EE 2115-01 - Lec 26 - 2026_10_26

Overview

Hiren Trada explains how an RC circuit and diode create positive or negative clamper circuits, then applies RC transients and comparator thresholds to build a one-shot pulse generator. He verifies the behavior in SPICE and introduces the 555 timer’s astable, monostable, and bistable modes, focusing on negative-edge triggering in the monostable configuration.

Key takeaways

A diode clamper stores charge during one half-cycle and places the charged capacitor in series with the input, creating a DC offset whose direction depends on diode orientation.
In the illustrated clamper, the grounded-anode diode conducts when its cathode reaches roughly -0.7 V, so the capacitor charges to about the input peak minus the diode drop.
A comparator-based one-shot creates a standardized output pulse by comparing an exponentially decaying RC voltage with a fixed threshold; the trigger interval can vary while the pulse width is circuit-defined.
With a 5 V supply and equal divider resistors, the comparator reference is 2.5 V; the example RC components, 110 kΩ and 1.3 nF, have a time constant of about 143 microseconds.
The 555 timer supports three distinct modes: astable for continuous oscillation, monostable for one pulse per trigger, and bistable for switching between two stable states.

Chapters

0:00 Midterm Grade Updates and Canvas Access

Hiren Trada says midterm grading needs another 24 hours because of competing workload.
Students will be able to view and download their midterms and points from Canvas; paper copies will not be returned.

1:10 Adding a Capacitor to a Diode Circuit to Create a Clamper

The circuit builds on earlier RC filters and diode applications such as rectifiers, clippers, and peak detectors.
Unlike a peak detector that outputs across the capacitor, this clamper takes its output across the diode and uses the capacitor to store charge.
The circuit can also be viewed as two inputs, one applied to the diode and one to the capacitor, combining at the output.

4:00 How Diode Bias and Capacitor Charge Shift the Waveform

With the diode anode grounded, the diode conducts when its cathode falls to about -0.7 V or lower.
During the negative input half-cycle, the capacitor charges to approximately the input peak minus the diode drop; during the other half-cycle, the reverse-biased diode largely blocks discharge.
Once charged, the capacitor acts approximately like a DC source in series with the input, shifting the waveform upward; reversing the diode produces a downward shift.
The DC offset depends on stored capacitor charge, and the diode orientation determines whether the circuit clamps toward a positive or negative level.

11:25 Clamper Simulations, Diode Reversal, and Non-Sinusoidal Inputs

SPICE shows the output initially following the input before capacitor charge builds, then shifting by the capacitor’s stored voltage.
Reversing the diode changes the circuit from a positive clamper to a negative clamper.
Clamping works with square, rectangular, triangular, and ramp waveforms, not only sine waves.
For a rectangular input switching between -3 V and +3 V, the capacitor charges according to the initial level and shifts the waveform.

19:00 One-Shot Circuits Convert Triggers into Standard Pulses

A one-shot produces one pulse of fixed width for each trigger, even when trigger timing or shape varies.
A conveyor-belt sensor can trigger a one-shot to provide a consistent pulse for counting passing items.
The output standardizes irregular or small input signals into rectangular pulses useful for downstream processing.
A 555 timer can implement a one-shot in its monostable configuration.

23:20 Building a Comparator One-Shot with an RC Transient

A diode clamper and an RC network feed a non-inverting comparator whose reference is set to half the supply voltage.
At the start of an input pulse, the capacitor voltage cannot change instantaneously, so the resistor initially carries the largest voltage.
As the capacitor charges through R1, the resistor voltage decays exponentially; the comparator output stays high only while its non-inverting input exceeds the reference.
A protection diode diverts negative voltage from the RC transient away from the comparator input.

33:00 SPICE Verification of the Comparator Pulse Width

The example uses a 110 kΩ resistor and a 1.3 nF capacitor, with a 5 V supply and a midpoint reference of 2.5 V.
Equal-valued divider resistors R2 and R3 establish the 2.5 V reference while drawing little current from the supply.
SPICE traces show the resistor voltage at node A briefly exceeding node B, driving the comparator high before the output returns to zero.
The input pulse spacing can vary independently of the one-shot pulse width, which is set by the circuit design.

41:20 555 Timer Structure and Its Three Operating Modes

The 555 is an eight-pin timer with supply, ground, trigger, output, reset, threshold, and discharge connections.
Three internal 5 kΩ resistors form the divider associated with the 555 name.
Astable operation continuously switches between high and low, monostable operation emits one pulse per trigger, and bistable operation has two stable states.
An astable 555 can alternate red and green LEDs, as in a flashing-light or clock display.

44:10 Negative-Edge Triggering in the 555 Monostable Circuit

The demonstrated monostable uses a 110 kΩ resistor, a 1.3 nF capacitor, and a 5 V supply.
A high-to-low transition is a negative edge; each such trigger causes the output to produce a single fixed-width high pulse.
The interval between output pulses follows the incoming trigger timing, while the pulse width depends on the circuit design.
Hiren Trada ends by noting that the remaining 555 circuit details will continue in the next class.

Transcript excerpt

0:01 All right. Uh I know first thing I want to address is the midterm grades. I had just too much going on. I worked on it some on Friday night and then Saturday, Sunday I just had just got too much piled on my plate. So Friday night I worked on it. I worked some on it uh last night, too. Um but I just need another 24 hours. Uh second thing a student had contacted me about paper copies. So uh similar to the quizzes, the midterm will be available on Canvas for students to view and download. Uh but paper copies will not be uh returned. But you should be able to every student should be able to view and download it from Canvas along with the uh points. Okay.

0:49 Uh let's see. All right. So we're going to look at an application of the RC circuit today and build upon the filter circuits that we saw in the last couple of classes. So All right.

1:45 So we've seen circuits where you have a resistor and a capacitor. Now we're going to put it all together. We're going to apply to a diode. So if you recall back to the diode, there was application called the clipper circuit. We had the peak detectors, the rectifiers, the clipping circuit. Um the circuit was exactly the same. We had a resistor, you had a diode. Uh and depending on where you took the output, whether you took the output across the resistor or whether you took the output across the diode, you have different um different waveforms. Here, we are going to add a capacitor. So, in this capacitor in this circuit on the left, we have the peak detector, where it charges the capacitor in one direction and then the output is taken

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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.