# Tag Info

## Hot answers tagged photodiode

28

It's all down to speed. What your circuit doesn't show is the self-capacitance of the photodiode: - Given that the signal produced by the photodiode is current (Iph shown above), if this is rapidly changing like in an optical data receiver, the junction capacitance will have a significant effect on rise and fall times. However, with a transimpedance ...

23

My advice is to not bother doing what you're doing, and spend the \$3.50US to get the part designed for the task, like https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/SFH-7060/475-3174-2-ND/6137022

22

According to this, the photodiode does indeed produce a current even when there is zero volts across it; it's the short circuit current. Note that the reference direction of $I_S$ in the question's diagram is opposite that of the $I_{SC}$ of the diode so the output voltage is: $V_{OUT} = - I_S \cdot R_F = I_{SC} \cdot R_F$ I found the above here. ...

20

Photodiodes are easy. You connect them reversed to the +5V (cathode!) and the anode to a resistor to ground. If light falls on the diode it will cause a current through the resistor, which will cause a voltage across it. So you can choose the sensitivity by choosing a value for the resistor. You'll have to make sure that there remains enough voltage drop ...

19

Different semiconductor junctions have different forward voltages (and reverse leakage currents, and reverse breakdown voltages, etc.) The forward drop of a typical small-signal silicon diode is around 0.7 volts. Same thing only germanium, around 0.3V. The forward drop of a PIN (p-type, intrinsic, n-type) power diode like a 1N4004 is more like a volt or ...

18

Photoresistors were the predecessors of photodiodes. Instead of acting as current source they were light dependent resistors (LDRs). Their main disadvantage is that they react very slowly to light changes.

15

Not really used in a similar way for stopping machines like in your question, but before there were phototransistors/diodes we had photomultiplier tubes. Under influence of a high voltage a single photon that collides with the light sensitive cathode will release several electrons. Then these electrons are attracted to the anode, on their way there collide ...

14

Fundamentals All materials in the chemical table and molecules of different combinations have unique electrical properties. But there are only 3 basic electrical categories; conductor, insulator( = dielectric) and semiconductor. The orbital radius of an electron is a measure of its energy, but each of many electron orbits formed in bands can be: spread ...

13

A photomultiplier tube is one such device. They are still used in some applications and as per a photodiode comparison with photomultipliers looking at the some disadvantages of a photodiode some areas where a photomultiplier has advantages: Larger detection area Has internal gain Much higher sensitivity Photon counting possible without special cooling and ...

13

Semiconductor laser effect is described by two coupled partial differential equations of carrier density and photon density, the rate equations. The solution of these equations result in a non linear current-intensity relationship causing relaxation oscillation when the diode is turned on. See here or following image: (image source: p. 45 of this document)...

12

Simpler than the photomultiplier tube is the basic vacuum tube photodiode: The curved plate is the photocathode, and the wire post in the center is the anode. Photons knock electrons free from the surfaces of both elements, but since the area of the cathode is so much larger than the anode, there's a net flow of them from the cathode to the anode — ...

12

You are confusing 'inverting' with 'negative feedback'. Open loop simulate this circuit – Schematic created using CircuitLab Figure 1: op-amp with open-loop inverting mode. In Figure 1 the op-amp will amplify the difference between its inputs by the open loop gain. Let's say the open loop gain is 1,000,000 and we apply +1 mV at the '-' input. Since ...

11

Several things: A long cable carrying the most sensitive node in the whole system is a bad idea. Carefully shield and then tightly couple the amplifier to the pickup. Then you can send the higher level lower impedance signal over the long cable. A 741 is a joke in this application. Look for a low noise opamp. There are amplifiers specifically for audio ...

11

It could be motion. Probably at the laser source, but it could also be at the photodiode end, even a fan somewhere can cause sensitivity. However, if you have no aperture, or the wrong aperture, it is also possible to get stray laser paths to the sensor as the laser crosses the metal shroud of the sensor or reflects around inside the arrangement. If you ...

11

It's a particular classification of nonlinear behavior that has important applications. In the same way as you can consider, say, a BJT as linear over a limited range, you can consider something like a diode as square law over a limited range. That simplification allows you to analyze functions such as RF detectors analytically. See, for example, this ...

