Photodiode Laser

Why does the output voltage from a photodiode with a load resistance of 100K ohms not fit the pattern?

A photodiode was placed the correct way round, with a 1.5V battery and a load resistor. A laser was incident on the photodiode, and we measured the output voltage across the load resistor. As the load resistor increased by factor x, the output voltage increased by factor x. However, when a resistance of 100K ohms was used, it did not fit the pattern. a 10K resistor gave about 1.6V output and a 100K gave 1.7 ('fitting the pattern' would predict the 100K to give about 16V surely). We used resistors from 1K to 100K, and all apart from the 100K fit the pattern. Can anyone explain any sort of 'phenomena' that might give rise to the voltage 'limitting' when high load resistances are used? (for your info: the laser was rated as 1.5mW, 635nm, but we calculated the incident power to be 0.4mW for each load resistor (apart from the 100K obviously!) thanks so much!

It sounds as though your circuit consists of a photodiode connected in series with a resistor and a 1.5v cell. I would expect that the photodiode is reverse-biassed so that the current that passes through it is the photon-induced leakage current; this current will be proportional to the light intensity, and will be largely independent of the voltage across the diode so long as the diode-voltage is always a volt or so.

It should be obvious that in this circuit the voltage across the resistor can never exceed 1.5v, since that is the battery voltage. Perhaps you have misunderstood the way that the photodiode works. In this form of connection the diode does not behave as a photovoltaic voltage source. The voltage in the circuit comes from the battery. This drives a leakage current through the diode, and the resulting current, flowing in the resistor, generates a voltage. The result is that the voltage across the resistor can never be larger than the battery voltage.

A reverse-biassed photodiode would act like a constant-current source, provided the reverse bias across it were adequate (a volt should be enough). But it would depart from this characteristic if the reverse-bias becomes too small. So if you want to extend the range of output voltage you should use a battery with a higher voltage.

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How Do Barcode Scanners Work?

So you want to know how barcode scanners work. Well, we first want to narrow down which type of barcode scanner we are talking about. There are many types of barcode scanners from laser scanners, CCD barcode scanners, imager type scanners, and more. Our first focus will be on how a CCD barcode scanner works.

The CCD barcode scanner is a scanner that has no moving parts. CCD stands for Charged-Coupled Devise Scanner. The scanner has a light source that when pointed to an object or barcode, it illuminates that image. The image is usually a barcode. Once the barcode is illuminated, a reflection is created and the barcode scanner reads that image.

How does the barcode scanner read the image? Well, there is a linear photodiode within the scanner head. This photodiode can read the reflected light off the lines on the barcode. This reflection is a digital image that is then scanned electronically within the devise. When the image is scanned electronically, each bar on the barcode is converted to the corresponding number or letter.

The barcode scanner is connected to a PC or Mac and the CCD scanner then sends the sequence of numbers and/or letters to the PC or Mac to populate the field of entry. This connection can be made in a number of ways. On way is with a keyboard wedge. This is a Y connection where one end of the Y connects to the keyboard and the other end of the Y connects to the scanner with the bottom of the Y plugging into the PC where the keyboard would normally connect. This method is used many times when the PC does not have enough interfaces. Serial is another connection method and works with just a straight serial cable from the barcode scanner directly to the PC serial connection. USB is now about the most popular method, because most PC's and Mac's today have many USB ports. Just plug and play!

Now we can look at how a laser barcode scanner works. The laser scanner works by sending a low energy light beam or laser beam to read the spacing between a pattern on the image one space at a time. The beam is moving back and forth by using a mobile mirror which causes a blinking effect. You can usually see the read line moving over the barcode. The reflection comes back and is then read by the fixed mirror in the scanner. The scanner then generates analog and digital signals that match the pattern. A barcode reader decoder then processes the information and sends it through the data communications interface.

Wala! Knowing how a barcode scanner works answers a lot of questions about where you can use a barcode scanner. They are becoming so popular now, you can find them everywhere in retail stores scanning items.

About the Author

John Barth founded Adazon Labels and Barcode Equipment www.adazonusa.com in 2003 and has a wealth of information in the barcode arena from over 20 years of experience in distribution. John's experience allows companies to cut costs on barcode scanners, custom labels and total barcode solutions. Call 847-235-2700 for more info.