Look Over Yonder with a Distant Object Detector: Page 2 of 4

Building a security system? Robot? Or maybe you're just curious what's out there. Gadget Freak Sajjad Haidar shows you how to build your own distance object detection system.

ferrite bead (FB) in series and a ceramic capacitor (0.1uF) in parallel to the LD is used to achieve slow rise and fall time of the current through the LD, some voltage spikes are also removed. As mentioned earlier, oscillator output will also be used as a reference input for the lock-in-amplification. The buffered oscillator output cannot be effectively used directly for this purpose. Because of the LD protection circuit there will be a little delay in turning ON and OFF the LD; also there will be delay in sending the pulses and receiving the signal also known as time of flight (TOF).

Fig.3. PCB implementation of the oscillator, LD driver and delay circuit

At the receiving end, the phototransistor detector output is amplified and filtered, these circuits also introduced some delay. To match with the delayed pulses the reference signal also needs to be delayed. The pulse delay circuit is implemented by an IC, LTC6994. The amount of delay can be adjusted by a 150K pot (R9). Before being fed to the reference input, the DC component of the pulses is removed by a capacitor C3. The implemented PCB is also shown in Fig.3. The oscillator output or the LD driver input and the delayed output waveforms are shown in Fig.4 and Fig.5, respectively.

Fig.4. Oscillator and the delayed reference output waveforms


Fig.5. Voltage across the LD

Phototransistor Amplifier and Filter:

The pulsed laser light is incident on the object to be detected and only a tiny fraction of the scattered light reaches back to the detection system. This scattered light is detected by a phototransistor. The phototransistor is fitted with a metal pipe of 5 mm diameter and 15 cm long to make directional sensing possible. The phototransistor current is amplified by an op amp (OPA2111). Along with the pulsed scattered light from the object, ambient light or background light also generates photo-current. 60 Hz line hum is also present at the amplifier output to a little extent. To remove those unwanted signal and background noise a simple notch filter is used. The passive components are chosen to pass only the pulsed light signal at 2.1 kHz. Other signals and noises are heavily attenuated. The phototransistor and the amplifier-filter is implemented in the same PCB, and put in a small enclosure. The cable from this enclosure carries photo-signal to the lock-in amplifier and power also comes to it from the common power supply. The schematic and the implemented PCB is shown in Fig. 6 and Fig.7, respectively.

Fig.6. Phototransistor amplifier and band pass filter.


Fig.7. Phototransistor amplifier PCB, and the enclosure.

Lock-in Detection Amplifier:

The heart of the detecting system is a balanced modulator, implemented by IC-AD630 and is shown in Fig.8. AD630 has two inputs, we can name one a signal input and other a reference input. The signal obtained from the photo-detector is amplified by IC1A (OPA2171A) and applied to input RINA. The delayed reference signal is applied to the other input SEL. We can either apply to SEL_A or to SEL_B, depending on the polarity

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