<![if !supportLists]>1. <![endif]>Introduction
This LED dimmer works with PWM and can be used to dim LED strips rated at 12V up to 2 amps. The LEDs are driven by a FET transistor.
The dimmer can be configured to operate in several ways: with a rotary button (potmeter), with one pushbutton or with two pushbuttons.
The dimmer features soft start in all modes, and soft stop + dim memory in one or two pushbutton mode.
It also contains a low drop voltage limiter which limits the voltage on the LED strip to 12V, making it suitable for use in motorhomes, boats and so on because when charging the batteries the voltage can rise up to 15V.
I made four versions of the PCB layout: a square version, a slim-line version, both for PIC18F4550 and PIC18F2550.
This circuit is based around the PIC18F4550 or PIC18F2550 microcontroller. This chip is actually a small computer contained in a single chip, including RAM memory, EEPROM, I/O ports, CPU and so on. When you buy this chip, it comes empty with no program on it. You have to compile the source code and download the resulting machine code into it, using a PC and a small programmer attached to the PC and the chip. To get yourself familiar with this stuff, I suggest you first read this link: Getting started with microcontrollers.
<![if !supportLists]>2. <![endif]>Description
Using a microcontroller to design a LED PWM dimmer might look like overkill at first glance. The reason for using a microcontroller is mainly for implementing things like soft start, soft stop and dim memory.
The characteristics of the device are:
- 3 operating modes (choosen with dip-switches or wired connections):
+ pushbutton operation with 1 pushbutton
+ pushbutton operation with 2 pushbuttons
+ operation with potentiometer
- Soft start
- Soft stop (some configurations only)
- Remembers last dim setting (push button operation only)
- Slide switch for direct access to 100% light
- For use with 12V systems (input 12-16V, output 12V)
- LED strips up to 2A
<![if !supportLists]>3. <![endif]>Usage
Default operation mode is with a potentiometer. To use the device with one push button operation, connect PAD8 to PAD5. To use the device with two push buttons operation, connect PAD9 to PAD5. Pads 5, 8 and 9 can be found on the ICSP connector.
One button operation:
Alternatively mount jumper S3 and set to ON.
Press button for a short while to lit the LED. Keep button pressed to increase light. Release button and hold button again to dim light. Press button shortly again to set the light off (soft stop). Press shortly again to put light on again, the light will soft start to last dim level.
Last dim level is stored in EEPROM at each change of dim level.
Two button operation:
Alternatively mount jumper S4 and set to ON.
Press UP button for a short while to turn the light on (to last dim level). Keep the UP button pressed to increase the dim level. Keep the DOWN button pressed to decrease the dim level. Press the DOWN button for a short while to turn off the light.
Shortcut to 100 percent:
If desired, connect a switch between X and Y or solder a switch on the PCB. Operating this switch bypasses the current dim level and directly lights to 100 percent.
Connect power, LEDs and potmeter:
Solder in/out directly to the power regulator and mosfet leads (- = 12V ground, + = 12V, Led- and Led+ to LED strip – and +). If desired, solder a potmeter to green, yellow and orange wires or solder an SMD potmeter on the PCB:
<![if !supportLists]>4. <![endif]>Schematic diagram
To support multiple versions of the PCB (see later) also multiple versions of the schematic diagram were created. These can all be found in the Eagle zip file later on.
<![if !supportLists]>5. <![endif]>Explanation of the schematic diagram
(Refer to the schematic diagram).
The diagram is really simple. A potmeter is connected to an ADC port (RA0) as a voltage divider. The PIC constantly reads the value and translates it (using a gamma table) to a PWM value on port RC1. The PWM output is connected to the gate of MOSFET transistor Q1 which drives the LEDs.
In stead of a potmeter one or two pushbuttons can be used as a user interface. The PIC constantly reads the buttons and how long they are pressed (as described above in “Usage”) and translates this to a suitable PWM value. Every 5 seconds the value of the PWM is stored in EEPROM (unless it didn’t change) so that the light comes up to the last dim level the next time it is used.
<![if !supportLists]>6. <![endif]>The printed circuit board
The pcb is designed for smd components.
It is single sided and designed with Eagle (from Cadsoft).
Below is only one version of the PCB. Other versions can be found in the Eagle zip file later on.
The big transistor (IRLZ24N) and the power regulator (LT1085-12) are actually not SMD components, therefore 6 holes must be drilled to fit these. It is best to fit it at the bottom side.
PDF file (mirrored) to print on a transparent: Print-layout
Link to Eagle files zip file containing all versions of the PCB, schematic and pdf files.
<![if !supportLists]>7. <![endif]>The final device
Below some pictures of different versions of the final device.
The connector marked “-+MCD” is the in-circuit programming interface to connect with the programmer (connections: - =, ground + = +5V, M = reset, C = clock, D = data).
PIC18F2550 Slimline version (left is the potmeter):
PIC18F4550 square version:
PIC18F4550 Slimline version (PIC not yet mounted):
<![if !supportLists]>8. <![endif]>Programming
The software is written in C (PIC C18 from Microchip).
It was first prototyped on a Dwengo experimental board (http://www.dwengo.org). Since this board uses a PIC18F4550, I designed two projects: one for PIC18F4550 and one for PIC18F2550. The first can, if desired (uncomment #define DWENGO), use the Dwengo library to show some debug messages on the Dwengo LCD. The PIC18F2550 version of the project does not need the Dwengo library.
You do not need the Dwengo board and library to build this project, just the Microchip MPLAB IDE and a PICkit2 compatible programmer will do.
The software uses the PWM feature of the PIC18Fx550 to dim the light. It uses 64 steps to do so.
Zip file containing all source code, header files and project files: Link to MPLAB projects
Connection to a PIC programmer.
Pad 5-6-7-8-9 must be (temporary) wired to a PIC programmer (5=-, 6=+, 7=VPP, 8=clock, 9=data), e.g. a PICkIt2 compatible programmer.
The software uses a gamma table (g in the source code) to translate percentage to a correct duty cycle for the light that visually corresponds to that percentage. This table is suitable for LEDs, I don't know what it will look like on classical light bulbs. It might be needed to change the table in this case.
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