10

If you look at the data sheet for the LED it says that at 20 mA the forward voltage may be between 2.8 V and 3.6 V. Importantly it states that this is at an ambient temperature of 25 degC. So, how well regulated is the ambient temperature in your experiment? I ask this because you seem to be relying on generating a constant current by using a resistor and a ...

10

The HW5P-1 is in fact a phototransistor. This is the diagram of the example circuit from the datasheet: The circuit symbol is clearly a phototransistor, regardless of what the poorly translated text says. A phototransistor and a photoresistor (like a cadmium sulfide cell) both react to light, but they are fundamentally different in how they operate. A ...

8

The simplest way to connect a photodiode (even an LED can be used in this way as a photosensor) is as below: simulate this circuit – Schematic created using CircuitLab Note that the photodiode is reverse biased through the 1 Meg resistor R1. The photocurrent generated by the diode opposes the flow of leakage current through this reverse biased diode, ...

7

The signal is picked up with a high-speed photodiode array of several sensors to allow the tracking circuitry and focusing to work. Using closed-loop control allows the mechanical parts to have relatively loose tolerances compared to even an LP record turntable. As one Japanese Engineer said you could make the player chassis out of chopsticks and it would ...

7

You could modulate your LED by driving it at a particular frequency (20kHz for example). Then instead of just looking for a simple on/off signal from your photodiode, you look for that particular frequency instead.

7

The easiest way to get a photodiode, and to control it, is to use one of the built-in optocouplers. The base/collector junction of a phototransistor is a photodiode (just ignore the emitter): Or use the 4N2x.sub/CNY17.sub as a model to build your own subcircuit.

7

You can connect the photodiode in either direction (anode to GND or cathode to GND). Both variants work; the difference between both variants is just the polarity of the signal. (You need to look up in the datasheet of whatever diode type you are actually using to find out which of the leads is the anode/cathode; or just measure direction of conductivity of ...

7

Depending on the speed you need, you may well be out of luck. You state that your current diodes "with a picosecond level rise and fall time", while you link to a PD with 12 nsec rise and fall times. Furthermore, you say that your PDs are "expensive ", but the linked units are only about 6 bucks in onesies, and if you think 6 bucks is expensive for a ...

7

There's what's called "analog switches and multiplexers", which exactly fulfill the role of your pushbuttons here. Basically, all large silicon chip manufacturers have them (for example, Texas Instruments). However, the currents from photodiodes are really small and sensitive. You don't want to switch these, if it can be avoided. These switches can easily "...

6

The first electrical light sensors were selenium cells. Selenium was used for resistances at the receiving station on the transatlantic telegraph cable in the 1860s, and it was noticed that it gave erratic results in daylight. Selenium can generate a small photovoltaic current so it was used in pre-war lightmeters and (I think) "Magic Eye" demonstrations at ...

6

Taking a look at a datasheet for a popular IR LED, the Lumex OED-EL-1L2, we can see the IR light output vs. current curve: So if you only give it 1mA, it gives you 0.2mW/sr radiant intensity. At 20mA, you get 10mW/sr radiant intensity. So, to maximize battery life, you should use the minimum current that will produce a light intensity that your circuit ...

6

Your source, of course, is http://www.electrooptical.net/www/frontends/frontends.pdf. A direct adaptation is fairly simple. Rbias should be connected to -15, as shown, but the 20K resistor must be tied to a separate bias voltage, Vb. For a given diode bias voltage Vd, Vb = 1.67 Vd. In your case, this would call for a Vb of about -100 volts. All of this ...

6

Photodiodes need to be treated in a special way ! 1) use the diode in zero-bias or reverse mode (not in forward mode) 2) when dark, no current flows through the photodiode 3) a circuit is needed to "catch" the photocurrent and amplify it. Here's an example of such a circuit: This circuit is called a transimpedance amplifier. Through feedback the opamp ...

6

Why not replace the 10 MΩ resistor with 1 MΩ and get rid of the output divider? It's all about the thermal noise created by the 10 Mohm resistor. A 10 Mohm resistor at 20 degC will produce a noise voltage of 90 uV RMS across a 50 kHz bandwidth. Compare this with a 1 Mohm resistor - it produces a noise of 28 uV across the same bandwidth at the same ...

6

They are both semiconductor PN junctions and they both exhibit the photovoltaic effect so, principally they are the same at a fundamental level. See also this article that gives more detail. Another article (here) contains this information: -

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