Hello everyone! I’m, Marty, the other half of Lumen Jewelry. I do the design and testing of the circuits used in our jewelry. Today I’ll discuss how the Skull circuit was made. The Skull is the second circuit I developed for Lumen, and the basis for the Owl and Robot in our kickstarter campaign as well.
First off, the tools we use. For schematic capture we use TinyCAD. It’s free, easy to use, and generates netlist files for the PCB layout package. For PCB layout we use Free PCB. It’s also free, easy to use, and all the keyboard shortcuts really speed up the manual layout of a PCB. After a PCB has been laid out, I’ll then use a Gerber file viewer or a service like FreeDFM to double check that the PCB will look like what I want. Finally, BatchPCB and other sites allow easy ordering of prototypes.
And now for some techno-babble. The Skull is our lowest voltage circuit running from 0.8 to 1.5 volts. This low voltage operation is important to minimize the number of series solar-cells. S1, S3-5 are single cell solar cells or large area photo-diodes. (BPW34 photo-diodes in this case) They provide the power to run the circuit. D1 is a schottky diode, it’s function is to keep the solar cells from consuming power in the dark. C4, C2, and switch S2 are the main energy storage system. The switch lets you save the charge in super-capacitor C4 for later, or shut off the piece in the dark. U1 is a standard CMOS dual schmitt trigger inverter used with a supply voltage way below it’s specifications. I’ve only seen 74LVC2G14 parts in small and ultra-ridiculously small surface mount packages, a DIP/SOIC package 74HC14 can be used instead if the inputs to the four extra inverters are grounded or used for something else. Combined with R2 and C5, U1 becomes a low power relaxation oscillator and is the beating heart of the piece. The frequency of oscillation is set by the time constant of R2 and C5, it oscillates at about one half hertz as shown.
C1, D2, and D4 form one of two charge pumps to double the voltage available to drive LED D4. (C3 D3 and D6 form the other charge pump.) The charge pump makes a short bright flash whenever U1 switches, and draws no current otherwise. This keeps power consumption low and maximizes dazzle. D4 and D6 flash one after the other. LEDs D4 and D6 are high brightness LEDs, worth it because the brightness gain versus cheaper LEDs is easily seen. R1 and micro-USB-B connector A1 allow for quick charging using USB, leave these two parts off if quick-charging isn’t required. (A1’s pin-out also works with mini-USB-B connectors) R1 limits the charge current from USB, while the LED drive circuits limit the maximum charge voltage to a safe level for C4.
Oh so simple. 😉 U huh, right. Have any questions, please ask, we’re happy to help.