There are several different ways a remotely (10s of meters or 100s of meters) located object can be detected, or the object’s presence or absence can be ensured:
Sending ultrasonic sound and receiving the reflected sound from the object. Principle used: Time of Flight measurement or TOF
Sending laser or LED light and detecting the reflected light from the object. Principle used: Time of Flight measurement or TOF.
Using a CCD camera, comparing successive images. If there is any discrepancy between two images, there could be something wrong with the object.
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 4 th category. The basic block diagram of the project is shown in Fig.1.
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 to some extent. 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.
Here's a video of the project in action:
Fig.1. System block diagram
Oscillator and LD Driver:
The oscillator used here is based on simple 555 timer IC in astable operation. The circuit diagram is shown in Fig.2. Using a 50 K potentiometer (R8), frequency is set at ~ 2.1 kHz. The duty cycle is ~ 50%. The output of the oscillator is fed to a MOSFET via a buffer (OPA2171), which is driving the LD. The LD used here, is a cheap laser pointer diode with the existing lens of the pointer. While using an LD in pulsed operation, we have to consider its protection as well. Resistor R2 (87 ohms) is used to limit the current within the safe level of ~ 40 mA.
Fig.2. Oscillator, LD driver and delay circuit