The device, whose scheme is published in the journal “Popular electronics” (USA), produces a positive pulse when a sound (for example, firing of the gun) reaches the microphone B1 (see diagram). The variable resistor RZ adjust the sensitivity of the instrument, and the led V4 indicates it is turned on. Break contact S1, the finish associated with a ribbon, breaks the supply circuit of SCR V5 as soon as the athlete crosses the finish line.
In the device, you can use any silicon transistors of n-p-n and p-n-p conductivity, for example of the KT315 series and КТ301, SCR КУ101 or КУ502 with any alphabetic index, led АЛ307А, B. as a microphone recommend the use of “speaker” DEM-4m.
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