Heartbeat Sensor

The Heartbeat Sensor is an attachment featured in Call of Duty: Modern Warfare 2 and Call of Duty: Modern Warfare 3. The attachment allows the user to track multiple targets, somewhat like the UAV killstreak. It displays the positions of people who are fairly close to and in front of the player. It also will produce a beep whenever it picks up a hostile target, which will increase in pitch the closer the target is to the user.

Campaign
In the campaign, the display screen can be folded out from the side of the weapon and folded back in again. The first time it is used is in the level "Cliffhanger", where Roach finds himself in near white-out conditions. It is used also in the levels "Contingency", mounted on two M240s, and in "Just Like Old Times" where an ACR with Red Dot Sight, Silencer and Heartbeat Sensor can be found. The sensor likely uses the electromagnetic field created by the heart to discern personnel locations. It most likely uses an IFF system to tell the difference between friendlies and enemies. All friendlies are blue dots and enemies are white. In multiplayer games, friendlies appear as green dots instead of blue and enemies appear as red dots instead of white.

Multiplayer
The tier two perk Cold-Blooded cannot bypass the sensor, only the tier three perk Ninja can. The EMP killstreak will render it inoperative for the duration of the killstreak so that only static can be seen on the screen. This attachment is compatible with Assault Rifles, Light Machine Guns, and Sniper Rifles. It is also seen on the AA-12 in a few Special Ops missions such as Acceptable Losses. To unlock the Heartbeat Sensor the player must get 15 suppressed kills with the weapon.

Overall, when analyzing its advantages compared to disadvantages, it's best to use the Heartbeat Sensor as a reference and to continue checking the minimap and the player's perception at constant intervals.

Call of Duty: Modern Warfare 3
The Heartbeat Sensor returned in Call of Duty: Modern Warfare 3. It was redesigned from the original model from Call of Duty: Modern Warfare 2, being slightly slimmer. Another noticeable change is that the player can now see the Heartbeat Sensor while aiming down their sights of most guns. The Heartbeat Sensor is available at Weapon Level 13 for Sniper Rifles, Weapon Level 14 for Assault Rifles and Weapon Level 20 for most Light Machine Guns, with the PKP Pecheneg and the M60E4 being the only LMGs unable to accept the Heartbeat Sensor legitimately.

The refresh rate is slower than in Call of Duty: Modern Warfare 2, and because of this, the Heartbeat Sensor is more suited for a scouting tool. It's best used for flanking enemy locations when UAVs are not available to locate enemy players not using Assassin used to scanning your immediate area before advancing.

Trivia

 * The Heartbeat Sensor is a strong reference to motion sensors mounted on weapons in the movie franchise Alien.
 * While reloading an M240 with a Heartbeat Sensor, the player's closes the sensor before the reload animation begins, and re-opens it when the reload is done, lengthening the whole reload.
 * The Heartbeat Sensor has a smaller range of detection in multiplayer, compared to the campaign.
 * The Heartbeat Sensor is very hard to use in split-screen mode because the screen is smaller. Only a few weapons will show the whole screen, such as the Barrett .50cal or the M4A1.
 * When viewed in third person with a sniper scope, one can clearly see up and down arrow buttons, as well as "Menu" and "Power" buttons. There are also small speakers and writing that says "Motion Tracker MT-33".
 * When on a player's back and viewed from third-person, the Sensor will appear completely white.
 * The reload and pickup animations of many weapons, like the F2000, SCAR-H or SCAR-L are slightly different when using a Heartbeat Sensor.
 * The Heartbeat Sensor on the M14 EBR is not camouflaged with the rest of the gun.
 * The Heartbeat Sensor cannot detect players with the Assassin perk.
 * In early screenshots and in the trailers of Modern Warfare 3, the Heartbeat Sensor had a black finish. Ingame, it has a tan finish. It is unknown why this was changed.