# Direct Current

## Overview​

Direct current (DC) specifies a unidirectional flow of electrons; that is they flow in one direction only.

The simple nature of direct current lends itself well to creating digital logic circuits, because it can represent binary 1 when ON (within a given voltage range), and binary 0 when OFF (at ground, or 0V). It's also the type of current that batteries generate.

Because DC is used as the primary type of current in digital logic circuits, most of this tutorial will focus on DC circuits.

Direct curent is also used in long distance, high power, electrical transmission lines (usually >250kV) because of lower electrical losses.

Meadow provides two power rails that supply 5V and 3.3V direct current. The 3.3V rail can be found on the left header, and the 5V rail can be found on the right header:

### DC Power Symbols​

A general DC power source symbol looks like the following:

But many specific power sources have their own symbols, such as batteries, as we'll see later.

### Digital Logic Levels​

Due to the nature of electronic circuits, digital signals are rarely exactly 0V or 3.3V (or whatever HIGH value is defined for a particular circuit). For this reason, HIGH and LOW signals just need to be close to the defined values for a digital circuit to function correctly.

The allowable range of values for digital 0 and 1 in modern circuits are based on what kind of logic level the circuit uses.

The logic level is based on what HIGH means in a circuit, and falls into two categories, CMOS and TTL.

#### Complementary Metal-Oxide-Semiconductor (CMOS)​

CMOS is the technology that nearly all modern integrated circuits use. CMOS HIGH signals can vary but they're defined as the VDD level, which is usually 3.3V in most common circuits. In fact, for nearly all circuits we'll create, we'll use the CMOS standard with VDD = 3.3V as the upper end of the voltage supply source.

Internally, most microcontrollers, CPUs, and other complex chips use a lower VDD, often 1.8V or less, which allows them to operate at faster speeds. However, most microcontrollers use 3.3V on their IO pins.

#### Transistor-Transistor-Logic (TTL)​

TTL is based on older transistor technologies in which the HIGH signal is defined as VCC, which is usually 5V.

#### VCC/VDD​

While CMOS and TTL logic define differing terms for upper end of their supply voltage (VDD and VCC, respectively), they both refer to the same thing.

#### Acceptable Logic Levels​

The following levels are acceptable for CMOS and TTL circuits:

TechnologyAcceptable LOW VoltageAcceptable HIGH Voltage
CMOS0V to 1/3VDD2/3VDD to VDD
TTL0V to 0.8V2V to VCC

Therefore, for a 3.3V CMOS circuit, the acceptable levels are between 0V - 1.1V for LOW, and 2.2V - 3.3V for HIGH:

#### Logic Level Shifters​

When connecting two circuits together that have differing logic levels, logic level shifters (also known as logic level converters) are typically employed. These are small, integrated circuit chips that convert one or both ways between varying logic voltages.