Another probe schematic error on page 38

[Mullihaka]

For those who are building the device using strip board.

Pin 1 of LM311 is connected to the ground on the PCB, but is not shown on the schematic.

[Qiaozhi]

For those who are building the device using strip board.

Pin 1 of LM311 is connected to the ground on the PCB, but is not shown on the schematic.

The output of the LM311 comparator can drive loads referred to ground, the positive supply or the negative supply. In the case of the Probe design, the comparator is being operated in the single-supply mode, and (Mullihaka is correct) pin 1 (which is effectively the reference) needs to be connected to ground. The PCB has the correct connections already, but if you plan on duplicating the design using stripboard, you need to be aware that this connection has been omitted from the schematic.