Analog switch converts 555 timer into pulse-width modulator

摘要

This Design Idea describes a new approach to producing a variable-duty-cycle waveform from a 555-based free-running oscillator. The circuit's wide modulation range, highly linear control over a wide range of duty-cycle values, and excellent linearity make it ideal for PWM (pulse-width-modulation)-based control applications. Figure 1 shows the basic circuit, which works as follows: When IC{sub}1's output goes high, switch S{sub}1 closes, and IC{sub}1's internal discharge, switch S{sub}2, opens. Capacitor C{sub}1 charges through R{sub}1 and R{sub}2. When IC{sub}1's output goes low, S{sub}1 opens, and S{sub}2 closes, discharging C{sub}1 through R{sub}2 and R{sub}3.