On Youtube, this guy named Xee2 or Xee2vids posts short videos of schematics, projects and circuits. There is no motion, just slides, and no narration, no voice in his videos. There is no way to comment to tell him if I’ve found a mistake. But so far, most of his vids are very good with very few errors. And most of the stuff is original and innovative. One project caught my eye, a blinking Joule Thief. The video gets to 35 seconds then freezes and won’t go any further, so I don’t know what the rest of it is about. But I got the schematic and now I’ve built on to see how it works.
It’s a simple circuit, with all the parts of a conventional JT and adds a 10uF capacitor to get the circuit to flash. I built it on a small perfboard but I’ve been having difficulties with it. I put a 68k resistor instead of the 100k resistor called for in the schematic. When I finished and powered it up, I got irregular flashes at about 2 per second. But I couldn’t find the irregularity in the flash rate. The schematic in his vid does not show a bypass capacitor across the 1.5V battery, so I installed a 10uF bypass capacitor. While I was doing that I removed the 68k and put in a 120k which I figure should be closer to 1 a second. When I finished and fired it up, nothing. Voltages look normal and I didn’t change anything since the first time it flashed. I’m puzzled why it’s not working. Maybe the heat from unsoldering then soldering it back has damaged something.
Try It Again I decided that it wasn’t worth troubleshooting the problem and built a second one, same as the first. I’ll go over a few things. These changes apply to both. I changed from the MPSA06 to a PN2222A, which is not that different. The original resistor was 100k, I changed to 120k, mainly because the flash rate was too high. Even with 120k the flash rate is still fast, about 2 per second. A better choice might be 180k or more. The capacitor was 10 uF, just like the original.
The Coil The original used a “1 inch core from Electronic Goldmine, 5 for $1.00”, which has been sold out for some time, and will no longer be available, since they sell mostly surplus and it’s extremely doubtful they will ever be able to find them surplus again. I still have some of these, so I wound 20 turns of telephone wire onto one core and measured the L and got 4.52 millihenrys. But this core is too large for a low power Joule Thief. Instead, I used a tiny 1/4 inch core from surplussales.com, which was about the same price. It is the (ICH) T231212T, available for $.25 apiece. As stated in the ad, they are high permeability, 5000 or more, so it doesn’t take many turns of wire to make a reasonable amount of L (inductance). I used 7 inches each of 28 and 30 AWG enameled wire, wound bifilar onto the core. The inductance was about 480 microhenrys, which is just fine for a Joule Thief. I used a yellow LED, mainly because a flashing white LED is totally boring – white LEDs are made for illumination – and I have a dozen Blue Blinky circuits madly flashing so I really don’t need another blue one.
The second one went together quicker than the first, mainly because I had the original to use as an example. The perfboard was only 6 holes by 6 holes, where the hole spacing was 0.1 inch. Everything fit nicely.
Test It Out I connected it to the power supply and it flashed, about ten flashes every 7 seconds. Still too fast. But what was disappointing was the light output. I could barely see the yellow flashes on the palm of my hand in a fully lit room. The light output is so dim I can look directly into the LED chip and clearly see it flashing. At this low current, the prediction he made about the AA cell should last for 8 years is very likely true.
But with the 120k and 10 uF, the flash rate was more than twice a second. I removed the 10 uF and put in a 33 uF, and it now flashed considerably slower, but still flashed at 1.4 flashes per second. The capacitor needs to be at least 47 uF to get down to close to 1 flash a second. In order to increase the light output. the capacitor must be much larger – he suggests 10k and 100 uF, but I think 100 uF is still too low. This brings up an important point. This resistor is the only source of base bias current for the transistor. The normal JT uses 1k, but here we have 100k, so it stands to reason that the base bias current is much less than a normal JT. This looks to me like the main reason the LED is so dim. The solution to more base current is a lower value resistor (the reason why he suggested using a 10k in the video). But the more base current, the faster the capacitor charges. So we have to increase the value of the capacitor to keep the flashes at about 1 second.
Conclusion I don’t see a future for this circuit, due to the low light output and need for a large, expensive capacitor. My Supercharged JT Flasher uses a 4.7uF cap, my Blue Blinky uses a 1 uF, and some other flasher circuits use similar sized capacitors. Also, we are back to a circuit that uses a single transistor, so the LED output, the frequency and length of the flash are all dependent on a single transistor and a few other parts; changing one may change the circuit in unexpected ways. Right now, I like the Blue Blinky circuit because it puts out bright but very short pulses, and does a respectable job of making the AA cell last a long time (2 years).
Back to experimenting…