|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.|
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 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.
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).