Internal elements of a 555 timer. A 555 timer is an integrated circuit that
produces various types of timed signals, such as a simple pulse or a continuous
square wave. The type of signals produced is determined in two ways: by which types
of components are externally connected to the IC and by how the external
components are connected to the IC. To thoroughly know how a 555 timer operates
it is necessary to understand the internal elements inside the IC package.
A 555 IC has five different types of internal elements. let's examine how each
element operates. Voltage divider network: resistors r1 r2 and r3 are connected in series.
Each resistor is 5k ohms so one third of the five volt supply voltage
drops across each one. The function of these resistors is to form a voltage
divider that applies two-thirds of the power supply voltage at the inverting
negative input of comparator A. At the same time one-third of the power supply
voltage is applied to the non-inverting positive input of comparator B. Voltage
comparator: Two voltage comparators are inside the 555 IC. Whenever the voltage
at a non-inverting positive input is more positive than the voltage at the
inverting negative input, a positive voltage is produced at the comparator
output. Whenever the voltage at the inverting negative input is equal to or
greater than the non-inverting positive input, zero volts is produced at the
comparator output. Hint - whenever the voltage is higher at the positive input
the output value is positive. The output is zero volts under all other conditions.
the outputs of both comparators are never positive voltages at the same time.
however, there are times that they are simultaneously at zero volts.
The non-inverting input of comparator A is connected to external
pin 6 which is called the threshold terminal. The inverting input of
comparator B is connected to external pin 2 called the trigger terminal.
The voltages at the comparator outputs provide logic levels that control the flip-flop.
RS flip-flop: the output of comparator A is connected to the R
input of the flip-flop and the output of comparator B is connected to the S input
of the flip-flop. The Q output of the flip-flop is connected to the output
buffer in the base of the NPN transistor. Whenever the output of comparator A is a
positive voltage, the Q output becomes a positive voltage which is called a logic
high. whenever the output of comparator B is a positive voltage, the Q output
becomes zero volts called a logic low. whenever logic lows are at both R and S
inputs the output of the rs flip-flop remains unchanged. logic high is that the
RS inputs are never present to simultaneously. NPN transistor: the NPN
transistor acts like a switch, it turns on and passes current between the
emitter and the collector when a positive voltage is present at the base.
it turns off and blocks current between the emitter and the collector when zero
volts is present at the base. the voltage applied to the base is from
a Q output of the RS flip-flop. Output buffer: the output buffer produces a high
current voltage which provides sufficient output to power external
circuitry. The output buffer receives its input from the Q output of the RS
flip-flop. the buffers output voltage goes positive when the q terminal of the
rs flip-flop is the logic low. the output of the buffer approaches zero volts what
a logic high is produced at the Q output.
You have completed this activity internal elements of a 555 timer.