The circuit shown in the figure is a three-digit tachometer that lets you measure low-frequency events recurring at intervals of 0.235 to 15 seconds--that is, from 4 to 255 rpm.

When 5,000 rpm came and went the Summit Racing tach was once again on the low side, with a 4,800-rpm reading, and the Sunpro tach gave us our first high reading of 5,100 rpm.

The versatile, multi-function DM8000 serves as a tachometer, counter, totalizer and zero speed switch–in a space saving 1/8 DIN housing ...

The tachometers are designed primarily for use on shaft ends to measure speeds of up to 100,000 rpm and extremely low torque burdens of less than 1 ounce-inch.

The light beam has a 12-in. range for fast and safe measurements with 1-rpm accuracy. The HT-4100 can measure up to 50,000 rpm. An easy to read LCD display provides results.