Documentation
New Hardware
The main new piece of hardware we’re using is a custom designed image sensor (pictured below)
The sensor PCB contains a 30x40 array of 1200 phototransistors along with a 32:1 analog mux (used for selecting rows) and two mezzanine connectors (50 pins each) for communication. The sensor is connected to a breadboard with a 48:1 analog mux (used for selecting columns). The analog muxes (controlled by the MCU) are used to select a single phototransistor to read from by applying voltage to the corresponding column and reading the output voltage from the corresponding row. The phototransistor is setup in a common collector configuration with a XX k resistor between the emitter and ground.
Schematic Source: electronics-notes.com1
The phototransistors and muxes are all run on 5V. Note that the sensor PCB theoretically supports any 0805 phototransistor, but we are using the PT0805CS due to its low cost and availability. See schematics for the sensor PCB here.
The new MCU peripheral we are using is the ADC which reads analog signals (in this case from the phototransistors in the sensor) and converts them to a digital signal.
Schematics
Breadboard
Sensor PCB
Here is the schematic of the sensor PCB. It contains the array of 1200 phototransistors, 32:1 analog mux, and mezzanine connectors. See the bill of materials for more information on the parts used and links to the source files for the PCB.
Source Code Overview
The source code for the project is located in the Github repository here.
Bill of Materials
| Item | Part Number | Quantity | Unit Price | Link |
|---|---|---|---|---|
| Sensor PCB | Custom | 1 | $8.32 | source files* |
| Mezzanine Breakout PCB | Custom | 2 | $0.69 | source files* |
| SMD Phototransistors | PT0805CS | 1200 | $0.08 | link |
| 16:1 Analog Mux & Breakout | CD74HC4067 | 3 | $5.95 | link |
| Female Mezzanine Connector | 0513380574 | 2 | $2.30 | link |
| Male Mezzanine Connector | 5017450501 | 2 | $2.11 | link |
| 32:1 Analog Mux | ADG732BSUZ | 1 | $13.11 | link |
| 5k Resistor | N/A | 1 | N/A | stockroom |
Total cost: $145.48
*Note: The custom PCBs were manufactured by JLCPCB.
References
Phototransister Circuts Applications. https://www.electronics-notes.com/articles/electronic_components/transistor/phototransistor-circuits-applications.php Accessed 7 Dec. 2023.
Qoi Image Compression. https://qoiformat.org/ Accessed 7 Dec. 2023.
Technical Data Sheet 0805 SMD Phototransistor: PT0805CS. http://szsgir.com/data/PT0805CS.pdf Accessed 7 Dec. 2023.
Acknowledgements
Profs. Josh Brake and Tina Smilkstein for all of their help, support and teaching as we attempted this crazy project. Sean Hodgins for inspiring us to do this in the first place.
Inspiration
The inspiration for this project came from a simlar project found here.
We aim to expland on this project by making it higher resolution, adding a bayer filter for color imagery, and adding on-device image compression.