1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice
alec_t

PWM pulse generator

This is the base design (not a full working drawing with pin-numbering etc) for a simple PWM pulse

  1. alec_t
    This is the base design (not a full working drawing with pin-numbering etc) for a simple PWM pulse generator using logic gates in a cheap, readily-available CD4011B or CD4049B IC. The pulse duty cycle is variable from 0% to 100%.

    PWMgenerator.gif

    Circuit Operation

    Inverting gates U1b,U1c plus resistors R4,R6 form a Schmitt trigger with upper and lower threshold voltages determined by the ratio of R4:R6.
    C1 charges slowly via R1 and discharges rapidly via R5,D1 under the control of the Schmitt trigger, thus providing an oscillator with an approximately sawtooth waveform and a pulse period governed primarily by the time constant R1C1 (since R1 is much greater than R5). With the component values shown the pulse repetition frequency is ~ 20kHz.
    A DC control voltage Vcon (in the range 0 up to the supply voltage V+) and the sawtooth voltage across C1 are summed via weighting resistors R2,R3. Inverter U1a changes state when the sum voltage crosses its logic threshold (~ half of V+), thus giving an output pulse with a width determined by Vcon. The weighting resistor values shown enable a full scale (0% to 100%) adjustment of the pulse duty cycle (mark:space ratio). If a reduced adjustment range is required then R2 can be increased in value.
    Vcon can be derived from a potentiometer (in the range 10k to 100k) connected directly between V+ and ground, or from any other source.
    R5 limits the current flow through U1b when C1 discharges and its value is chosen according to V+ using the formula R=100 x V+.
    Variation of pulse width with control voltage is non-linear because of the exponential charging of C1. It can be made linear by replacing R1 with a constant-current source if necessary.