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Unknowns came up while I was testing the EC circuit of the Ladybug Shield Alpha 1:

• The characteristics of the shrunken AC Waveform.  My calculations have been assuming Vin doesn’t change.  Does it?  If it does, the variability will have a significant impact on the EC measurement.
• Rectification.  I’ve observed a drift in results.  Does the peak detection part of the circuit need to be reset?  I use the circuit discussed in this blog post for rectification.
• Rectification.  The Vpp I observed on the scope was around 80mV.  Is this the best I can get?  If it is, the digital values will be noisy.  Certainly it is best to go into an ADC with the cleanest DC signal.

# The Goal

The Goal of this post is to investigate and understand the unknowns.  The outcome will evolve the design of the EC circuit.

# The Files

The Kicad files that I used to create the test prototype is located at this GitHub location.

# Thanks To Those That Went Before

A very grateful thank you to Chris Gammel.  He helped me with the direction to take with this prototype.  The new Contextual Electronics course is about to start.  I can’t wait to participate and continue to learn!

# What to Test

As I walked through what I was observing in the EC Circuit with Chris, He broke up the circuit in a very understandable way. He also pointed out a test circuit in which switches are used to decide which circuit to test:

The AC Waveform created by the Wien Bridge Oscillator is shrunk to ~ 200mV Vpp.  The shrunken signal is Vin to the Gain loop that has the EC probe as a variable resistor.  Vout goes into a rectifier whose output is fed into an ADC.

Chris had a great suggestion.  Why not test the shrunken signal through the rectifier and also test the affect the Gain loop has?  Brilliant.  The unknowns I noted at the beginning can be easily tested this way.  The way I will do this is to put a SPDT switch on either side of the gain loop.

# Schematics

As noted earlier, the Kicad files are located at this GitHub location.  I created a hierarchical schematic.  I’ve been thinking about how best to combine the process of drawing a block diagram (as shown in the image above) with the high level of the schematic.  Here’s what I came up with.

I liked Kicad’s ability to include an image onto a schematic.  I put the SPDT switches off to a breadboard.  This is represented by the blue drawing.  Blue text and drawing are annotations – not part of the board’s design.  It is nice to be able to do this for this prototype, since part of the functionality:

• SPDT switches

will be on a breadboard connected by wires to the prototype board.

# On To Metrix Create:Space

I am not thrilled with the layout.

There are too many vias.  I went through the process that goes into the making of the prototype boards in a previous post.  Unlike the PCBs done at OshPark, the vias are not plated.  This means I must solder a piece of wire in a via.  What I ended up doing is making the vias the size of the wire I use in my breadboard.  This way soldering is easy.

Hopefully, I’ll be able to test this weekend.