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Timing Circuit - Buzzer

by Masafi
Tags: buzzer, circuit, timing
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Masafi
#1
Jan25-10, 06:13 AM
P: 65
In a timing circuit, an electronic processor operates so that the buzzer sounds when Vc is greater than Vs.

The switch S is normally open. Explain in detail what happens in the circuit after the switch S is closed for a moment then opened again. Your answer should include an appropriate calculation and a sketch graph.

Answer is:

S closed --> C charges
up to Vs
Instantly/very quickly
S open: discharge starts
Exponential discharge
Vc = Vs e^(-t/RC)
Vs = Vs e^(-t/RC)
ln = -t/RC (1)
t = 29.7 s

Buzzer sounds for 29.7 s

However, shouldnt the answer be 2 x 29.7, as the buzzer also sounds during the time the capacitor is charged and Vc > 0.75Vs?
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