Abstract: Device for fail-safe testing of a condition indicating device including at least one infrared sensor. The infrared sensor is connected by amplifiers to comparators which have different threshold values and have outputs connected to a microprocessor. An infrared signal transmitter is actuated by the microprocessor at periodic intervals to produce signals which are detectable by the infrared sensor. The microprocessor delivers an alarm signal if the test signal is not received by the infrared sensor within a specified period after triggering of the transmitter. The microprocessor also monitors the time occurrence of noise signals produced by the infrared sensor, and produces an alarm if no noise signals are received within a specified period. During monitoring of reception of transmitted test signals, monitoring of the noise signals is interrupted to prevent the test signal reception from being processed as a noise signal.