 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 5 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. Int. 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 highs at the RS inputs are never present 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 the 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 buffer's output voltage goes positive when the Q terminal of the RS flip-flop is a logic low. The output of the buffer approaches zero volts when a logic high is produced at the Q output. You have completed this activity internal elements of a 555 timer.