In Vietnam, US forces employed several types of unattended ground sensors the overall programme was called Igloo White. The idea was to:

• Build a sensing device that would detect movement, sound, or even infra-red emissions (heat from vehicle engines);
• Camouflage that device so it looked like a normal piece of vegetation;
• Deposit the device into an area of known or suspected enemy activity;
  monitor the device for activation;
• Make some sort of action when the sensor was activated.
  
  Types of Sensors
  There were several types of unattended ground sensors deployed in Vietnam. Sensors could be divided into categories depending on sensing method and construction.

Sensing methods

Seismic: These sensors contained seismic detectors. That is, they detected movement in the earth -- similar to earthquake detectors -- such as vehicle(s) moving past or footsteps. Seismic devices required a small spike to be driven into the ground; this spike contained the sensing element. If the ground moved, the sensing element moved and activated a radio transmitter that transmitted a beep, alerting the monitor to the fact that something was moving near the sensor location.
Acoustic: Acoustic sensors are similar to sonobuoys -- they contain one or more sensitive microphones attached to a transmitter and they transmit whatever sounds they pick up.
Infra-red. These sensors are sensitive to changes in ambient temperature. If a warm body -- human, truck engine, water buffalo, tiger -- came close to the sensor, it sensed the rise in air temperature and caused an internal radio transmitter to transmit a beep.
Construction

Sensors came in various sizes and shapes.

Sensor pods: These devices were fairly large. As I recall, they were 6 to 8 inches in diameter, 3 to 4 feet long, and contained one or more microphones or other sensing device, a radio transmitter, a battery, and an antenna.
The device was in a metal canister, painted with camouflage paint. The antenna was built to look like a small sapling or jungle plant.
Pods contained acoustic sensors (microphones) and seismic sensors. Useable life was battery life.
The canister had a pointed end, was dropped from an airplane, and would (hopefully) bury itself in the ground, leaving the microphone exposed and burying the seismic detector. The transmitter would transmit any sounds or ground vibrations that it picked up to aircraft or other monitoring station.
The idea was to drop several of these along a road or trail -- hoping that the dropping aircraft could get them delivered -- then monitor their output. When sounds such as truck engines were heard or when heavy seismic activity was detected, then artillery or air strikes could be called in on the area.

Scatterable sensors:
These were small devices, mostly seismic.
They consisted of a seismic detector, a transmitter, and internal antenna, and a battery in a fiberglass or plastic case made to look like a broken branch, a big leaf, or other piece of forest litter. One type of sensor was designed to look like -- I am not kidding -- animal droppings. (Most of these things were Seismic Intrusion Detectors -- SIDs. These latter types were immediately named "TURDSID" by the guys using them.)
They were usually delivered by air, just dump a batch of them out along a trail, road, or in a suspected base camp area.
Because they were small, their batteries were small and transmitters were small. Battery life was not long -- a few days.

Manned sensors:
As an artillery forward observer, I operated with infantry units. One unit had some test sensors that were used in perimeter security. The sensor consisted of a small box containing a battery, transmitter, and antenna. Attached to it were several (four, I believe) long wires, each with a seismic stake on the end. The stake was about the size of a tent peg. There was also a receiver, tuned to the frequency of the transmitter. The idea was that the sensor box would be put, say, along a trail leading to an ambush site and the spikes pushed into the ground. Each spike transmitted a different beep. When something came down the trail, it would activate the seismic sensors, a distinctive beep would be transmitted to the receiver, and people in the ambush would know where the movement was.

SOG teams would be used to insert sensors in "over the fence" missions. The item pictured is dated 1972 and is a REMBASS unit so I think its post war, but may have been used earlier. If anyone has any dated sensor units or more information please contact me. paul@howsplendid.com

More information can be found here;
http://home.att.net/~c.jeppeson/igloo_white.html