (+135°F [+57°C]) Heat Detector and D257A Rate-of-Rise and Fixed-temperature (+190°F [+88°C]) Heat Detector. The detector base also operates in a circuit controlled by the Bosch D291M Addressable Smoke Detector. In this circuit configuration, the D291M Addressable Detector Base controls a supervised 24 VDC circuit containing up to 19 detector bases that accept any D291M listed head. In this case, the detector base is not limited to heat detection heads.