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rgb_dimmer

Building up an RGB Dimmer

Needed Parts and PCB

You need a PCB. You may use the layout from Github and etch it yourself or order the PCB or a complete kit.

Partlist (in buildup order)

Amount Part Placing Picture
1 PCB RGB Dimmer 1 pcb_rgb_dimmer.jpg
1 Diode 1N4148 D1 diode_1n4148.jpg
4 Resistor 560R green-blue-brown R1-R4 resistor_560r.jpg
1 Resistor 1M brown-black-green R5 resistor_1m.jpg
1 IC holder IC1 ic_holder.jpg
1 Voltage regulator LP2950Z IC3 ic_lp2950z.jpg
3 Transistor BC337 T1-T3 transistor_bc547c.jpg
1 LED 2mA LED1 led_yellow.jpg
1 Connector AVR ISP 6pin JP1 connector_isp.jpg
1 Connector pin header (for serial TX debugging etc.) JPx connector_pinheader.jpg
1 Capacitor 10µF C1 capacitor_10uf.jpg
1 Capacitor 100µF C2 capacitor_100uf.jpg
1 IC ATMega328 IC1 atmega.jpg
1 Transceiver PCB RFM12B IC2 ic_rfm12b.jpg
1 Antenna (82,2mm wire) antenna.jpg

You have to decide on the LEDs you want to use. Depending on the type, you have to use different resistors R6, R7, R8 for the LEDs. See further down for a recommended one.

The capacitor C2 is only to buffer the input voltage in case you connect LEDs that draw much power and you want to avoid problems with some programmers that may have problems flahing when the voltage changes much.

Buildup of PCB

As always: start from flat to high. Go through the partlist and solder the parts from the top ones to the last ones.

For some parts, you have to consider something special:

  • IC holder: Look at the notch and place it in the right direction.
  • LED: Solder it to a wire if you want to place the PCB in a housing later. Read the instructions on how to solder the LED to a cable. The longer wire of the LED is +. It goes into the hole more in the middle of the PCB.
  • ISP Connector: The notch points to the side of the PCB.
  • 10 uF Capacitor: The marked line is -, which points to the mid of the PCB. On the PCB, + is labelled.
  • ATMega: Before inserting it, you may want to check the voltage levels when switching the power on. Pin7 should have VCC (3V) against pin8 (ground). If you have different voltages: don't panic, nothing is broken, nothing is fried. Search your error. To insert the ATMega, bend the pins at 90 degreed by placing the ATMega on the table and bending it carefully. Then insert it into the IC holder. Be sure that you are not charged with electricity (ESD!) when touching the IC pins.
  • RFM12B: You should also check the voltages first before soldering the module. At the place for the radio module the pad beside the antenna and at the opposite side the third pad should read about 3.3 V. For soldering, read the RFM12B mounting instructions.

(Image directly loaded from external GitHub source. If it doesn't work, fix link in wiki!)

Flashing the firmware

If you have a new ATMega where nothing is flashed onto, download a prebuilt binary package or build your own firmware. If you bought a hardware kit, the ATMega should already be flashed.

Power the Device

The device has an internal voltage regulator with 3.3V output to power the ATMega and the RFM12B. It's recommended to power the whole device with a 5V power supply. You can use a cheap one that is meant as phone charger. They have typically 500mA output current.

Connecting LEDs

You can power three single LEDs or a multi RGB LED with common anode (+). The maximum current is 0.6A, limited by the transistors. Calculate the resistor you need by the following formula:

R = (U_in - 0.3 - U_LED) / I_LED

with U_in the input voltage of the device (e.g. 5V), U_LED the voltage your LED needs (e.g. 2.8V) and I_LED the current you want to do through the LED (e.g. 250mA).

One LED type that is very bright with a moderate current are the Cree XP-E LEDs. They are available as a module in the smarthomatic shop. The module makes assembling easier and ensures good cooling.

If you use this module, the following sets of resistors are recommended:

Placing Part normal bright max
R6 Resistor for red LED 39 Ohm 27 Ohm 7,5 Ohm
R7 Resistor for green LED 27 Ohm 18 Ohm 5,6 Ohm
R8 Resistor for blue LED 22 Ohm 15 Ohm 3,9 Ohm

The “normal” resistors are enough if you use a thin glass housing (as shown on the homepage). You can use normal 1/4 W resistors and the LED module won't heat up much. No heat sink required. This is the safest and easiest choice.

The “bright” resistors result in LED currents of ~120mA and a power dissipation at the LED module of ~1W. It should not need an additional heat sink, but it gets hot already (you can touch it, maybe ~50°C). The resisors have a calculated power dissipation of ~0,3W. 1/4W resistors max be enough, but I recommend to use metal oxyde resistors (1W). The overall current of the RGB dimmer is 380mA, so a typical power supply for smartphones with 500mA max. current fits perfectly.

With the “max” resistors, you definitely need an extra heat sink at the LED module and resistors which can cope with the higher current (1W).

Use resistors which can tolerate the power they consume. The power is calculated as:

P = U_Res * I_LED

with U_Res the voltage at the resistor.

rgb_dimmer.txt · Last modified: 2015/02/22 20:15 by breaker27