An asynchronous photodetector circuit is provided to interrogate incoming signals and determine whether the frequency of those incoming signal pulses is acceptable. The circuitry of the present invention permits a series of incoming pulses to be interrogated to determine whether the frequency of those pulses is acceptable and can be assumed with confidence to be emanating from an appropriate light source. Upon the receipt of a first input signal pulse, a time window is created by the present invention to define a period to time during which a subsequent input signal pulse is to be expected. Other than during the duration of the time window, the present invention will not accept an input signal pulse and will not count that pulse as having been received. Each properly received pulse creates a subsequent time window until a predetermined number of consecutive pulses is received during their time windows. When that predetermined number is received, a signal is provided. In a particularly preferred embodiment of the present invention, the signal that the predetermined number of consecutive pulses have been received is used to lower the threshold voltage of a comparator for the purpose of facilitating receipt of subsequent signals once the appropriateness of the incoming stream of pulses is determined.