The Little Detecto



It's a wonderful device - a detector that detects the things you need! There is a detector for everything - Applied Phlebotinum, Green Rocks, guns, bombs, spaceships, ghosts, mice, cheese, socks... If you live in a high-tech setting and need to find something, there is a type of The Little Detecto that will detect it!

The physics behind The Little Detecto is never explained. It just detects the thing that is the local Serious Business.

The trope namer is a gadget from one Tom and Jerry cartoon that was used to detect mice.

Compare Thing O Meter, a comedy trope. Contrast with Everything Sensor. The Little Detecto is different from Everything Sensor because it's not an EVERYTHING sensor. It detects only the one thing that is Serious Business here.

Anime and Manga
"The whole thing is fine at first. We're presented with a mystery, and for a little while there the story makes some very logical progressions...but then everything just goes all to hell when Junsa (and, later, the Rocket-Dan) pulls out a freakin' sleep wave detector. Because, y'know, that's what police officers just happen to carry around with them at all times. I can totally imagine Junsa's orientation day at the police station. "OK...here's your badge, your gun, and your standard issue sleep wave detector. Good luck.""
 * The dragon radar in Dragonball, made by Bulma to find the Dragon Balls. Handwaved by saying that the Dragonballs emit a special kind of radiation in the first chapter of the series.
 * On Pokémon, Officer Jenny (Junsa in the original) just happened to have a sleep wave detector in her pocket when the situation called for it. Dogasu (he compares the dub to the original) says here:

"Hiei: I only came to find the source of the vile noise﻿... and kill it."
 * Read or Die had this.
 * In Love Hina, Kaolla Su produces a device that detects turtles... and some kind of pot, the two words being homonyms in Japanese.
 * She later upgrades it to detect Keitarou and Naru.
 * About the only good thing about the Brand of Sacrifice from Berserk is that it lets Guts know when the demons are coming for him by bleeding.
 * In Digimon Tamers, the D-Arc is able to detect where are the Digital Fields, which contain Digimon that crossed to the real world.
 * In Yu Yu Hakusho, when Yusuke and his teammates need to locate Hiei, Botan mentions the Spirit Detective Tools. The Demon Compass finds demons by measuring demon energy. The Mystic Whistle is a whistle for demons that functions like a dog whistle; it causes Hiei to appear.


 * In addition to being the source of a Magical Girl's power and, Soul Gems in Puella Magi Madoka Magica are also handy Witch detectors as shown in the second episode, glowing brightly when they're close to a Witch's presence.

Comics

 * The HEYDAR and the BTRS Meter on The Middleman.

Film
"Ripley: What's it key off?
 * Of course, the PKE (psycho-kinetic energy) Meters, Ecto-Goggles, Giga-Meter, and the ghost-sniffer, all from Ghostbusters.
 * The motion detectors in Aliens.
 * Kind of explained, if I remember right. The motion detector uses changes in air pressure to detect movement.
 * From the first movie:

Ash: Micro changes in air density."


 * Very small changes in air density are what we call sound. So, it's a microphone?

Literature

 * "Numbers" Riktor, from the Discworld books, designed numerous Little Detectos for different things. The only one that became relevant to the books was the resograph, which alerted the wizards to a serious incursion in reality.
 * Probably named after the Richter Scale.
 * Frodo's sword Sting from Lord of the Rings glows bright blue when the orcs draw near.
 * As do Glamdring and Orcrist, the swords Gandalf and Thorin acquired at the same time Bilbo picked up Sting in the Hobbit.
 * And the swords of the Nac Mac Feegle of the Discworld, when they're near lawyers.

Live Action Television

 * Molly Walker from Heroes can find people.
 * Bunsen Honeydew once invented a gorilla detector. It completely failed to detect the gorilla until after it had already attacked his computers, and him. Bunsen originally attributed the failure to the fact that the attacker was not actually a gorilla.
 * Doctor Who memorably featured the timey-wimey detector. It goes ding when there's stuff. The sonic screwdriver is used this way at times, too.
 * The Cloister Bell is capable of detecting when the Doctor is going to regenerate (except when it doesn't), when there's going to be a hole in space-time the size of Belgium, or when the Doctor is driving erratically.
 * In the original series (during the late Tom Baker era) and the TV-Movie, the Cloister Bell is less of a detecto and more of a major league Red Alert klaxon. It signals when the TARDIS itself is in danger of destruction.
 * The classic serial "The Android Invasion," in which androids are passing as humans, has a robot detector. It's a small box with a single red indicator light on it.
 * Stargate Atlantis features the Life Signs Detector, about the size of a PDA and highly useful in many situations.

