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Photocell Amplifier
This is a low frequency amplifier with an adjustable transimpedance that is
 intended to be used to take relative measurements of a wide range of photo currents.


Not having many parts, this amplifier can be put together in a short amount of time.

Find updates at www.projects.cappels.org

Introduction

I needed to make some relative optical measurements. I had some small but relatively large area photo diodes, that when operated as photovoltaic cells generated between 20 nanoamps and 200 microamps with the range of optical power available from a light source I needed to test.  That is why this amplifier came into being.

The output of the amplifier is to be measured on a digital voltmeter. Besides being able to accept 20 na to 200 micro amps and output voltages that range form 100 mv to 2 or 3 volts, the amplifier needed to have a low input resistance so the diode could be run at or near zero bias so that its output would be a linear function of light on the photocell. It also has to have plenty of connectors. Along the bottom of the board in the image above, from left to right, are a power input connector because it would use an isolated unregulated DC power supply that is sometimes used for other things, a voltage output connector so that its output could be plugged into a digital voltmeter, and an input connector so that it can accept current from a variety of photodiodes.

The objective of the design was an amplifier that could be built quickly and use "ordinary" parts, except for the op amp, which needed to have low input offset current and a decently low input offset voltage. I also did not want to worry about low noise design, hence the 22 uf capacitor on the input and the use of a digital voltmeter to read the output.

The Circuit

ss
Simply an amplifier and a +/- 4 volt power supply.


Starting with the power supply. An input voltage of 12 to 16 volts DC is regulated to + 8 volts. R1 assures that a minimum of 10 milliamps flows through the 1N5227, 3.6 volt Zener diode. All together, this makes approximately 8.6 volts. This would have been simpler with a 7808 regulator, and a low power version of the regulator would have been plenty sufficient, but one works with what one has on hand.

The 8 volts is split by R2 and R3 to produce a ground reference voltage at half the regulated supply voltage. This results in power to the opamp of +4 volts and -4 volts. Very little current flows between the power positive and negative rails, so U1A, which provides a low impedance ground reference, is not really needed, but the opamps came two to a package, and not having to pay much attention to the effects power supply current returning through the ground made life a little simpler, so I went ahead and used U1A in this role.

The photocell is connected to the amplifier through approximately two meters of Belden RG-174/U miniature coax cable. All I want to measure is the average current so the current from the photocell is filtered by C3 to reduce the amount of hum and other modulation of the current. The capacitor, along with R4 form a 20 Hz low single pole low pass filter which after examination of the output on a scope, is adequate for my needs.

The actual amplification is done by the marvelous TLC27l2 opamp. Feedback from the output to the inverting input through 3.3k R5 and the 500k pot allows transimpedances from very low 3.3 k ohms to 500 k ohms. R4 is to limit noise gain while R5 limits the closed loop gain, thereby assuring that some loop gain remains and that the amplifier remains stable.


My experience with the amplifier

I have used this amplifier along with the photodiode it was intended to be used with a few times now for comparative measurements (measure a reference, then measure the device under test) and am very happy with its performance.  I did not expect it to be as good as it is, and I give all of the credit to the TLC27L2 opamp. Short term drift was not apparent, and the offset with no current input only varies about 1.5 millivolts as the transconductance is varied from 3.3k to 500k.


If you see this posted on a web site other than www.cappels.org, please let me know at the email address below.

A few keywords to help people find this via search engines:
Photocell, solar cell, DIY light meter,  DIY LED measurement tool.


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Contents ©2010 Richard Cappels All Rights Reserved. Find updates at www.projects.cappels.org

First posted in October, 2010

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