Simple light sensor adapter for oscilloscope

Summary of circuit features

• Brief description of operation: Detects light level, light pulses or modulated IR
• Circuit protection: No special protection needed
• Circuit complexity: Very simple and easy to build, no need for circuit board
• Circuit performance: General purpose test instrument which is not very accurate but useful in many quick tests
• Availability of components: Should basily available (if you are active electronics builder you propably have them)
• Design testing: Built quickly from the parts found on my lab, the first prototype was usable, some more version built on other places as needed
• Applications: IR remote controller testing, stroboscope testing, light level measurements
• Power supply voltage: No power supply needed (passive operation)
• Component cost: Few dollars

Circuit description

This circuit is a simple visible light or IR detector for testing any equipment which outputs light. The circuit can be also used for measuring ambient light levels.

This circuit is based on one photodiode which is used in photocell mode so that the photodiode generates a voltage and current signal when it get light.

Circuit diagram

       +--------------o Center pin of BNC connector
       |
     __|__
 -->  / \
 --> /___\
       |
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       +--------------o Shield of BNC connector

Components

I built my circuit from components found in junk box (I don't have it's type number). The values are not critical and you can use almost any photodiode you can find (the performance might vary on type you are using). If you don't find any photodiode, you can try a LED instead (it works also, bit not as well). I have built the circuit using different photodiodes and LEDs and it has always worked at least somehow.

Component list

D1    Almost any photodiode or LED
CON   BNC connector

Using the circuit

The circuit is best used when you connect it directly to the input of your oscilloscope. Then set the input to the DC inout setting. When you have set the triggering right, you will get to see a signal DC level which somewhat resembles the ambient light level and also the other light signals you are testing, for example the modulated IR from you IR remote controller. The circuit can be very easily used for looking at the signal IR remote controllers send when you use in in somewhat darkened room. Just place the IR remote controller to the sensor and make it point to the sensor, pus one button and look at the oscilloscope screen.

Other possible application are looking at the light flickering generated by different lamps. When you move the sensor near the lamp, you will see the changes in the light level on oscilloscope screen.

I have also used the circuit to measure the flas length of different stroboscopes. When the sensor is placed near the stroboscope, you will see a very easily noticable pulse on oscilloscope screen every time the stroboscope flashes. From the pulse length you can make some assumptations of the flash duration of the stroboscope (the pulse lasts at least same time as the stroboscope flash, propably some time longer because of the capacitances inside the sensor).

Using the circuit to measure light leves

For measuring light levels, I recomment you to use a multimeter instead of oscilloscope for measuring. A photodiode works as wuite linear current generator when it operates in photocell operation mode. This means that the output current if a photodiode quite accurately resembles the light coming into the photodiode. If you connect the photodiode to the multimeter current input and measure the current (using micropampere range), you will get a numeric value of the light level. This numeric value can be used to judge agains how muvh the light level has changed (for absolute ratings you need to calibrate the system with a known light source). The circuit below shows a wiring how to wire the photodiode to the multimeter:

       +--------------o Multimeter current input +
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     __|__
 -->  / \
 --> /___\
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       +--------------o Multimeter current input -