Joule Thief Night Light Update

I plugged it in and measured voltages across some of the components using a super cheap volt meter. With black tape on the photoresistor (LEDs on): (all approximate)

• across projector LED: 2.8V
• across other LED: 2.7V
• across proposed joule thief location (see last post): 6.25V
• across 1Ohm resistor: 0.017V
• across 33Ohm resistor: 0.57V
• across photoresistor: 1MOhm

With the tape off (LEDs off):

• across proposed joule thief location: 1.1V
• across 33Ohm resistor: 0.6V
• across transistor (collector-emitter): 0.5V
• across photoresistor: 4kOhm, 0.43V

What this is telling me is that some sort of voltage regulation is happening. I think it has something to do with the zener diodes. The AC-DC converter looks a lot like typical capacitor (transformerless) DC power supply, but with zener diodes as part of the rectifier. Looking at the circuit diagram in last post, can anyone tell me how this power supply works?

I was close with the 5V guess. It's actually around 6.2V when the LEDs are on. But the problem is that this drops to 1.1V when the LEDs are off (light room). While that's good for power dissipation while off (estimating about 20mW using 33Ohm resistor current and 1.1V), it means that if I have a 6.25V joule thief hooked up to it in a light room, a lot of current will have to be dissipated by the 33Ohm resistor and the transistor, which is bad.

One (very non-elegant) work around would be to cut a trace, have a way to switch between JT or AC operation, have a transistor inline with the JT, and a tiny microcontroller with leads spliced across the photoresistor. In JT mode, the microcontroller would read the photoresistor and tell the JT transistor to turn on or off. In AC mode, the JT would be taken out of the circuit by a switch and it would operate as it was designed to. 

Another thing I might be able to do is splice off of the photoresistor-base transistor connection and use that to turn a transistor in the JT circuit on or off directly. 

I'll have to think about this some more. Figuring out how the power supply works would probably help. 

To do:

-learn how the power supply works

-design JT circuit for 6.25V and ~20mA

-figure out efficient way to integrate JT circuit 

I mapped out the circuit today. I discovered lots of free online circuit drawers, which was cool. I used the digikey one this time.

Stock Circuit. Note: there are two crossovers that look like 4-ways.
The only 4-ways that are actually 4-ways have dots.

For comparison. I have L1 and L2 flipped on my circuit diagram.

I attempted to lay it out in a similar manner to how the components are on the board (single layer PCB). I also attempted to understand what was going on. I am no EE though, so please correct me if I got it wrong. Starting from the red "+AC" line (left), there is a 150Ohm resistor, followed by a 250V film cap in parallel with a 1MOhm resistor (hidden under cap), followed by a shottky diode (forward) and a zener diode (reversed). These diodes are copied on the -AC line side. I think those components all make up the AC to DC converter, guessing 120VAC to approx. 5VDC. The DC side of the shottky diodes are linked and the DC side of the zener diodes are linked, giving the +5V and 0V references respectively. Following the +5V reference through a 33Ohm resistor (now about 3V), we get to the LED that makes the ball glow (the one labeled L1 on the board and L2 in my diagram). In series with that is a 1Ohm resistor and the projector LED, which then terminates into the 0V reference.

Now for the control part: Photoresistor resistance decreases with light and increases in dark. So if the room is lit up, power flows from the 5v reference node to through the low-resistance photoresistor, into the base of the transistor, which means it is on. I'm guessing the 20KOhm resistor in series to 0V with the photoresistor (with the transistor base tapped off between them) acts as a discharger or voltage divider, but I'm unsure. With the transistor on, the path that branches off between the 33Ohm resistor and the L2 (schematic) diode is now a low resistance path to ground, so all of the power (~0.25W?) goes that way instead of through the LEDs. If it is dark, the photoresistor resistance is high, causing the transistor to be off, preventing power to flow through it, allowing the LEDs to be powered. It's probably a fairly efficient circuit when it is on, but when it is off it is constantly bleeding some power (guessing about 0.25W) through the 33Ohm resistor and the transistor. Probably a good idea to unplug these when not in use. (see next post for update on this).

Modification plans: I want to add an option to power this using a Joule Thief circuit.

The "JT" voltage source represents the Joule Thief circuit. The switch will allow me to switch between AC and DC mode. It should be fairly safe this way. I'll probably put a diode in the JT circuit to prevent power from flowing across it in case I plug it in with the switch on. The good thing about this setup is that the photoresistor should still function normally in DC mode. However, the power drain when the transistor is on is concerning...it may kill the already partially dead AA's I'll be using for the JT circuit in a single day. We'll see. If that ends up being a problem, I could probably cut a trace somewhere and install another switch.

Challenges moving forward:

• I wasn't able to get exact values for many of the components, so I will have to use a scope/nice multimeter to figure out what voltages are where in the circuit. I'm not sure how I'll power it since everything will be exposed...I need some sort of low current 120VAC source.
• Knowing the voltage difference across the LED's will give me the JT design voltage.
• I want to figure out how much power the stock unit draws when plugged in (off and on).
• Designing the JT circuit (lots of internet tutorials).

The goal of this project is to modify a "Projectables" brand plug-in projection night light with a Joule thief so that partially dead AA batteries can be used to light it up, while maintaining plug-in functionality. Basically, I wanted an excuse to make a Joule thief and to use up a bunch of partially-dead AA's. You can buy these from Amazon, Home Depot, directly from the manufacturer, and other places. I have the solar system one that I picked up from Home Depot for $10.

You can see the light sensor (photoresistor) here

They work by shining an LED through a image and a lens, all of which is located in the globe, which can be rotated 360 degrees thanks to springy-metal contacts. The following pictures detail the take-apart process.

It's held together with three TA20 (triangle) right-hand threaded screws. These puzzled me for a bit because I figured they'd be standard threaded, but nope...they have reverse threads.

Lots of patents

Insides:

The LED under the globe is for making the globe glow.

Spring contacts allow for 360 degree rotation. Clever

 To disassemble it more, you have to take needle nose pliers and pull out the little gray piece of plastic holding the wall plug contacts in. Then you can push the wall plug contacts out.

Bottom: gray piece that holds in plug contacts. The clear piece is the photoresistor protector.

I'll map out the circuit later. It seems pretty simple. I want to be able to tie the Joule thief circuit into the two LED's. I'm hoping to be able to add a simple switch somewhere to allow me to switch between wall mode and battery mode.

Circuit

Popping open the globe (snaps together like one of those plastic Easter eggs) reveals the projector. It was a bit more complicated than I thought it would be and splits into a few pieces. Starting from right and moving left: lens, tube, solar system picture (the tabs keep the picture in place), lens, tube, LED.

The projector