Performance-Based Assessments for Digital Circuit Competencies

Something omitted from too many basic digital electronics texts is a thorough discussion on interfacing IC logic gates with high-power devices, usually using relays. This is a very important subject, however, because many devices we wish to control with digital logic circuits are too power-hungry to directly drive with the logic gate outputs! Here, students get the opportunity to experiment with how to make a logic gate (CMOS, preferably) drive an electric motor.

One component value you may wish to have your students size themselves is resistor R₃, being the base current limiting resistor for transistor Q₁. It must be sized such that the transistor is saturated with the gate output in the HIGH state, yet not allowing so much base current that the transistor becomes damaged. Figuring out an appropriate size for this resistor is a very practical exercise, forcing students to review transistor theory (calculations with β) as well as consider characteristics of the load.

It may be advisable (especially if the logic gate is TTL and requires a precise 5.0 volt power supply) to have a separate source of power for the electric motor.

It should be noted that the input states in this circuit are defined by the voltage levels, not by the contact status. In other words, a closed contact equals a “low” (0) logic state.

I strongly recommend having students build their logic circuits with CMOS chips rather than TTL, because of the less stringent power supply requirements of CMOS. I also recommend drawing a combinational circuit using four gates, because this is the common number of two-input gates found on 14-pin DIP logic chips.

I strongly recommend having students build their logic circuits with CMOS chips rather than TTL, because of the less stringent power supply requirements of CMOS. I also recommend drawing a combinational circuit using four gates, because this is the common number of two-input gates found on 14-pin DIP logic chips.