Luxmeter-Experiment-Seite auf Deutsch
Luxmeter-Experiment-Seite auf Deutsch

Luxmeter Experiment

I granted myself an experimental project working with photo diodes. I must emphasize that I never had to work with photo diodes (but I've seen one, though). A simple PCB has been developed to test two versions:

Both versions can be build on the same PCB. The circuit is supplied by 5 V DC and generates the -5 V for the symmetric supply of the OpAmp internally. It has a low-gain-output capable to reflect fast variations of intensive light pulses and a high-gain-Output that reflects low light situations amplified by a factor of 100 with acceptable speed response. Although I have some experience in developing analog circuits I must again state that I never have been in contact with photo diodes and my circuit may not be optimal. Nevertheless, some of the principles are general and may help you to build a better circuit.

Have a look at the circuit and the Target layout file.

General Considerations

Photo­diodes can be operated in reverse direction or in photovoltaic mode. In reverse direction they have a leakage current proportional to lighting over several decades. In photovoltaic mode they generate a photo current which is also proportional to the lighting. In each mode, we are talking about a few nA per Lux. Current in the range of nA is pretty low and is subject to get lost somewhere!

The measurement will primarly target to eliminate the leakage or photo current and thus keep a constant voltage over the diode. So you can avoid to charge the juction capacitance of the diode which can be relatively high. The BPW21 for example has a junction capacitance of more than half of a nanofarad. Charging it with a few nA would seriously degrade the bandwith of the circuit!

If we do not want to limit the circuit to high intensities, the opamp amplifying the signal must meet some requirements:

The layout is also a challanging task. 100 MΩ of insulation resistance at 5 V cause a leakage current of 50 nA! Guard rings are needed to avoid high differential voltages with respect to the sensitive signals. The manufacturing has to be clean since dirt of all kinds (especially skin sweat, remains of the liver sausage from the morning break etc.) may have serious impact on the insulation resistance of the board! Of course you can wash the boards afterwards but it is better not to contaminate them at all.

The circuit requires a symmetrical supply. From the +5 V supply voltage, -5 V are generated on board and are low noise filtered so a single +5 V supply is sufficient. You must not connect a negative supply at the -5 V pin! On this pin, the internally generated -5 V are available for measuring purposes or to supply a few mA to the following circuit!

Option 1 uses a BPW21 which is optimized for visible light (in the datasheet the sensivity is specified in nA/Lux, accordingly) and a relatively slow and cheap opamp type LTC2051). It has a very low offset value of 3 µV as well as a low bias current of 50 pA (at room temperature). The costs are at 4€ what is, well, affordable.

The diode operates in photovoltaic mode here and the opamp compensates the photo current keeping the voltage over the diode at 0 V.

Option 2 uses a BPW24 which is significantly faster. Its sensivity is mostly in near infrared and is therefore specified in A/W (photo current per radiant power). It is biased with -5 V to lower the junction capacitance and thus rise its speed. A higher voltage would make it even faster but the following circuit would have to match this (and you have to generate the voltage somehow). Fitting this demands, a much faster opamp type LT1469 was selected. It will raise the costs of the cirquit significantly! Until now, I could not verify how fast the circuit really is since I do not have a light source with the required short (and defined) rise time.
A circuit like this would probably be used for fast data transmission.

A simple trim potentiometer is used to calibrate the sensivity. If you happen not to have a reference light source, just measure the value of the pot (without supply, of course) and set it to 1 kΩ. For option 1, the low gain output has about 10 µV per lux and the high gain output about 1 mV per lux. 1 mV may seem very low but illuminated with one lux you could hardly see anything! In your office there is a minimum illumination of 500 Lux and then we have already 0.5 V. When the sun shines it will be some more and if you turn the diode towards a bright surface or even towards the sun, the high gain output will soon meet its limit!

The low gain output is made to show short, bright flashes like those used in photography.

Experiment: measuring a Photo Flash

Have a look at some oscillograph curves of a Canon 420EX flashlight, recorded with the low speed option (with the BPW21).

I find it fascinating how deep you can zoom into the oscillogram on modern oscilloscopes. In fact, the pictures of the measurement and main flash are just magnifications of the picture time lapse which has been recorded at 25 megasamples per second.

It is also clear that the measurement flash must not be used to expose the picture as it would look like a double exposure. Exposure begins just with the main flash. The energy of the measure flash will be lost for the exposure. This is mandatory as even the measure flash might be too bright for a good lighting. The 75 ms also include the time necessary to completely open the shutter of the camera.