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This is the first real life circuit I’ve ever made (well, except for a boring ordinary astable multivibrator or two some time in the past that slowly blinked a couple red LEDs).
So, it all started when I replaced an LED bulb because it was flickering annoyingly and then decided to pry it apart to see what was inside. Within it I found a delightful set of 40 yellow colored LEDs (it’s probably the phosphorescent material in the white LED that makes them look yellow), 10 in series and 4 of those in parallel. Within each yellow LED appeared to be three LED junctions in series, so there’s actually 120 LED junctions in there. After marveling at it for a while I decided to pry further and pulled out the piece of plastic the LEDs were attached to which revealed a PCB within with what appeared to be a few capacitors and a coil (either an inductor or a transformer), but I haven’t yet been able to pull the PCB out for further examination. Now that I had my hands on a loose plate of plastic with 40 LEDs in it, I decided to have some fun turning it on.
Fortunately, I happened to have an Electronic Playground which had 7 resistors, a potentiometer, 2 red LEDs, 4 capacitors, 3 NPN transistors, a diode, a 10:1 transformer (300mH primary, 3mH secondary), a speaker, a pushbutton, a variable capacitor (for radio frequencies, unspecified capacitance), a ferrite antenna (unspecified inductance), and finally a 9V battery holder, all packaged with spring wire connectors and a bunch of loose insulated wires with stripped ends.
First I connected the 9V battery to a single one of the 40 LEDs through a small resistor and measured 7-8V (after replacing the battery on my multimeter which was reading a voltage higher than that of the 9V battery, and climbing too as the multimeter’s battery continued to decline). This large voltage is due to the presence of not one but three junctions wired in series in the LED, so the junction voltage is about 2.5V.
In an attempt to drive the whole array (which needed some 70-80V) I connected the pushbutton, 300mH coil, and a small resistor in series with the power supply and the LED array in parallel with the coil (after double checking polarity so as not to fry my precious new toy). Now it flashed whenever I let go of the pushbutton, but that’s not really any good unless I want to grow some serious endurance in my finger muscles. Next was an attempt at a joule thief, but I didn’t remember well how to do that and I didn’t have my iPad on hand at the time so it didn’t end up working, although after putting the same circuit into EC as I remember implementing it it did work in the simulator so I don’t know what happened with the failed attempt.
After that I searched through the Electronic Playground manual for an astable multivibrator circuit and then attached an extra transistor to take the place of the pushbutton in the original inductor circuit. Now it flashed periodically, but the 10μF and 100μF capacitors were way too big. I wanted it faster, much faster, so it would appear to be on continuously. Fortunately the other two capacitors were 4.7nF and 47nF, so I swapped the big ones out for the little ones and enjoyed a lit up LED array. Some tweaking and a bit of discovery later, we now have the final version shown here, complete with a diode to save the transistor if the wires connecting to the LED array accidentally touch each other (without the diode the transistor got very hot very fast). Unfortunately, as you may have noticed, the circuit does not oscillate particularly stably in EC (EC seems to have trouble with astable multivibrators), but I have no reason to believe that there is any instability in the real circuit.
With this final circuit, I can get it to light the whole array fairly brightly (nowhere as bright as the original bulb, but this is just a boost converted 9V battery, not a 120V mains power supply). By adjusting the potentiometer I can change the behavior of the circuit. Adjusting it over one half of the potentiometer dial adjusts the brightness with the far end being fully off, and the power draw adjusts accordingly, although lower brightness leads to lower efficiency as the multivibrator part has a constant power draw. The other half over the dial has a more or less constant brightness with the far end being only slightly dimmer than the middle, but the further it was towards the end the more power it drew. Therefore, the sweet spot is right in the middle and it can be adjusted by hand to maximize brightness and efficiency.
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