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UT71B Interface
The digital multimeter series UT71 from Uni-Trend
provides a cheap measurement solution with reasonable accuracy. The set includes an USB Interface Cable and a Windows based software. Unfortunately the USB Interface doesn't work with Mac OS X. But the optical interface can be easily soldered for less than 5.00 €.
| Qty | Part | Cost |
|---|---|---|
| 1 | PL2303HX or similar  | 3.00 € |
| 1 | IR-Fototransistor  | 0.10 € |
| 1 | IR-LED  | 0.10 € |
| 1 | 4k7  | 0.10 € |
| 1 | 1K0 | 0.10 € |
| 1 | PCB (very small piece)  | 0.50 € |
| 1 | Housing (3D print or laser cut)  | 0.50 € |
Schematic
The USB to Serial converter PL2303HX provides 4 wires for the signals GND, +5V, RxD and TxD as shown in the schematic The TxD diode is optional and not supported by the software (yet).
Housing
I used a laser cut housing based on 2mm white acryl. The design was prepared with Inkscape.
Depending on the thickness of your wiring you may not need all layers. In my example I only used 4 pieces.
SVG file
Assembly
Cut and engrave on your laser cutter of choice. Prepare the small piece of PCB (perfboard). Glue the IR transistor (dark blue)and the IR LED. The flat part should be on the left for each. Solder the parts and the SMD resistors as shown on the picture. Last but not least connect the four wire of the PL2303HX.
Software
Based on the protocol information I prepared a tiny Python program to receive the measured values, plot and store them as well as export in CSV format. This allows long-term measurements of any unit. The measurement values are stored in a SQLite database and can also be easily access with Open-Source tools like DB Browser for SQLite.
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