Digital tachometer

ConclusionA simple digital tachometer has been constructed with standard t.t.l. integrate circuits. The circuit needs no calibration and its accuracy is sufficient for heartrate measurements. The mean heartrate value for four periods was used. This approach eliminates rapid changes of the value caused by respiration, and yet provides a stable value during surgical operations when strong electro-magnetic interferences make the e.c.g. signal temporarily not suitable for processing. The circuit measures the frequency in the range 15–250 impulses/min with an accuracy of ±2 impulses/min for 15–119 impulses/min, ±3 impulses/min for 120–199 impulses/min and ±5 impulses/min for 200–250 impulses/min. The tachometer can also be used for frequency measurements of other signals, e.g. respiration.