Performance-Based Assessments for Digital Circuit Competencies

Here, I let students choose appropriate values for R_pullup and R_limit, rather than specify them as given conditions.

Here, I let students choose appropriate values for R_pullup and R_limit, rather than specify them as given conditions.

Pushbutton switches are recommended over toggle switches for demonstrating a latch circuit. When the contacts are momentary, the latching characteristics of the circuit becomes more evident to see.

I have intentionally requested students try different resistance values for R_limit so they may see the effects of gate output loading, and the importance of proper logic level voltages. Students should try undersized resistors (10 Ω, perhaps) on both LEDs to generate this problem, and then use oversized resistors (1000 Ω, perhaps) to make the problem go away. Large-valued limiting resistors will cause the LEDs to be dim, but will also restore workable voltage levels so that “high” output states are actually interpreted as “high” when fed back to the gate inputs.