The operation of the unit revolves around three !Cs: a 4093 quad NAND Schmitt trigger, a 4066 quad analog switch, and a 7555 timer. When a high is fed to probe 1 in, it is inverted to IC1a and once again by IC1b, so that the input to IC2a is high. Thathigh causes the switch contacts in IC2a to close. With the contads closed, a high-level output is presented to the input of IC2b. The high output is fed to probe 2 in,
That signal is tben inverted by IC1d and routed to IC2d, causing its contacts to open, and tbe unit to output a logic-level high. The output of IC2d is tben fed to IC2c.