EE 2115-01 - Lec 10 - 2026_09_16
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Overview
Hiren Trada explains how CD4066 voltage-controlled switches can implement logic gates, then introduces voltage comparators and the role of reference points in defining positive and negative voltage. The lecture connects truth tables to circuit behavior, including switch resistance and noise margin, and shows how a comparator can control a thermostat by comparing a measured signal with a set point.
Key takeaways
- A CD4066 switch is not an ideal short when on: its on-state resistance creates a voltage drop, so a 10 kΩ pull-up with up to about 500 Ω switch resistance can leave a low output near 0.5 V.
- A pull-up with series switches creates NAND behavior because every switch must close to establish a path to ground; parallel switches with the same pull-up create NOR behavior because any closed switch can pull the output low.
- Changing from a pull-up to a pull-down changes the default output from high to low, enabling AND with series switches and OR with parallel switches.
- A comparator is a threshold decision circuit: its output is high when V+ > V− and low when V− > V+, making it useful for thermostat control around a temperature set point.
- Positive and negative voltage values depend on the selected reference node; using the midpoint of two series 1.5 V batteries yields rails of +1.5 V and −1.5 V.
Chapters
- The CD4066 contains four separate switches, each with two signal terminals and a control terminal, such as pins 1 and 2 controlled by pin 13.
- Pins 7 and 14 provide power; Trada notes the supply can include 5 V and 10 V and may extend to about 18 V, subject to checking the datasheet.
- Unlike a mechanical switch, a semiconductor switch has on-state resistance that creates a voltage drop when it is closed.
- The inverter uses a pull-up resistor and a voltage-controlled switch to ground, with input A controlling the switch.
- When A is low, the switch is off and the pull-up holds the output high; when A is high, the switch turns on and pulls the output low.
- An inverter is the only basic logic gate with one input; AND and OR gates accept at least two inputs.
- With a 10 V supply, a 10 kΩ pull-up, and a switch resistance of up to about 500 Ω, the closed-switch output can remain slightly above 0 V.
- The voltage divider produces a low-level output of roughly 0.5 V or less, rather than an ideal 0 V.
- Digital systems use noise margins, voltage ranges interpreted as logic low or high, to tolerate non-ideal output voltages.
- Trada assigns a practice calculation for the same inverter circuit powered at 5 V.
- A two-input truth table lists the four possible input pairs: 00, 01, 10, and 11.
- An AND gate outputs 1 only for input pair 11; its Boolean relationship is analogous to multiplication.
- A NAND gate inverts the AND result, so its output is 0 only when both inputs are 1.
- Logic 1 corresponds to the system's positive level, such as 5 V or 10 V in the examples.
- Two CD4066-style switches in series connect the output to ground only when both input-controlled switches are on.
- A pull-up resistor holds the output high for all other input combinations, matching the NAND truth table.
- The two switches' on-state resistances add, increasing the low-level voltage drop compared with a single switch.
- A three-input NAND can be made by adding a third series switch; every input must be high to pull the output low.
- With a pull-up resistor and parallel switches to ground, either switch can pull the output low.
- The output stays high only for inputs 00 and goes low for 01, 10, or 11, which is the NOR truth table.
- The circuit illustrates how the switch arrangement, series versus parallel, changes which input combinations create a path to ground.
- A pull-down resistor holds the output at logic 0 by default, unlike the pull-up circuits that default high.
- With series switches connecting the output to the positive rail, both inputs must be high to produce an AND output of 1.
- With parallel switches and a pull-down resistor, either high input can connect the output to the positive rail, implementing OR.
- The closed-switch voltage may fall short of the 10 V rail; Trada estimates an output near 9.1 V in the example.
- A comparator symbol is typically a triangle with positive and negative signal inputs, power rails, an output, and sometimes a step marking.
- An operational amplifier can be used as a comparator, but comparators are not necessarily operational amplifiers.
- An op amp is a broader building block used for operations such as summation, difference, integration, and filtering.
- When V+ exceeds V−, the comparator output goes high; when V− exceeds V+, the output goes low.
- The high output is limited by the circuit's positive supply, such as +5 V or +10 V, while the low output is near the negative rail.
- A thermostat can compare a temperature signal with a set point, such as 70°, and switch a cooling compressor on when the measured temperature exceeds it.
- Voltage is measured relative to a chosen reference point; changing that reference changes the reported positive and negative values without changing the differences between points.
- In the one-dimensional example, choosing A as 0 V makes E equal −1 V and F equal −3 V; choosing E as 0 V shifts A to +1 V.
- Two 1.5 V batteries in series provide 3 V across the outer terminals, or +1.5 V and −1.5 V when their midpoint is used as the reference.
- Trada reminds students that the quiz is at the beginning of Friday's class and is posted on Canvas.
Transcript excerpt
0:01 All right, good afternoon. Getting better. Still about a minute delay. Uh so today we'll continue with lecture 9, wrap it up. Uh the what we are leading into is u applications using logic gates and comparator circuits. Uh comparator circuits is the next uh next lecture which is uh in the slides it's lecture 10. Okay. Uh let's pick up where we left off. Okay, so we were talking about this the CD 4066 and there is one in your lab kit. So we'll also perform experiments based on this microchip. So just as a brief recap,
0:56 this chip has four four separate voltage control switches. Each has three terminals. Two of the terminals are the actual poles, meaning they are the end points of the switch. For example, one and two. And the controlling pin is a third pin. In this example, it's pin number 13. So depending on the signal that arrives at pin number 13, either the switch between one and two is off or it's on. Okay. Same thing for the other three switches also. And then pin number seven and 14 are for the power supply. And that power supply uh if you look up the spec sheet can be 5 volts, 10 volts, and I believe
1:44 it goes up to 18 volts also. I'll have to recheck the data sheet, but it can go up to 18 volts. Now, when the switch turns on, if it's a mechanical switch like the one we have in most homes or most rooms here, it will have some contact resistance, but the resistance is as low as possible. Okay, it is kept as low as possible. These voltage control switches, however, using semiconductor devices. So there is a tiny amount of resistance built into these switches and that is specified
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Related notes
Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Hiren Trada.