There are several different ways a remotely (10’s of meters or 100’s of meters) located object can be detected, or the object’s presence can be ensured. Some of these are namely:

1. Sending ultrasonic sound and receiving the reflected sound from the object. Principle used: Time of Flight measurement or TOF
2. Sending laser or LED light and detecting the reflected light from the object. Principle used: Time of Flight measurement or TOF.
3. Using a CCD camera, comparing successive images. If there is any discrepancy between two images, there could be something wrong with the object.
4. Sending pulsed laser light (Visible or IR), and detecting the scattered light from the object by lock-in amplification.

There are other methods as well. Each of the above methods mentioned, have some advantages and disadvantages. Considering several factors I decided to work on a project based on the 4th category.

An oscillator (~2.1 kHz) drives a laser diode (LD) through a suitable driver and a LD protection circuit. The pulsed laser light is positioned to shine on a part of the remote object of interest. The faint scattered light is detected by a phototransistor, fitted with a 15 cm long metal pipe (dia. ~5mm). The purpose of this pipe attachment is to make the detection directional. The faint scattered light produces a faint signal at the photo transistor output, which is amplified and filtered (band-pass) to remove unwanted signal and noise. The signal is then applied to one input of a balanced modulator (AD630). The other input of the balanced modulator (also called the reference input) is fed with the delayed original oscillator signal, driving the LD. At the output of the balanced modulator, a DC level is obtained which is proportional to the signal detected at the photo transistor. Any unwanted signal or noise will be eliminated to a large extent by this balanced modulator. This technique of extracting signal also called lock-in-detection. The output is filtered and amplified further. At the final stage a comparator is used to trigger an alarm. When the object is moved or relocated, the signal goes below a certain threshold level and the alarm sounds to alert.

This design idea is a cheaper solution of detecting the presence or absence of a remote object. Similar kind of expensive commercial system available only for special use, for instance detecting the inception of a volcanic eruption from a distance. This cheap system can be commercially made at price less than $100, can become a household device for security. It can be used to detect the presence or absence of car or other objects ( several meters to 100’s of meters away), detection of movement of a remote object etc. This project uses widely available cheap electronic components. The project shown here is a workable prototype. To make it commercial and easily usable, some modification needs to be done. The directional receiving sensor tube can be replaced by easily manoeuvrable optical lens and sensor array.