OPTI 380B Lab #5 “Op-Amps II, 555, and the Linear Power Supply” Dr. Mike Nofziger Instructor College of Optical Sciences University of Arizona Dr. Mike Nofziger 2010

The “555” Timer IC: Designed in 1970 > 1B sold (2003 data), still in wide use 8-pin DIP, contains over 20 transistors, 2 diodes, and 15 resistors Single voltage power supply (4.5 – 16 V) Frequency stability ≈ 1% 3 Modes of operation: - Monostable Mode - “one shot” pulse generator - Bistable Mode - basic flip-flop - Astable Mode - oscillator (continuous stream of rectangular pulses, specified frequency)

Dr. Mike Nofziger 2010 PinNamePurpose 1GNDGround, low level (0 V) 2TRIG OUT rises, and interval starts, when this input falls below 1/3 V CC. 3OUTThis output is driven to +V CC or GND.+V CC 4RESET A timing interval may be interrupted by driving this input to GND. 5CTRL "Control" access to the internal voltage divider (by default, 2/3 V CC ). 6THR The interval ends when the voltage at THR is greater than at CTRL. 7DIS Open collectorOpen collector output; may discharge a capacitor between intervals. 8V+, V CC Positive supply voltage is usually between 3 and 15 V. The “555” Timer IC: DIP Layout:

Dr. Mike Nofziger 2010 The “555” Timer IC: Monostable Mode - “one shot” pulse generator - pulse begins with the trigger input < Vcc/3 - pulse width determined by the RC time constant - pulse ends when the voltage across the capacitor (pins 6&7) = 2/3 Vcc - t ≈ 1.1RC

Dr. Mike Nofziger 2010 The “555” Timer IC: Astable Mode - oscillator, rectangular pulses - pulse frequency determined by the RC time constant: - T = (0.693)(R A + 2R B )C - Duty Cycle ≡ t H /(t H +t L ) = (R 1 +R 2 )/(R 1 +2R 2 ) {≥50%} = (R 1 )/(R 1 +R 2 ) {<50%, with a diode ‘clamp’}