Timer circuit diagram

Crystal oscillator

Photocoupler

As shown in the figure, it is a timer circuit composed of a 555 timer, a display terminal, a second clock generator, a photocoupler, a calculator, and the like. The circuit can be used in countdown occasions such as sports competitions, quizzes, etc. The readings are accurate, intuitive, and have a pause function.

Timer circuit

In the figure, 555 and R2, RP1, C2, etc. constitute an astable multivibrator with an oscillation frequency of = 1.443 / (R2 + RP1) C2.

Adjust RP1 to adjust its oscillation frequency. When the switches K1, K2 are closed, the astable multivibrator begins to oscillate. The pulse output from the output of the 555 circuit (pin 3) is applied to the optocoupler, and the isolated output of the optocoupler is added to the 838 calculator "=" key. Before timing, first operate the calculator to accumulate 1 count, that is, press the “ON/C” button to the “+” to “1” button, then turn on K1 and K2. In the timing process, if you want to pause, you can press the “CE” button to stop, or disconnect K1, and the timer stops.