Electronic – PCB pancake slip ring brushes

A guard ring is traditionally used to protect high impedance nodes in a circuit from surface leakage currents. The guard ring is a ring of copper driven by a low-impedance source to the same voltage as the high impedance node. This would typically be the input pin of an op-amp.

Here's an example of a classic guard ring layout for a metal can op-amp from National Semi's AN-241:

And here's an example of how it would be connected, from Analog's Analog Dialogue magazine:

The key feature is that the guard ring is connected to a node that will be driven to the same voltage as the high impedance node being protected, but with a much lower source impedance.

Note that not all vendor websites are created equal. Microchip's AN1258 recommends using the high-impedance net to create a guard ring around the low impedance nets --- don't do this.

Now to your specific case. While the undriven side of your capacitor is not strictly a low-impedance node, since the ammeter itself should provide a fairly low impedance path to ground when you're measuring, it's still going to cause measurement errors if any current should try to reach ground through that node instead of by another path. It wouldn't hurt to add a ring around the node like this:

Unlike in another answer, I wouldn't include the driven side of the capacitor within the ring, since that's a low impedance node, being driven to a fairly high voltage. However, you've indicated the net in question isn't even physically located on the PCB, so this advice is largely moot. Being as the high impedance net is basically floating in air, it should be well-protected from leakages in any case.

The Chip on Blob board, From left to right, on the top:

1. Ground, unconnected
2. V+
3. Green Led (Off when Logic High) (1kΩ resistor to PNP Base)
4. Yellow Led (Off when Logic High)(1kΩ resistor to PNP Base)
5. Red Led (Off when Logic High) (1kΩ resistor to PNP Base)
6. Regulated Ground
7. Unknown, Connected to Pin 8 with a resistor, probably a mode select or delay based on the resistance.
8. See 7.

The capacitors are bypass 0.1uf capacitors, simply to filter out some noise on the power supply, standard value on nearly everything.

The Transistors are PNP transistors. Collector to the LED/Resistor/Ground, Emitter to V+. They are on when the base is logic low (ground), and off when the base is logic high (v+).

To use it with an arduino, you would simply remove the CoB board, connect a digital/pwm pin to 3, 4 and 5, as well as connecting the ground of the board and the arduino together. Since you would have it in output only, there isn't any issue with the V+ voltage level.