Getting closer
So here we are again. This time I'm trying to gate the LED drive directly with series pass transistors (Q5-Q6). It's sorta working, sorta not. Here's the schematic:
**broken link removed**
Sorry it seems such a mess. See the attached LTspice result display below. This shows at least three major problems:
- 1. A "filling-in" of the end of the multivibrator off cycle with increasing JT oscillations (more about this below).
- 2. An incomplete turning off of the JT output.
- 3. And what's with those high voltages?!?!? The peaks of the JT outputs appear to be at more than 60 volts! That's probably a little bit more than one wants to put across their LEDs.
I suspect that most, if not all, of these problems are caused by the RF from the JT (Joule thief) oscillators leaking into and interfering with the multivibrator, which runs at a much lower frequency. Actually, until the end of the "off" cycle, the multivibrator outputs (the 2 lower traces) look remarkable clean, the best I've been able to get out of this mash-up so far. The JTs appear to be running at somewhere in the neighborhood of 300kHz, so all those solid areas are 300kHz HF.
I've tried to block the RF from getting back into the multivibrator (MV) by using 100pF bypass caps. They helped, but as you can see they haven't eliminated the problem. In other attempts I put in series chokes to try to keep the HF out of the multivibrator, but couldn't get better results than here. Is this something I should do? or does my problem lie elsewhere?
The high voltages make absolutely no sense to me. The JTs, when run as stand-alone modules, generate spikes of maybe up to 10 or 12 volts; the average output is closer to 6 volts. How could a series transistor
increase the voltage so severely? (Hmm, maybe this is a "feature" that could be exploited for another circuit?) Now I'm kinda afraid to try to actually breadboard this circuit!
By the way, this took several
hours to simulate in LTspice. (.asc file attached below for your viewing pleasure.) The resulting .raw file was 3.3 gigabytes. The reason it runs so slow and took so long is no doubt because of all the HF oscillations it has to track, so no fault of LTspice here.
So if you have any ideas, I'd really appreciate any light you could shed on this situation. I feel I'm very close to getting this working.
Specifically, what I want to do is use the low-frequency output from the MV (1-5 Hz) to alternately enable/disable the two JTs. This could be done either by enabling the JT oscillator (as I
thought I was doing here), or by disabling or "quenching" it on the off-cycle. (I also hope not to waste too much energy when the JT is just sitting there idle.) Several possible methods have occurred to me:
- Gate the LED output as I've tried to do here
- Gate the JT collector
- Gate the base drive of the JT
- Inhibit the JT by injecting a voltage somewhere that'll "quench" it (???)
- ???