|Need a few quick pre-gens for a game convention or organized play group at your local game store?
Your cousin from out of town wants to sit in and join this week's game?
The party insists on hiring that NPC to join them for the adventure?
That failed stealth check got half the party killed and now the fallen are playing back-up reinforcements?
You don't have time to min/max your way through a character build but want to try something new?
Fast Character gets you playing fast.
On the main Fast Character Dungeons & Dragons 5e "A Bit Of Everything" page, you'll find a pulldown menu to change the default technology level of a character:
These semi-official options to include weapons and armor from other technology levels are based on Dan Helmick's Unearthed Arcana: Modern Magic (classes and spells) and My New D20 Modern Campaign (armor) conversion notes on Wizards.com - expanding on the "Firearms" and "Alien Technology" rules already in the 5e Dungeon Master's Guide.
LOW: Primitive / Stone Age (no metal, monster hides). This tech level fits primitive societies such as those of Athas in the D&D Dark Sun setting and other "savage" worlds such as Barsoom (of Edgar Rice Burroughs' John Carter of Mars fame). Armors are replaced by descriptions of monster hides, and weapons are made from stone, bone and wood. DMs are encouraged to house rule a "fragile" property on such items so that critical fails with weapons and critical hits on armors pose a risk of damaging or ruining them. (See the "Objects" rules, DMG p. 246.)
MEDIUM: Medieval / Steel Age (default D&D). This tech level fits most D&D core products and campaign settings (Forgotten Realms, Greyhawk, Eberron, Exandria, Dragonlance), as well as many of the most popular fantasy novels and movies (Lord of the Rings, Game of Thrones, The Witcher, tales of Camelot). Knowledge of forging steel is not uncommon, and crafting experts know the secrets to working with mithril and adamantine.
HIGH: Renaissance / Gunpowder Age (muskets, early pistols). See "Firearms" rules, DMG p. 267; automatically adds "Renaissance weapons (pistol, musket)" as an extra proficiency. This tech level only adds a couple ranged weapon options; armor and other weapons are unchanged. Such tech fits the "Age of Exploration" era (pirates, traders, colonizers, conquerers) reflected in the old "Red Steel" and "Savage Coast" publications of the Mystara D&D campaign setting. (Think "Pirates of the Caribbean Fight Dracula.") For some DMs, this tech level might also fit in a "steampunk" fantasy world.
VERY HIGH: Modern / Digital Age (firearms, Kevlar armor). See "Firearms" rules, DMG p. 267; automatically adds "modern weapons (firearms)" as an extra proficiency. This tech level fits "monsters among us" fantasy settings such as the old d20 Modern base settings (Shadow Chasers, Agents of Psi, Urban Arcana, DarkMatter). DMs could use D&D5e to emulate Shadowrun or World of Darkness campaigns if they don't mind replacing the unique nuances of those game systems with 5e's "heroic action hero" focus.
In addition to gear, note the three "Modern Magic" (UA:MM) character classes to unleash spells on powered machines and computer networks: Cleric (Domain of the Modern City), Warlock (Ghost In The Machine), and Wizard (Technomancer). DMs also may want to tweak skill proficiencies to use "Nature" as "Natural Sciences" (biology, chemistry, physics, earth sciences, astronomy) and "Religion" as "Social Sciences" (culture, economics, psychology, politics, religious lore).
ULTRA HIGH: Futuristic / Starfaring Age (beam weapons, sci-fi suits). See "Firearms" and "Alien Technology" rules, DMG p. 267-268; automatically adds "futuristic weapons" as an extra proficiency. Warning: Laser pistols and rifles do triple base damage compared to bows and crossbows, meaning their impact on low-level games will be significant and unbalancing for most by-the-book encounters.
This tech level mixes space opera with high fantasy, something found in the original D&D Blackmoor campaign setting and a few one-off products (most famously, Expedition to the Barrier Peaks).
For DMs and groups looking for deeper 5e-based science fiction systems, check these out: