Author Options:

Joule Thief w/ 555 timer Answered

Hello, I have a question about using a 555 timer with a joule thief. I'm trying to build a circuit for some solar powered bike lights but I have a couple of questions...

1. Am I correct in assuming that it's a bad idea to try to run a TLC555 timer chip from the output of the joule thief? Without a load, it seemed to be pumping out at almost 80 volts.

2. I tried building the circuit so that the two rechargeable 1.2V batteries would power the 555 directly, and then the output would sink and source two transistors alternately, which in turn would switch the LEDs alternately, using the output from the joule thief, which was also powered directly from the battery. That resulted in no LEDs lit and some very hot transistors... Bad idea.

The TLC555 chip is supposed to run on a supply voltage of 2-18V, but when I tested it with the 2.4V coming from the batteries, it would source, but not sink...

Is there any way to build this circuit so the 555 could blink 2 LEDs alternately while the LEDs are powered by the joule thief?

The only alternative I can see that might work would be to power the joule thief from the output of the 555 and just blink all of the LEDs at the same time from the joule thief's output.

Any help would be appreciated!


1. The circuit on the breadboard. The MVB is on the left, and the JT is on the right. 2. Front bike reflector with white LEDs installed. 3. Back of the reflector. 4. Plastic project case with solar panels attached (with ugly hot glue) More to come...

BikeLight 005 (Small).jpgBikeLight 002 (Small).jpgBikeLight 003 (Small).jpgBikeLight 007 (Small).jpg

acmefixer, Thank you SO much for that information! I built the circuits the way you suggested and it worked amazingly well on my first try. I've seen the MVB schematic before, but I didn't know how to incorporate it into the plan with the JT. I got most of the parts from Radio Shack, so I had to use a MPS2222A transistor, which according to the book in the store is the same as a BC337-25. I also had to use the 1N4148 diode because they didn't have any 1N5817s. For the MVB, I used 10K resistors with the 10uf caps so they blink pretty fast (the only reason I wanted blinking in the first place was to run twice the amount of LEDs at 50% of the cycle each for the front and back ones. Instead of one LED for each side of the MVB, I have a group of two white ones for the front and three red ones for the back, and they seem to be balanced out pretty well as far as power goes. Is there any advantage to which side of the transistor you put the LEDs? I have them the way the schematic shows because it seemed to fit my layout better. The next step is to connect the solar panels. I'm using the solar panels from two yard lights, and the included batteries. The batteries are in series, so (correct me if I'm wrong) I want to hook the panels up in series with each other, and then in parallel with the battery (including a diode to prevent leakage)? Also, if I'm using the NiCd batteries that came with the yard lights, and the solar panels are always connected to them (independent of the rest of the circuit), will there be a problem with the memory effect in the batteries? Should I switch to NiMH instead? I'm going to take some pictures and show you the progress :)

Personally, no idea, but if you could add diagrams or photos of what you've actually done, other people will be able to give a lot more help.