The chronometer has four 7-segment displays, which can show a time lapse of 000.0 s to 999.9 s with a resolution of 0.1 s or a count from 0000 up to 9999. The chronometer is based on the type 74C925 counter IC with integrated display driver from National Semiconductor. The device draws a current of about 40 mA from a +5-V supply. On power-up, the counter is set to 0000 by network R14-C4 or by S1. The IC can derive a clock from two different sources: the internal oscillator or an external one via the COUNT input. The oscillator is formed by IC2c and IC2d and is enabled via the timer input. The enabling is effected manually by S3 or by inputting a given level, high or low, depending on the position of 53, into CHRON. The timer and counter inputs are identical, but are separated electrically from one another and from the signal source by optoisolators. This allows input potentials of up to 25 VP-P to be applied to either of them. As with the timer input, the level at the COUNT input can be either high or low and is selected by S4. The position of S2b determines whether the time or count function is active. Switch section S2a inserts the decimal point between the third and fourth display digits when the timer is selected. Whereas a simple level is needed to start the oscillator, the signal at the COUNT input needs more if the module is to work error-free. The signal must have steep edges; it must be free of interference; at low level, it must be well below 1V; and at high level, it must well above 2V. Moreover, if a switch is used at the COUNT input, it must be debounced adequately.
Required Field 
