YAJT = yet another joule thief
I bicycle ~5 mi / day for fitness and errands.
"You can find every thing you need on the side of the road"
Ive found all kinds of lost electronics and sundry other things.
Ive found FORD model cell phones. (Found On the Road Dead)
usually run over, and chewing tobacco tins.
This got me to wondering if they could be combined and re purposed.
Well, cell phones are complicated beasties, and
most of the innards would be too difficult to re purpose, but
the LEDs used in the back light are simple easy things.
What if we could use these with a tobacco tin to make a utility / flash light?
Alternatively, an Altoids tin or similar case can be used.
Now, you could say why bother and just go buy a cheapie.
But hey, this is Instructables where We get off on DIY!
The phone LED back lights use a few leds connected in series.
They do this as all of the leds will have identical current.
Paralleling of leds is not recommended as
they wont share current equally due to uneven forward voltages.
One could add swamping resistors for each led, but
this wastes power / efficiency and adds cost / size to the circuit.
In this case there were 7 leds, so at 3V / white led
the total voltage for the string is ~20V
This project can be used for strings of 5 to 10 leds.
Now, these could be directly lit with a series string of 3 x 9V batteries,
and a series resistor (470 ohms), but
these are more expensive than AA cells.
Too simple! I wanted to do the Joule Thief thing, and
convert the 1.2V of a rechargeable NiMH AA cell
up to the 20V needed to operate the leds.
This is where things get interesting for a techie.
Indeed, there is a booster circuit in the phone to do this, but
it runs off of the 3.7V LiPo cell and
probably wouldnt run off of a 1.2V NiMH cell.
Also, who knows where that ckt is an how to use it?
The average joule thief circuits are basically blocking oscillators that
use transistors in saturated mode, either fully on or fully off.
Energy is stored in the inductor as a ramping current during the on period and
transferred to the leds during the off period.
Usually and most simpy, the on-off switching point occurs when
the switching tranny can no longer support the ramping inductor current and
is ripped out of saturation.
This occurs at the worst possible time when
the energy being stored in the inductor and current are at maximum.
Losses in the tranny are worst in this mode.
This leads to less than desired efficiency / battery charge life.
The solution is to turn off the tranny before it gets ripped out of saturation.
Hence, this oscillator circuit is a little more complicated, but
not outrageously and the parts are cheap.
Here, the tranny is turned off before it gets ripped out.
Qled is the main switch.
Qfl is for fast and hard turn off of the switch tranny.
Qfh with Cfb determines the oscillation frequency and
initiates the turn off.
Lbst is the boost storage inductor.
Dbst is to transfer energy to the leds.
Cbst is to smooth current to the leds.
The inductor current is way to large (400mA max) to slam right into the leds.
This circuit will supply ~10mA to the led string, for ~200mW of power.
It runs at ~80% efficiency so
input from the battery is ~250mW or ~200mA of current.
For a 2000mAh cell, itll last ~10hrs.
At ~25% LED efficiency, this is 63mW of real light.
An old school flashlight bulb rated at 250mA and maybe 3% efficiency
would only produce ~8mW of light.
The led version is a major improvement.
I developed and simulated the circuit using LTspice IV:
I highly recommend this SW. Its easy to learn and use,
a general purpose spice simulator, and a wonderful way to learn electronics.
It is multi platform now in both Windoze and OSX versions.
It also runs well on Linux with the use of Wine, my set up.