Page History: Backpack Tracker curve tracer
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Page Revision: 2020/07/23 20:14
This article describes the design of a electronics debugging tool for electronic circuits.
It's main purpose is to give a graphical representation of the impedance at a node of the circuit, similar to a commercial product, the Huntron Tracker.
The tool allows for very fast fault finding in electronic circuits, especially when a known good circuit is available, so graphs can be compared.
If there is no known good circuit is available, still comparison can be made to similar nodes in the same circuit, and differences point to the culprit.
In example microprocessor pins are all similar, if one pin shows a different impedance, there is reason to doubt the microprocessor as being defect.
Creating an impedance graph is based on a very simple principle: apply a voltage to that pin, and measure the current that flows in the pin.
By applying different voltage, the impedance at each voltage can be shown in a graph.
A good escription of the circuit can be found here:
OctopusTypical test voltages are +/- 15V at currents of a few mA, to avoid damage to the circuit under test.
As explained in the article, for resistors, Ohms law U=IxR will show a linear relation between voltage and current. A straight line.
For a diode, at positive voltages the impedance will be low, and negative voltage will be high.
This kind of characteristics are visible on the screen of the Backpack Tracker.
The name "Backpack" is from the core building block of this tool: the Micromite Backpack.
The Micromite Backpack is a design of Geoff Graham, and is based around a PIC32 processor that can be programmed in Basic. Information can be found here:
BackpackThe Micromite Backpack is a 2 circuit board sandwich existing of a PIC32 board, and a commercial ILI9341 LCD touch color display 320x240 pixels.
In this design the LCD will display the impedance curves, and will act as a very simple control panel with only 2 (touch) buttons.
The PIC32 microcontroller will generate the voltage applied to the circuit under test, measure the current (*) and convert the measurements to an impedance graph.
The software is written in MMBasic, the native language of the Micromite Backpack.
Since the PIC32 processor can only output voltages in the range of 0V..3.3V, an additional amplifier is needed to achive the +/- 15V.
This amplifier, and it's power supply, are located on a third printed circuit board.