A gradually brighter light can mimic the sun rise. It will allow for gentle wake-up in the morning.
Experience with the Philips HF3463 showed me that you shouldn't count on it to get up on time. I sometimes find myself waking up by the alarm sound and facing the alarm clock in full brightness. When you're sleeping on your side and the light is behind you, it won't wake you either.
Needed output power
A Philips HF3463 sunrise clock uses a Philps 100W 1200lm Softone lamp. Replacing the bulb with a 60W 700lm still gives enough light. So our light should also be able to output about 700lm.
Selection of the lighting source
- Lumileds Luxeon 3535L & Lumileds Luxeon 3535L HE Plus
- Devices available at Mouser & Digikey:
- LEDs cost about €0.42/pce
- 300mA max, lowest cost per lumen. Other 180lm/W devices cost about the same but don't allow such high currents.
- If driven at 200mA, then 7 LEDs would yield about 800lm.
- "Could" be soldered to custom Al substrate PCB.
- AllPCB : 120x20mm or 100x30mm is $17 / 5pcs
- How hard is it to solder LEDs on a metal substrate?
- 280x24mm, which is too long for the alarm clock
- 19.5V, 350mA, 1230lm
- Digikey €3.12/pce (not available)
- COB LEDs
- DopDea DPD-36120-0428
- DopDea DPD-9450-0414, ZF-9450-0414
5050 LED panel GU5.3 Base fitting for MR16 lamps
- 12V lamps
- typically 500lm, which is rather limited
- heatsink is part of the lamp
- AliExpress has many on offer, very cheap
- In a local store it was hard to find LED lights with a GU5.3 base. GU10 (230V) is the most common type of lamp. The best GU5.3 base LED lamp suitable for this application:
- 12V AC/DC
- €11.95 (no kidding)
Wayjun Epistar 10W
- available on AliExpress from many vendors
- commonly used in floodlights
- cheap : $0.29/pce (cheap version 900mA), $0.94 (real version 300mA)
- A sample of each has been ordered. I received both but don't know which is which. I asked the seller and they replied that the "heavier" one is the real one. The "real" one is indeed about 4g heavier.
- They were both announced as warm white, but the "real" one is cold white.
- The warm white one has a terrible thermal efficiency. Without heat sink, the current needs to be lower than 100mA, or the LED will run too hot (>70°).
- The light output of the warm white one is disappointing too.
- The cold white LED performs a little better, but it's still disappointing. Too much of the power is lost in heat. These LEDs can't be used.
- AliExpress "specs"
- limited efficiency : 58lm/W (worst case for the cheap version)
- 11V, 1050mA, 675lm
- heat sink needed
- expensive: 100x100mm = €3.84
- limited light output
220V light with dimmer
- requires 220V, which could be dangerous
- consumes a lot of power because of the inefficient lamps
- No need for color changing the light
Controlling the room light
- Standard light with controllable dimmer: You need a dimmer that can be remotely controlled
- Philips Hue White Wireless bulbs : if you have a lot of money to spend
- Inefficient because of the many series resistors in the string
Light source cooling
The above mentioned LED COB sources require that the aluminum substrate is not hotter than 70°C. If we want to maximize light output, we'll have to add a heat sink to get rid of the excessive heat.
There's no reliable info about the thermal resistance of the LEDs. Some tests will need to be done:
- Attach thermo couple to the LED strip
- Measure room temperature T1
- Leave LED panel hanging in free air. Don't lie it flat on a table or other surface.
- Attach current source to the LED and regulate current (I) so that steady state temperature of Al-substrate of LED is 70°C (T2)
- Measure voltage over LED (U)
- Calculate thermal resistance of LED panel = (70-T1)/ (U * I)
- Attach heat sink to LED panel.
- Repeat step 1-5.
A 100x60x10mm : €1.55/pce heat sink will be used.
This is not the cheapest heat sink around. Care should be taken to the thermal resistance of the heat sink. The LEDs can work up to 85°C. Any hotter and their lifetime will decrease rapidly. At a room temperature of max. 40°C, this allows for an extra 45°C temperature rise due to LED heating. The total resistance of the thermal path should be smaller than 45°C / 10W = 4.5°C/W. The thermal resistance of the LED is already 1.4°C/W. This leaves about 3.1°C/W for the heat sink.
AliExpress vendors don't publish the thermal resistances of their heat sinks. We'll have to measure thermal resistance by ourselves.
Another way is to go with some simple rule of my old colleague : "If your heat sink is too hot to touch, then it's too small".
LED driver modules
The plan is to power the clock from a 5V USB power adapter. The LEDs require about 12V/500mA. So we need a boosting LED module, not a buck module.
If we decide to make the module ourselves, we need to select an IC:
- easy to solder : SOT23 is ok, QFN isn't
- over voltage protection (when LED is not connected or broken). Proper solutions directly sense the output voltage using an extra pin. This brings the minimum number of pins for the IC to 6 : gnd, vin, vout, sw, fb, en.
- VIN min < 5V
- VIN max > 5V
- VOUT min > 12V
- IOUT > 1A (we need 500mA)
- low power. Remark that the output LEDs continue to draw current when the LED module is in shutdown. External circuitry is needed to fully shut down the LED module.
- dimming capability (analog & digital dimming are explained in the MP3202 datasheet)
Digikey selection : RT4533GJ6 and others