Analog switch converts 555 timer into pulse-width modulator

This design idea describes a new approach to produce a variable duty cycle waveform from a free-running oscillator based on 555. The wide range of circuit modulation, the highly linear control over a wide range of values of duty cycle and excellent linearity make it ideal for control applications based on PWM (pulse width modulation). Figure 1 shows the basic circuit, which works as follows: when the output of IC1 goes up, switch S1 closes and the internal discharge of IC1, switch S2, opens. Capacitor C1 is charged through R1 and R2. When the output of IC1 falls, S1 opens and S2 closes, discharging C1 through R2 and R3.

Figure 1 An external analog switch and a 555 timer provide a free-running oscillator with a fixed duty cycle.


The generic configuration works well to produce a fixed value duty cycle. To obtain a continuously variable duty cycle, Figure 2 shows how to connect potentiometer R4 to the common junction of R1, R2 and R3. The duty cycle of the output waveform, DT C, follows the equation: DT C=(R1 +R2 +RVAR )/(R1 +2R2 +R3 +RPOT ), where RPOT is the end-to-end resistance of the potentiometer, and RVAR is the fraction of RPOT between the rotor and R1. As the equation shows, DT C depends linearly on RVAR. Switch S1 comprises a section of a quadruple bilateral SPST switch 4066 CMOS, IC2.

Figure 2 Add a potentiometer, R4 , to produce an output pulse that has a manually variable duty cycle.


You can use the circuit in Figure 3 to evaluate the linearity of the work cycle. A rotary switch and a 16 kO series resistor chain provide a 10 kHz signal with nine discrete and equally spaced duty cycle values ranging from 2 to 98%. For accurate results, use a 5½-digit multimeter to match the values of resistors R4  to R11 and a Tektronix 3012 oscilloscope or equivalent to collect DT C data.

Figure 3 To obtain fixed-duty-cycle values for linearity evaluation, you can replace the potentiometer with a rotary switch and a series-connected string of precision resistors.

Microsoft Excel spreadsheet software includes a linearity analysis that returns the following trend line for work cycle measurements: DT C=0.7565×RVAR +2.1548; R2 =1. The value of 1 for R2  as Excel calculates shows that the transfer function is perfectly linear. The ignition resistance of the switch S1 and particularly its leakage current slightly affect the slope and interception of the DT C-versus-RVAR equation, but the equation remains strictly linear. The use of only one of the four IC2 switches eliminates the leakage and crosstalk effects that would occur if other circuits used the remaining switches. In addition, the use of moderately low values for the resistance network further reduces the effects of the leakage current on the circuit performance.