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  VBPW77 Phototransistor

I could not help it, I loved the VBPW77 phototransistor the first time I had the chance to use it. Housed in a high reliability hermetically sealed TO-18 metal can package with gold plated lead and a cute curvy glass lens protruding out of its cap, its appearance alone gives me a lasting impression of its superiority compared to its cheaper plastic cased counterpart.

Its glass lens limits its field of vision to about 10 degree range, a useful characteristic because in my application, I want the sensor to keep its focus on the job in front of it, and not be distracted by stray lights that may come in elsewhere. It also has respectable sensitivity, in fact, in most cases; I can use it without the aid of an amplifier. I know right that instant that this is the kind of phototransistor I want to use in my projects whenever and wherever it is needed.

Line Follower Sensor using VBPW77 phototransistor

I got immediate inquiries to people wanting to use this device. Apparently, everybody is building a line following mobot, and want me to give an application circuit using the BPW77 as line sensor.

To get the job done quickly, I assembled my first prototype on a prototyping PCB. I connected a 10K resistor in series with the collector terminal of the phototransistor, and used a 5mm LED with a 100 ohm series resistor as the illuminator, as shown in the figures below.



Last Update

Figure 1. Sensor Test Circuit. Circuit too simple? Donít worry, we will complicate the circuit soon enough.

Figure 2. The phototransistor sight is directed towards the light spot emitted by the LED.

 The VBPW77 transistor and the LED are physically arranged such that the phototransistor is directed towards the center of the light spot on the reflecting surface being emitted by the LED.


Figure 3. Sensor Test Setup-  The sensor is tested by swiping it over a white bond paper overlaid with black electrical tape strip of varying width. The light meter measures the light ambience.
Figure 4. Close up view of the sensor circuit. I borrowed a prototyping board being used in another (different) project; all other components not shown in this picture are not part of the sensors and should be ignored.

A white bond paper overlaid with a short strip of black electrical tape was used to test the line sensor.  Three tapes, each with widths measuring 18, 9, and 4.5mm, respectively, are used as test tracks. The output is monitored by Tektronix TDS744A oscilloscope. Tested at a distance of about 5 mm from the surface, and with a light ambience of 100 foot-candles, the sensor yanked out an output waveform as it is swiped from right to left as shown in the figure below:


Figure 5. Sensor response at 100 foot -candles ambience.

The first pulse is the sensor response for the 4.5mm width line, the second pulse for the 9mm line, and the third and widest peak for the 18mm line. I increased the light ambience to about 1000 foot candles and repeated the test, resulting in a waveform as shown below:


Figure 6. Sensor response at 1000 foot candles ambience.

The first pulse is generated by the passing of the sensor over the 9mm width line, the second pulse over the 18mm width line. The pulse for the 4.5mm line is altogether gone!

Recommended MOBOT Line Sensor Circuit

 A well-lighted office is about 25 foot-candles bright. Under a shade on a mid afternoon sun is about 1000 foot candles bright. The oscilloscope waveforms shown in figure 5 and 6 tells me that, under these two conditions, the sensor can be used to reliably detect line widths as small as 8-9mm. Hence with just the addition of a Schmitt trigger buffer as shown in figure 7, we can complete a working line sensor!


Figure 7. The recommended line sensor circuit. Not shown, for circuit clarity, are the IC power supply connections. Do not forget to connect them. It will also do much good if you connect a 0.1uF capacitor as close as possible to the Vcc-GND pin of your IC.


A 74HCT14 sensor is chosen because of its relatively low trigger point compared to the other Schmitt triggers in the high speed CMOS family. Note that you cannot use a TTL logic here! Also note that the sensor output will be on low state when it is over the black line.

Due to component value tolerance (or some other reasons), it may sometimes be necessary to tweak the value of R1 to obtain better results. Experiment with the value of R1. Higher resistance results in greater sensitivity but makes it more affected by stray unwanted light. Lower the resistance value if your sensor is to work in very brightly lighted area.

A camera flash may momentarily trigger the sensor. In our experiments, we measured a camera flash triggered output of as long as 10mS wide. Hence, your mobot software must take this into account Ė it should ignore pulse detection of less than 10mS. Otherwise, the flash may dazzle your mobot and send it off course.

The line sensor will not work under direct sunlight! (July 29,2003)

I got news that IC stores ran out of 74HCT14 stock. You can use CD4093 or MC14903 schmitt trigger NAND gate as a substitute, noting the consequence that the circuit may not work well under very bright ambience. You can experiment to improve the circuit by using a super bright LED for D1 and lowering the value of R1 to, say, 2k2 ohms. Select the final value of R1 so that the VBPW77 fully saturates when the sensor points to the non-black portion of the track. (Aug. 5,2003)


Written By: Henry Chua

e-Gizmo Mechatronix Central

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