USB controlled mini lava lamps

Step 5 Make circuit

The heart of this project is simply using software to toggle the keyboard LEDs, and hijacking the LEDs to switch larger currents. I should mention here that some LEDs are switched on by applying a positive voltage to the anode, while other circuits (like the one in this keyboard) float the LED anode high (so the LED is off) and tie the cathode of the LED to ground when it is activated (pull-down switching). You could modify this circuit to work with either, but I will diagram the negative tie-down method. The LED being turned off (set high) turns on a transistor switch, which closes a relay, and finally the lava lamp is activated. This means that the software toggle for the lamp will be reversed from the LED. I adapted the circuit from windmeadow, but it is based on a simple transistor switch as described in most electronics texts (p. 50 in Mimm's book, or on wikipedia). I have included a ktechlab file so you can see how the transistor switch works, and a full schematic (in svg and png formats). Be sure to use the original schematic or the svn file for printing out, as the smaller images are not clear.

You will need to remove the LEDs from the keyboard circuit board and determine their polarity and which side is switched with a multimeter. Run one wire from each LED that will be involved in a switching circuit (these are labeled SCROLLLOCK and CAPSLOCK on the schematic). Desolder the +5v and ground from the keyboard circuit to provide power to the perfboard, and then run wires to return power to the board (this is the +5V and grounds on the schematic). Then follow the schematic to make the circuit, using the pinouts of the components to identify connections. Assemble this circuit on a breadboard first, and test the circuit. This was the step where I determined that the LEDs were pull-up instead of pull-down switched, and generally reached the point of maximum chaos on my workbench. When the circuit works (testable by hitting the Caps Lock Key or Scroll Lock Key and seeing if the lava lamps toggle), figure out how to pack the components onto the perfboard. Wirewrap can be used, but the simplest assembly method is to simply shove components through holes, wrap the leads together, and solder them together. Test that the assembled circuit works on the perfboard.