A new take on the LED coaster with wireless power charging and supercapacitor energy storage.
Adobe Portable Document Format - 50.75 kB - 07/01/2019 at 07:28
kicad_pcb - 231.08 kB - 07/01/2019 at 06:22
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With two sets of stacked super caps, the total storage is brought to 60F.
The below is the current draw of each major component. This doesn't encapsulate everything, especially with changing supply voltages, but it is a good show of how the LEDs have the majority of the current draw.
555 Timer: 0.25mA
4-bit binary counter: 1uA
4-16 bit decoder: 8uA x 2
Single LED: 27.5mA
I selected the regulator to be 2.75V (just under the 2.8V rating of the caps). Since the forward voltage of the LEDs is 2V, this device has a useful range of only 0.75 volts. Once below 2V, the LEDs will no longer light up and the device will slowly bleed out the remaining energy.
Upon powering the device with the charging coil, the caps quickly charge to 90-95% of the max voltage. I originally thought this was "close enough" to being charged. When removing from the supply after less than a minute of charging, the device would run out of power within a very short period of time.
Charging it for longer and monitoring it shows that the voltage quickly ramps up, but flattens out near the peak and takes quite a while to actually reach 2.75V. When giving it a long time to charge completely, the device was able to run for about four hours.
Possible ways to increase run time:
-increase supply voltage (limited by capacitor specs $$$)
-more capacitance ($$$)
-choose LEDs with lower forward voltage
-run LEDs dimmer (larger series resistance)
-find a timer with lower current draw (if available)
I ordered my PCB through OSH Park.
The board came out very nicely, but I did notice one error I made. Several of the footprints were custom made, including the supercapacitors. For these components I only considered the pad dimensions, but not the full package size (which is wider than the pads).
I was still able to fit two of the capacitors on the board, and further added to this by stacking another two on the first ones.
The first tests seemed to work as intended, but after being removed from the charging coil, the LEDs would go dark, and a few seconds later turn on again for only dozens of seconds. I had not considered in designing that the charging circuit would be back-fed by the capacitors and waste the energy that was just stored. I added a diode in series with the 5V coil output to stop any current flowing back to the charging module. This solved the issue and allowed for much longer operation time.
I designed the schematic and did the PCB layout with KiCad.
The visual representation has some parts missing as a lot of the footprints were custom made.
The idea is to have an LED coaster powered from super capacitors and wirelessly charged. As capacitors don't have the storage capabilities of batteries, it is important to design this to have as low of power draw as possible. For that reason I won't be using a microcontroller, or having more than one LED on at a time.
Wireless charging coil:
-5V in, 5V out
-15F devices, rated for 2.8V
-brings in 5V from the charging coil, regulates to 2.75V
-set a frequency for a 4-bit binary counter
4-bit binary counter:
-count from 0-15. Feeds 4-16 bit decoder.
4-16 bit decoder:
-a counter feeding into this allows to pulse one LED at a time (16 total LEDs)
16 LEDs along the edge of the coaster. The effect of one on at a time will create a spinning effect.