A low-cost, adjustable PWM signal generator built around an LM393 dual comparator. One half of the comparator runs a sawtooth oscillator, the other slices it against an adjustable threshold, and a discrete push-pull follower drives the output to 100 mA.
No microcontroller, no firmware — two trimmers and a jumper.
Complete CircuitStudio design: schematic, PCB, and a released manufacturing package (Gerbers, NC drill, pick & place, BOM, STEP).
Full-size schematic, PCB drawings and 3D views:
PWM Generator Rev 1.PDF
| Supply voltage | 5 – 15 V DC |
| Output current | up to 100 mA |
| Output swing | GND to the input rail, less the input diode drop and one VBE |
| Duty cycle | 0 – 100 %, set by trimmer R2 |
| Frequency | 15 Hz – 300 kHz across three jumper-selected ranges |
| Soft start | duty ramps up from zero on power-up (C6) |
| Shutdown | active-high SD input forces 0 % duty |
| Board | 25.0 × 28.0 mm (0.98 × 1.10 in), 2-layer |
| Assembly | SMD on the bottom side only, plus 4 through-hole parts |
| Placements | 40 (19 unique parts, C18 not populated) |
Range is selected by jumpering one pair on J2, which switches the oscillator's timing capacitor:
| J2 jumper | Timing cap | Range |
|---|---|---|
| 1 – 2 | C4, 2.2 nF | 55 kHz – 300 kHz |
| 3 – 4 | C5, 100 nF | 1.5 kHz – 10 kHz |
| 5 – 6 | C7, 10 µF | 15 Hz – 120 Hz |
Within a range, R9 sets the frequency. Exactly one pair should be jumpered at a time.
J1 — 1×4, 0.1 in pitch:
| Pin | Net | Description |
|---|---|---|
| 1 | PWM_OUT |
PWM output, up to 100 mA |
| 2 | Vin |
5 – 15 V DC supply |
| 3 | GND |
Ground |
| 4 | SD |
Shutdown, active high — forces 0 % duty. Leave open or tie to GND for normal operation |
| Part | Function |
|---|---|
| R9 | Frequency, within the range selected by J2 |
| R2 | Duty cycle |
| C6 | Soft-start time constant — increase for a slower ramp, decrease for a faster one |
Both are 10 kΩ top-adjust trimmers (CT94EW103).
- Input — Vin enters through D1 (MBR0560 Schottky) for reverse-polarity protection. U1, an HT7550-1 SOT-89 LDO, derives the +5 V rail that the comparator and timing network run from. The output stage runs from the input rail, not the 5 V rail, so the PWM output swings as high as the supply allows.
- Sawtooth oscillator — One half of U2 (LM393) charges the selected timing capacitor through R9 and discharges it at a fixed reference, producing a 0 – 2.5 V sawtooth. J2 picks the capacitor and therefore the range; R9 sets the charge current and therefore the frequency.
- PWM comparator — The other half of U2 compares the sawtooth against a DC control voltage from R2. Where the control voltage sits within the sawtooth's 0 – 2.5 V span is the duty cycle.
- Soft start — C6 sits on the control node and charges through R5 at power-up, so the control voltage ramps from 0 V and the duty cycle ramps with it.
- Shutdown — Driving SD high turns on Q5, which pulls the control node to ground and holds the output at 0 % duty.
- Output stage — Q3 (NPN, SMBT2222A) and Q4 (PNP, MMBT2907A) form a complementary emitter follower. R6 (10 Ω) is in series with the output and R14 (1 kΩ) pulls it down.
- Indicator — D2, a green LED, sits across the output through R7 (10 kΩ), so its brightness follows the duty cycle.
Key parts: U2 LM393DT · U1 HT7550-1 · Q1–Q3, Q5 SMBT2222A · Q4 MMBT2907A-7-F · D1 MBR0560
Melanson Tech - PWM Generator.PrjPcb CircuitStudio project
Melanson Tech - PWM Generator.SchDoc Schematic
Melanson Tech - PWM Generator.CSPcbDoc PCB layout
PWM Generator Rev 1.OutJob Output job - regenerates everything below
docs/schematic.png Schematic image used above
Default Configuration/Outputs/
PWM Generator Rev 1.PDF Schematic and PCB drawings
Gerber/ RS-274X, 2 layers, bottom-side assembly
NC Drill/ Excellon drill files and drill report
Pick Place/ Placement data, .csv and .txt
BOM/ Bill of materials with Digi-Key part numbers
ExportSTEP/ 3D model of the assembled board
Open Melanson Tech - PWM Generator.PrjPcb in Altium CircuitStudio (the outputs here
were generated with 1.5.2) or in Altium Designer, which reads the same formats.
The project's library search path points at a local folder, so schematic symbols and footprints may not resolve on another machine. The design files themselves carry everything needed to view, plot, and fabricate the board — the libraries are only required to edit components or re-run an ECO.
The manufacturing package under Default Configuration/Outputs/ was generated on
2024-04-06 and is what you send to a fab house:
- Gerbers — RS-274X, metric, 2 layers (
.GTLtop copper,.GBLbottom copper), with.Outlineas the board outline - Drill —
.DRL(Excellon) plus.DRR; 40 plated holes, 4 tools, smallest 0.3 mm - Assembly — pick & place plus the
.GBPbottom paste layer. Every SMD part is on the bottom side, so this is a single-sided reflow. J1, J2 and the two trimmers are through-hole. - BOM —
BOM_PartType-Melanson Tech - PWM Generator.xls, with Digi-Key part numbers for most line items. C18 is marked DNP — do not populate.
If you change the design, re-run PWM Generator Rev 1.OutJob rather than editing these
files by hand.
CERN Open Hardware Licence Version 2 — Strongly Reciprocal (CERN-OHL-S-2.0).
Copyright © 2024–2026 MelansonTech (Shawn Melanson).
This source describes Open Hardware and is licensed under CERN-OHL-S v2. You may redistribute and modify it under the terms of that licence. This source is distributed WITHOUT ANY EXPRESS OR IMPLIED WARRANTY, INCLUDING OF MERCHANTABILITY, SATISFACTORY QUALITY AND FITNESS FOR A PARTICULAR PURPOSE. Please see the licence for applicable conditions.
In short: you may build, modify and sell this design, but if you distribute a modified version — or a product made from one — you must publish your modified source under the same licence.
The full licence text is in LICENSE, or at https://cern.ch/cern-ohl.