PSoC Based Photometer

Introduction

In this post we want to describe the realization of a simple “general purpose” photometer, based on a photodiode sensor and on a Cypress PSoC5LP microcontroller. The photometer is the instrument that measures the intensity of a light emission, its applications range in many different sectors, including chemical and biochemical analyzes and the study of light sources. To make the instrument “flexible” we have chosen to keep the sensor separate from the body of the instrument : in this way it is adaptable to different modes of operation.

Detector

The detector is made of a Hamamatsu S1337-66BR photodiode. This photodiode is characterized by a sensitive area of ​​5.8×5.8mm, it has a spectral range from 340nm to 1100nm with a sensitivity peak at 960nm, and is therefore suitable for use in the detection of light emissions of low intensity from UV to near IR.
The sensor sensitivity curve is shown in the image on the left.

The photodiode was mounted at the end of a plastic tube, painted in black, and connected with a shielded cable. This setup allows us to move and position the sensor easily. The image below shows the sensor.

Hardware

The hardware is based on the FreeSoC2 development board of the Cypress PSoC5LP microcontroller. The photodiode signal is amplified by a trans-impedance amplifier, the scheme is shown below. The operational is the fast OPA656, with double power supply + 5V, -5V. The gain is changed via software by selecting the feedback resistance. The amplifier signal is sent both to the ADC for digital acquisition and to the outside on a BNC connector for display with oscilloscope.

The PCBs have been placed inside a classic electronic box: the photo below shows the PCB with the operational (the green one) on the side there is the PCB with the IC for the creation of -5V starting from + 5V. Below is the FreeSoC2. On the front of the container there are the control buttons and an LCD display.

Software

The PSoC software provides the acquisition of the sensor signal (after appropriate amplification) with an 18-bit precision ADC. The signal range reaches 6V while the conversion rate is 1000 samples per second. The value of the sampled signal is sent via serial to a data logger (Raspberry PI with sw Python).

The numerical value shown on the display is instead the result of a temporal integration made on about 500 samples. This is done to reduce “noise” and statistical variations in the signal.

The instrument can therefore be used in three different ways :

• Fast Detector : in this modality the signal is taken directly from the amplifier output and sent to an oscilloscope or a fast ADC . In this mode the signal acquisition electronic part is not used.

• Real-Time Detector : this mode can be used to study phenomena that vary over time with a maximum frequency of 500Hz. The signal is acquired by the ADC and sent to the data logger (Raspberry PI with sw Python).

• Precision Measurements Detector : in this mode the signal to be acquired is considered constant over time, and the value read is integrated on a considerable number of samples in order to improve the signal to noise ratio. The value is shown on the display.

Measures

The image below shows an example of precision measurement : the gain is configured to the value 10 (corresponding to Rf = 10kΩ), the measurement is given on the display in millivolts. The measurement is activated and stopped with the start/stop button.

In the image below you can see the oscilloscope trace obtained by using the sensor in “fast” mode and acquiring the signal produced by exposing the sensor to a simple domestic lamp : note the ripple produced by the alternating current at the 50Hz network frequency.

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