The Pennsylvania State University
Memo
DATE: 2/11/2014
TO: 300W Memo Writers
FROM: Vladimir Bakirov
SUBJECT: Laboratory 2 status report
This memo is a status report about the progress of "Lucky 7" team in the EE 300W laboratory
last week. The objective was to build a Theremin musical instrument using the photodiodes,
myDAQ and TL074 Low-Noise JFET-Input Op-Amp. In the memo explained how design should
be implemented at a high level. In addition to, there is a paragraph about circuitry
implementation. In the attachments section provided schematic for the circuit.
Design implementation
The goal of the task is to control generated audio wave by adjusting pitch and loudness of the
sound wave according to the light intensity in the photodiodes. The photodiodes generate
leakage current proportional to the applied light intensity. This implies that the photodiode can
act as a current source. The myDAQ generates the audio wave in the form of a sine wave and
regulates amplitude and frequency in agreement with data from the photodiode. Since current
is not best for data acquisition in myDAQ, it was suggested to convert current to a voltage using
transimpedance amplifier. The myDAQ is going to collect voltage measurements from the
output pin of the amplifier.
Schematic design
Our team decided to implement the schematic using design of transimpedance amplifier. The
main goal was to identify the resistor for our circuit. Consequently, first was determined to
identify voltage and current margins for the circuit. The amplifier model is TL074CN, which is
produced by Texas Instruments. The device consists of 4 amplifiers with a maximum supply
voltage of 18 V and maximum input voltage 15V. The myDAQ has maximum 10 V of the voltage
supply. Therefore, output voltage can’t exceed voltage margin of 10 V. The maximum leakage
current can be obtained by connecting the DMM to the photodiode. The current is 0.03uA for
the room light and 300uA for the flashlight from the camera. It was decided to proceed with
0.03uA as a maximum current, because the Theremin operates under general room light. Three
resistors with values of 1 kΩ, 1 MΩ and 100 MΩ were picked for the experiment.
Conclusion
According to table 1 in the attachments, the 1M ohm resistor gives the greatest voltage (3.36V)
compared to 1 kΩ and 1 MΩ ones. Consequently, myDAQ can have a wider range of data for
acquisition. The fact that the voltage margin (10V) is greater than the output voltage (3.36V)
implies that the device can operate with the brighter light intensity, then experimental one.
Attachments:
Table 1. Resistor identification experiment.
Resistor Values
1kohm
1Mohm
100Mohm
Current of the Photodiode
0.03uA
0.03uA
0.03uA
Figure 1. Circuit Diagram for our design.
VDD
4
5V
U1B
5
7
6
11
I1
0.033uA
TL074CN
VEE
-5V
R1
100MΩ
Remark: pin 7 goes to the myDAQ.
Output Voltages
33uV
0.034V
3.36V