This CMOS circuit was found in a Popular Electronics from the 80's. The circuit consists of a counter that has its output activated when the selected key combination is pressed.
September 2017 - Littelfuse's Polyswitch SMD Resettable PTCs can now be purchased from Mouser...
The purpose of this circuit is to prevent TTL circuits from receiving a voltage greater than 5 V...
This type of amplifier can be found in many other points in this section. This is from a 70s...