Well, Christmas is over again, and so it is time to buy cheap lights and modify them for next year 
I picked up a number of "candy cane" LED lights for a song, and I want to make them do more than they were designed to do.
The originals have 4 candy canes, each with 10 white LEDs in parallel, each cane paralleled (so 40 white LEDs in parallel), then a 22 ohm series resistor, and a switch connecting the string of LEDs to 4 AA batteries.
Using NiMH cells, I see (under load) a voltage of 5.31 volts across the battery, 2.91 across the LEDs, and therefore 2.4 volts across the resistor. The current drawn is measured at 104mA (calculated as 109mA from voltage drop across the resistor - the difference is probably due to the voltage drop across the meter). This gives an average LED current of 2.6mA.
The batteries live in a separate unit, and once the batteries are removed, there's heaps of room for more electronics
What I'm going to do is power them externally, and use a pic to make the lights a little more interesting.
Here are the things I'm thinking about...
1) What voltage should I run the device from? A higher voltage will give me lower current (good if I have long runs of cable) but I then may need some sort of regulation for the PIC's power.
2) Should I rewire the LEDs to be in series in each cane (that will require about 30 volts) or maybe series/parallel to use a lower voltage If so, where is the "sweet spot". Currently almost half the power is wasted in the series resistor. I would prefer to do better than that.
3) What about "smarts"? I want to be able to (at least) control each cane individually, but is there a smart way to control each LED in each cane? It might be really cool to be able to do that, but then you're back to driving each led individually...
4) And can I control them remotely? Can I inject a signal onto the power supply that can be used to send commands to each unit?
5) Can I do this all really cheaply -- or at least with the parts I have on hand?
6) Can I do it in a generic way that can be applied to other lights with minimal customisation of the controller?
7) Can I do it by next Christmas?
I realise that it will be hard for people to address some of my questions, because you don't know (for example) what parts I have on hand... But if anyone has any bright thoughts, I'll be very interested to hear them.
I picked up a number of "candy cane" LED lights for a song, and I want to make them do more than they were designed to do.
The originals have 4 candy canes, each with 10 white LEDs in parallel, each cane paralleled (so 40 white LEDs in parallel), then a 22 ohm series resistor, and a switch connecting the string of LEDs to 4 AA batteries.
Using NiMH cells, I see (under load) a voltage of 5.31 volts across the battery, 2.91 across the LEDs, and therefore 2.4 volts across the resistor. The current drawn is measured at 104mA (calculated as 109mA from voltage drop across the resistor - the difference is probably due to the voltage drop across the meter). This gives an average LED current of 2.6mA.
The batteries live in a separate unit, and once the batteries are removed, there's heaps of room for more electronics
What I'm going to do is power them externally, and use a pic to make the lights a little more interesting.
Here are the things I'm thinking about...
1) What voltage should I run the device from? A higher voltage will give me lower current (good if I have long runs of cable) but I then may need some sort of regulation for the PIC's power.
2) Should I rewire the LEDs to be in series in each cane (that will require about 30 volts) or maybe series/parallel to use a lower voltage If so, where is the "sweet spot". Currently almost half the power is wasted in the series resistor. I would prefer to do better than that.
3) What about "smarts"? I want to be able to (at least) control each cane individually, but is there a smart way to control each LED in each cane? It might be really cool to be able to do that, but then you're back to driving each led individually...
4) And can I control them remotely? Can I inject a signal onto the power supply that can be used to send commands to each unit?
5) Can I do this all really cheaply -- or at least with the parts I have on hand?
6) Can I do it in a generic way that can be applied to other lights with minimal customisation of the controller?
7) Can I do it by next Christmas?
I realise that it will be hard for people to address some of my questions, because you don't know (for example) what parts I have on hand... But if anyone has any bright thoughts, I'll be very interested to hear them.
