If Vs = 5V, R1 = 1kΩ, R2 = 2kΩ, and C = 1mF, determine the following: a) The voltage level at which the 555 Timer output switches from the source voltage (high) to 0V (low). b) The voltage level at which the 555 Timer output switches from low to high. c) The amount of time that the output of the 555 Timer is high when the circuit is first powered
A 555 timer is an integrated circuit (IC) that utilizes RC circuitry to produce either an oscillating
output (square wave) or a single pulse. In the astable configuration (see below), the 555 timer
produces a square wave output with the peak at the supply voltage and the bottom at ground.
1) When the timer is initially turned on, the capacitor C is completely discharged and begins
charging through the combined resistance of R1 and R2 (series) and the output is equivalent
to the source voltage, Vs.
2) When the voltage on the capacitor C reaches 2/3rd of the supply voltage, the internal
circuitry of the 555 Timer IC causes the DIS pin to go to 0V, causing the capacitor C to
discharge through resistor R2 and the output to go to 0V.
3) Once the voltage on the capacitor C falls to 1/3rd of the supply voltage, the internal circuitry
of the 555 Timer IC causes the DIS pin to no longer be 0V and the capacitor C begins
charging through resistors R1 and R2 and setting the output voltage back to the source
voltage.
NOTE: if the capacitor is initially charged (Vo), the capacitor charging equation becomes:
Vc = Vs + (Vo − Vs
)e
−
t
RC
4) Steps 2 and 3 are repeated until power is removed from the circuit and the capacitor fully
discharges.
If Vs = 5V, R1 = 1kΩ, R2 = 2kΩ, and C = 1mF, determine the following:
a) The voltage level at which the 555 Timer output switches from the source voltage (high)
to 0V (low).
b) The voltage level at which the 555 Timer output switches from low to high.
c) The amount of time that the output of the 555 Timer is high when the circuit is first powered
on.
d) The amount of time that the output of the 555 Timer is low during a regular cycle of the
output.
e) The amount of time that the output of the 555 Timer is high during a regular cycle of the
output.
f) A graph of what the output waveform of the 555 Timer would look like. This can be done
by hand or using a computational tool like Excel or MATLAB. You must include at least
the initial period of high output as well as one complete regular cycle of high/low output.
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