Re: The purpose of R1
What is the purpose of R1?
4000 series logic incorporates two mos transistors (one p-channel and one n-channel) connected in complementary fashion to function as an inverter. Because of the symmetry of these two transistors, appropriate negative feedback around the complementary pair will cause the pair to self bias itself to approximately 1/2 of the supply voltage. When the probe input is open, feedback resistor R1 causes N1's output to go to vcc/2, which will turn off both leds (or at least dim them to the point where they appear to be off).
Why is the value 2.2 M chosen here?
If R1 is too high, N1 will not have enough negative feedback to function as a linear amplifier and it will oscillate when the probe is open. When the probe is connected, however, R1 is the primary source of probe input impedance seen by the probed circuit; if R1 is too low, probe measurement accuracy will be degraded due to external circuit loading. It is likely that 2.2MΩ was selected because this is the highest resistance that will reliably enable N1 to operate in its linear region.