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Discrete PWM Generator Schematic Circuit Diagram

DC Motor Speed Control with PWM Waveforms

PWM waveforms are a common method used for regulating the speed of DC motors. The ratio between the mark (on) and space (off) periods of the digital waveform can be determined either through an adjustable analog voltage level (as seen in NE555 based PWM generators) or digitally using binary values. In most cases, digitally derived PWM waveforms are generated by timer/counter modules in microcontrollers. However, if you prefer to avoid incorporating a microcontroller in your circuit, it’s straightforward to produce these signals using discrete logic components.

Generating PWM Waveforms without Microcontrollers

An enhanced version of the depicted circuit can create two PWM waveforms using an 8-bit digital input word. Each signal encompasses 15 different values. This 8-bit word can be generated, for instance, from an expansion board installed in a PC, an 8-bit port of a processor lacking built-in PWM capability, or even from a laptop’s printer port. This approach enables the creation of PWM waveforms without the need for a microcontroller in the circuit.

Discrete PWM Generator Schematic Circuit Diagram

Limited Programmable Ratio

In this design, the mark/space ratio can only be programmed up to 15/16 instead of the full 16/16. An input of 0000 results in a continuous low on both outputs, effectively turning off both motors. While many similar circuits employ a dedicated ‘enable’ input for motor shutdown, this particular design doesn’t necessitate it.

Circuit Components and Channel Expansion

The provided diagram illustrates the circuitry required for generating a single waveform. To achieve a complete two-channel circuit, an additional 74HC193 is necessary. The clock signal from the HCF4060 generator can be utilized to drive both channels. Furthermore, the unused flip flop in the 74HC74 package can be employed for the second channel, denoted by corresponding pin numbers in brackets. Remarkably, the entire two-channel setup can be constructed using just four ICs.

Features of 74HC74 and 74HCT74 Flip-Flops

The 74HC74 and 74HCT74 are dual positive edge-triggered D-type flip-flops. These components feature individual data (nD), clock (nCP), set (nSD), and reset (nRD) inputs, along with complementary nQ and nQ outputs, enhancing their versatility and usability in various applications.


  • Output Drive Capability: 10 LSTTL Loads
  •  Outputs Directly Interface to CMOS, NMOS, and TTL
  • Operating Voltage Range: 2.0 to 6.0 V
  • Low Input Current: 1.0 A
  • High Noise Immunity Characteristic of CMOS Devices
  • In Compliance with the JEDEC Standard No. 7A Requirements
  • ESD Performance: HBM 2000 V; Machine Model 200 V
  • Chip Complexity: 128 FETs or 32 Equivalent Gates
  • Pb−Free Packages are Available

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