An interesting application that finds applications in industry, especially when fluctuations in the frequency of a power grid can occur, is the one shown in the figure. It is a frequency indicator that uses a lamp (LED or other type of indicator) output. The first NE5467 is set to a frequency 6% above the frequency you want to monitor while the other is set to a frequency 6% below. This frequency value is chosen precisely because of the normal selectivity of the NE567. When both NE567s are locked, this means that the frequency of the monitored signal is within 1% of the adjusted value, as shown in the graph in the figure below.
When the frequency goes out of the indicated range, the lamp goes out, remaining only the indicator connected to the IC output that was still locked.
