Halloween is time to be Geeky
What is an Electronics Geek/Hobbyist supposed to do in Halloween? Bllinking lights and sound effects are the staple of the hobby, so there's plenty of 'opportunity' here... I keep in my lab a drawer full of old incandescent bulbs I got in bulk a few years back, along with a mix of electronic components bought on sale. Incandescent lights are quiqkly being replaced with more efficient LED tachnology, but what to do with these old devices? Can we at least use them once a year for a seasonal project without feeling guilty about the energy waste? What if I could use an off-the-shelf low-power electronic candle to drive multiple of these lamps inside plastic pumpkins... Hmmm, I think we have a project idea here!
Figure 1 - The Old Lamp Drawer
I had on hand a few of these cheap, battery operated "electronic" candles (Figure 2). These are the type that provide a dim, yellowish flickering light and that you can buy even on a dollar store. Opening the box revealed that the inside circuit was no more than a 3V button cell powering a "flickering" LED. Remarkably simple... (I was expecting there would be some circuit driving a "normal" LED). Instead, it seems like the LED itself is of the 'flickering', self-contained variety; no external circuitry needed. All I had to do then, was find a way to power this from a 12V supply (an old laptop "power brick") and drive the incandescent lamps with an amplified version of this flickering signal. Figure 2 shows the circuit I used.
Figure 2 - Electronic Candle
The circuit is quite simple. A 2.7K series resistor with the 12V supply powers the candle LED. The flickering signal as seen at the LED is coupled through a 47 uF capacitor to a simple one transistor amplifier built around the 2N3904 transistor. I used a high value for the coupling capacitor because the frequency of the "flicering" signal is rather low, so the 3dB high-pass corner needs to be farily low. An IRF540 N-Channel power MOSFET is then used to drive the lamps. Though the circuit only shows one lamp, I actually used two in parallel and more are possble (only limited by the power supply and FET maximum current, which is a whopping 33 A, though a heatsink may be needed at that stage). Even though the two lamps I used consume a total of 1A peak, the IRF540N remains quite cool and no heatsink was needed. The on-resistance for this MOSFET is quite low (Ron < 0.044 Ohm @ VGS = 10V) so that very little power is dissipated in the device. Note that the gain of the first stage 2N3904 amplifier is such that the MOSFET is mostly fully ON or fully OFF, so it doesn't operate in a high Ron region.
Putting it all together
I assembled the circuit in a simple prototype pcb and plastic box (both bought at Radio Shack). Figures 4 and 5 show the finished "product". Note that the power enters the box through the "right side" of Figure 5, though it was not connected yet at the time I took this photograph. Figure 6 shows how I used a plastic bottle cap to mount the lamp on the bottom of the pumpkins. A generous amount of hot-glue keeps things together. If this was a more permanent project, I might have opted for a metal mount, but this method works fine and is simpler. Make sure that the glass doesn't touch any parts that might melt. Incandescent lamps get quite warm...
Figure 4 - The PCB Assembly
Figure 5 - PCB and Enclosure
Figure 6 - Mounting the Lamps
Figure 6 and 7 shows the two pumpkins, happily flickering in my living room table. Notice the control box to the right of Figure 6... This is a fun project to build in one of those cold Fall afternoons. Hope you enjoyed it, and happy Halloween!
Figure 6 - Pumpkins and Control Box
Figure 7 - Happy Halloween
Comments, questions, suggestions? You can reach me at: contact (at sign) paulorenato (dot) com