Electronics

Vocabulary flashcards covering the architecture, internal components, pins, logic rules, and RC timing principles of the 555 timer integrated circuit.

555 Timer

A widely used integrated circuit (IC) device that relies on resistor-capacitor ($RC$) timing to operate timing circuits such as monostable and astable multivibrators.

Monostable Multivibrator

A 555 timer circuit mode that generates a single output pulse.

Astable Multivibrator

A 555 timer circuit configuration that functions as a continuous square wave oscillator.

Comparator

A device whose output is HIGH when the voltage on its $+\text{ input}$ is greater than the voltage on its $-\text{ input}$, and LOW when the $-\text{ input}$ voltage is greater than the $+\text{ input}$ voltage.

Inversion Bubble

A circular symbol connected to the input or output of a digital device that inverts a logic signal ($0 \rightarrow 1$ and $1 \rightarrow 0$).

Buffer

A circuit element used on the output of electronic devices that prevents anything connected to the output from interfering with preceding stages.

S-R Latch

A digital circuit whose output is normally LOW, is set to HIGH by a HIGH signal on $S$, and is reset to LOW by a HIGH signal on $R$.

555 Internal Voltage Divider

A series network of three 5\text{ k}\Ω resistors connected between $V_{cc}$ and GND that creates reference voltage levels of $\frac{1}{3}V_{cc}$ and $\frac{2}{3}V_{cc}$.

CONTROL Voltage Pin (Pin 5)

The pin connected to the internal voltage divider that can be used to externally adjust the trigger and threshold reference levels.

TRIGGER Pin (Pin 2)

The 555 timer pin that causes the output of Comparator B to go HIGH when its input falls below $\frac{1}{3}V_{cc}$, setting the S-R latch, driving the OUTPUT pin HIGH, and turning off the discharge transistor.

THRESHOLD Pin (Pin 6)

The 555 timer pin that causes the output of Comparator A to switch to HIGH when its voltage rises above $\frac{2}{3}V_{cc}$, resetting the S-R latch, driving the OUTPUT pin LOW, and turning on the discharge transistor.

DISCHARGE Pin (Pin 7)

The 555 timer pin tied to the discharge transistor that turns ON to drain charge from an external timing capacitor when the output switches to LOW.

RESET Pin (Pin 4 / R1)

An external reset pin featuring an inverting bubble where applying a LOW signal resets the device and forces the output LOW.

Time Constant ($\tau$)

The product of resistance and capacitance ($\tau = R \times C$), defining the time required for a capacitor voltage to change by $63\%$ from where it is to where it wants to go.

$$5RC$$

The total duration of time taken for a capacitor in an $RC$ circuit to go from its starting voltage to being considered fully charged to its target voltage.

$$1.1RC$$

The length of time required for a capacitor to charge $\frac{2}{3}$ ($66.6\%$) of the distance from its initial state to its target voltage.

$$0.693RC$$

The length of time required for an initially discharged capacitor to reach $\frac{1}{2}$ ($50\%$) of the supply voltage, or to charge between $\frac{1}{3}V_{cc}$ ($3.33\text{ V}$) and $\frac{2}{3}V_{cc}$ ($6.67\text{ V}$).