A brief history of two electronic design projects

1st Project

Then I built this first version for preliminary tests:

This is the wiring side:

The module could be powered from an external +15v 0v -15v lab power supply, but to make it more portable and user-friendly I incorporated a miniature DC to DC converter that supplies the required dual polarity voltages from any laptop USB socket or 5v charger.

But I wasn't satisfied with this first design, which used an adjustable attenuator to reduce the open loop gain of the op amp, because at the higher attenuations the output from the attenuator is only a few 100 µV, too low to be measured accurately. So I revised the design and made a new one incorporating several improvements:

Here is the reverse side of the Mk. 2 model. The wires are colour coded for ease of circuit tracing:

In this new design I created an inverting amplifier with variable closed-loop gain by cascading a non-inverting amplifier with switchable feedback resistors with an inverting unity-gain buffer. The adjustable closed-loop gain of this amplifier was thus the open-loop gain of the amplifier under study. Other improvements included the use of a precision 10-turn helipot in combination with a number of selectable range resistors instead of a helitrim to allow fine adjustment of the test voltages.

I used Inkscape to draw the final circuit diagram. Inkscape is a fantastic open-source vector graphics editor with a great range of features:

This simple product illustrates a number of elementary criteria that are important for user-friendly design. There is a high degree of correlation between the spatial mapping of the controls and the layout of the schematic diagram. In both cases, the principal signal flow is from left to right and the helipot, the DIP switches, the test points and the op amps have similar physical relationships to each other.

An exception is the main power supply input terminals, which are placed at the top of the module so that the leads connected to them don't get in the way of the user. Colour as well as labelling is used to ensure that the correct supply polarity is respected. The USB-B plug solves this problem because it imposes a physical constraint - it can only be inserted one way round. But even that is a somewhat mediocre design, since the user has to ensure that the plug is oriented correctly. Only now are symmetrical reversible connector designs such as USB-C becoming commonplace at last.

The helipot allows the user to optimise the operations. In use, it has to be set near one end to obtain a given output voltage, and then turned near the other end to set the same voltage of the opposite polarity, and this operation has to be repeated many times for different settings of the DIP switches. The usual way to turn a knob is with the thumb and index finger, which is laborious for a precision 10-turn component. But this small knob can be swept through its full range very rapidly by using the outside edges of both hands on either side of it, something that is impossible with a screwdriver set helitrim.

Here is a graph comparing the experimental results (data points) with the theoretical line graphs. The maximum difference is less than ±1%:

2nd Project

For my Physics HL Internal Assessment project, I designed and built a test module to investigate how the emission wavelengths of infrared and visible LEDs are related to their V-I characteristics. (You can use this method to determine Planck's constant experimentally):

Here is the wiring of the module, for which I used 0.3mm diam tinned copper wire and PTFE sleeving:

Again I used Inkscape to draw the schematic diagram:

Here are the measured average V-I characteristics of 24 LEDs of six different types:

And here is a graph of the emission wavelength against the reciprocal activation voltage derived from the V-I characteristics: