The 555 Timer – Part 1 | Monostable Mode

Philippians 4:6–7 (KJV) 6 Be careful for nothing; but in every thing by prayer and supplication with thanksgiving let your requests be made known unto God. 7 And the peace of God, which passeth all understanding, shall keep your hearts and minds through Christ Jesus. In this video, we're taking a look at one of the most famous integrated circuits ever made — the 555 timer. We'll start by looking at the 555 datasheet and then take a quick look at what a 555 actually looks like at the transistor level. Rather than trying to follow every transistor inside the IC, we'll see how a functional block diagram makes the circuit much easier to understand. Then we'll use that block diagram to follow the operation of the 555 in monostable mode, also known as one-shot operation. We'll see how: • Pulling the Trigger input below approximately ⅓ VCC sets the internal latch and sends the output HIGH. • The external timing capacitor begins charging through the timing resistor. • When the Threshold input reaches approximately ⅔ VCC, the latch resets and the output returns LOW. • The discharge transistor then discharges the timing capacitor through Pin 7, preparing the circuit for another trigger. We'll also build the circuit on a breadboard and use the monostable timing equation: T ≈ 1.1RC Our circuit uses a measured 44.7 kΩ timing resistor and a nominal 100 µF timing capacitor, giving us a theoretical pulse of approximately 4.92 seconds. But what happens when we actually build it? Our real circuit gives us about 6.27 seconds! That's a great opportunity to see why real-world circuits don't always behave exactly like the ideal calculations and to talk about component tolerances and other practical differences. And, of course, we'll have a little fun along the way. 😄 RESOURCES & REFERENCES Texas Instruments — NE555 Precision Timer Official product information and datasheet. TI's current NE555 documentation includes the functional diagram and monostable application information used in this lesson. Texas Instruments NE555 Datasheet (PDF) Texas Instruments NE555 Product Page NE555 Transistor-Level Internal Circuit Diagram Diagram by Wdwd, via Wikimedia Commons. Licensed under Creative Commons Attribution 3.0 Unported (CC BY 3.0). The Commons page confirms Wdwd as the author and permits sharing and adaptation with appropriate attribution. NE555 Internal Circuit — Wikimedia Commons Creative Commons Attribution 3.0 License Build Circuits With Rich Come along as we learn electronics by understanding the theory, building real circuits, and seeing what actually happens on the bench. Whoo Buddy!! 00:00 Introduction 00:32 What Is Monostable Mode? 01:28 Simple 555 Timer Block Diagram 03:42 Why Is It Called Monostable? 05:57 Exploring the NE555 Datasheet 10:14 Why Block Diagrams Are So Useful 10:49 Inside the 555 Timer – Transistor-Level Circuit 12:06 The 1/3 VCC and 2/3 VCC Reference Voltages 14:03 Breaking the 555 Into Functional Blocks 16:25 Back to the TI Block Diagram 25:45 Reset Pin and Preventing False Triggering 26:28 Other 555 Timer Applications 29:08 555 Timer Pinout 31:59 Our Monostable Circuit 33:06 The 1.1RC Timing Equation 35:01 Trigger Circuit and Pull-Up Resistor 35:54 RC Timing Network 36:57 Calculating the Output Pulse 38:09 How the Monostable Circuit Works 38:40 Triggering Below 1/3 VCC 39:19 Discharge Transistor and Capacitor Charging 40:13 Reaching the 2/3 VCC Threshold 42:26 Let's Go to the Lab! 43:03 Breadboard Circuit Walkthrough 47:18 Testing the Circuit 48:02 Real-World Component Tolerances 49:32 Monostable Operation Recap 50:41 Philippians 4:6–7 (KJV)