Close

Sunrise LED

A project log for easy-alarm-clock

Improvements for the ubiquitous alarm clock

Christoph TackChristoph Tack 06/24/2018 at 16:120 Comments

Background

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

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:

  1. Attach thermo couple to the LED strip
  2. Measure room temperature T1
  3. Leave LED panel hanging in free air.  Don't lie it flat on a table or other surface.
  4. Attach current source to the LED and regulate current (I) so that steady state temperature of Al-substrate of LED is 70°C (T2)
  5. Measure voltage over LED (U)
  6. Calculate thermal resistance of LED panel = (70-T1)/ (U * I)
  7. Attach heat sink to LED panel.
  8. Repeat step 1-5.

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:

Digikey selection : RT4533GJ6 and others

Discussions