Tabletop Games

 * In Dungeons and Dragons, many spells work like The Little Detecto. They have titles beginning with "Detect", as simple as that. Detect Magic, Detect Evil, Detect Undead, Detect Traps...There's also "Locate City", but that's mostly known for something else.
 * Though Locate City is most infamous for a trick called the Locate City Nuke. It involves placing 4 different metamagics on the otherwise harmless spell so that it becomes a viable target for putting a metamagic spell that throws people outside the range of the spell. Since Locate City's range is measured in miles and being thrown out of the range does 1d6 per ten feet, it kills pretty much everything in range, including the caster.
 * Lesser Globe Of Invulnerability, a normally useless spell, comes into play here as it can stop you being thrown the 400ish miles for over 200,000d6 damage.
 * The Detect advantage in GURPS, of course. In the Magic book nearly all the spell relating to this stem from "Detect Foes".
 * The most basic building block of Enhanced Senses powers in the Hero System. For example, radar is basically "Detect objects that reflect radar signals" with some modifiers tacked on to make it work like proper radar in terms of game mechanics.

Video Games

 * Cole from In Famous has the inherent ability to detect nearby Blast Shards and sources of electricity.
 * In Metal Gear Solid 2, a bomb disposal expert creates one which specifically searches for the aftershave his Apprentice turned Mad Bomber applies to all his bombs. It turns out he no longer applies it to all his bombs.
 * In the Pokémon games, there's also the Item Finder, which shows you when you're close to items... which can be everything from medicine, to Poke Balls, to glass flutes, to chunks of gold. How it picks these up and ignores everything else is never explained other than "It's just a game, relax."
 * In Diamond and Pearl, in the Sinnoh Underground, you also have a "radar" which lets you find things buried in the ground and walls. These can be both traps and "orbs," which you use to buy things down there.
 * Pokemon Colosseum has a character that can detect an "aura" around Shadow Pokemon. XD replaces her with a device that looks like a DBZ Scouter.
 * Beyond Good and Evil has both the Pearl Detector and the Animal Detector, which help you find Plot Coupons and animals for your First Person Snapshooter sidequest.
 * The Elder Scrolls: Morrowind had three detection spells, Detect Magic, Detect Creatures, and Detect Key. While the last was very handy, one wonders what aspect of keys makes them detectable vice any other particular object. (Although it does serve a kind of Mundane Utility versimilitude. One can only imagine a wizard inventing it after turning his laboratory inside-out trying to figure out where he left his keys last night)
 * In the Warhammer 40k game Dawn of War-Retribution, during the Ork Campaign when pursing teleporting Eldars, trusty second in command, Mister Nail-Brainz, set his git-finda to "Pansy" mode.
 * Sam Fisher's goggles in Splinter Cell: Chaos Theory gain the ability to view electromagnetic fields, which essentially shows the player electric/electronic devices they can interact with.

Web Original

 * In WALLE Forum Roleplay, Leonard Ghertivel has a device that detects all the specifics of various robot hardware/upgrades upon scanning of the robot in question.
 * The Return, pattern detectors. Little palmtop things that tell you if the creature currently ripping your heart out through your ribcage is a demon or not.
 * We have Bakura's Gaydar from Yu-Gi-Oh the Abridged Series

Western Animation

 * Bender once claimed to have built-in Gaydar.
 * Spoofed on The Ren and Stimpy Show episode "Marooned". Cadet Stimpy pointed one such device at various things on a strange planet, but all that he found out was that "this thing makes the coolest noises."
 * In The Fairly Odd Parents, Mr. Crocker has a "fairy detector" that he uses to hunt down Timmy's fairies.
 * In the Phineas and Ferb episode "Vanessessary Roughness", Dr. Doofenshmirtz "just happens" to have a pizzazium detector in his top pocket after he and his daughter stumble upon a sample at the mall. It comes in handy when Baljeet snags the pizzazium first.
 * In "The Chronicles of Meap", Phineas converts a GPS into a "cute tracker" to find an adorable alien the boys call "Meap". Isabella, who harbors a crush on Phineas, is annoyed that the device apparently ignores her, until Phineas reveals that he deliberately had it ignore Isabella's cuteness. When he adjusts the device to stop ignoring Isabella, it self-destructs.
 * At the end of Xiaolin Showdown's first season, Jack Spicer builds a Shen Gong Wu detecto-bot.

Real Life

 * Truth in Television: metal detectors, bomb detectors, gun detectors.
 * Geiger counters detect nuclear decay through beta particles or gamma rays.

Fanfiction

 * In the Harry Potter fic "Mr. Black and the Demoness", a mad scientist friend of Harry's made a "Dork Lord" detector out of an old alarm clock.