This is a continuation of the earlier blog. I wanted to change two parameters, the flash rate and the peak LED current. To do these, I experimented with the following.
Flash Rate
I built a sixth one (or seventh – I’m beginning to lose track, even though I labeled them). I changed the C1 capacitor from 1 uF to 0.1 uF. I tried it, but the flash rate was a bit too rapid. So I drilled two holes next to the 0.1 uF and soldered a 0.22 uF cap in parallel. The total was then about 0.32 uF, and the flash rate slowed down to just a bit less than 3 per second. This was about where I wanted the flash rate to be. R1 was 1.5 Meg, but I could have increased it to slow down the flash rate. I’ve used up to 2 Meg so far with no problems.
LED Current
When I pulled the parts, I changed R3 and R4 from 330 to 120 ohms. The flashes were going to be faster, so there was effectively less current per flash, given that C2 was the same value. So in order to charge up C2 faster, I chose 120 ohms.
When I reduced the R3 and R4, the Q2 switching transistor was now handling more current. So first off I used a BC338, which I know from experiments handles high Joule Thief currents very well. Then I tried a second BC338 in parallel with the first, and it seemed brighter, so I drilled three holes next to the original transistor and soldered the BC338 in parallel permanently with the original.
Results
The results look good. The flash rate is about 3 per second and the flash is very bright. I thought about lifting the base of the second BC338 and giving it its own 1k resistor, so the two transistors were not sharing the current from the original 1k. This would double the base current and cause both transistors to be even more saturated, with less voltage drop from collector to emitter. I was thinking that the amount of improvement would be several hundredths to a few tenths of a volt, which isn’t very much, so I haven’t done it yet. I think I’ll first try adding a second 1k in parallel with the first, to see if it helps. Maybe I can look at the collector voltage with the o’scope and see if there is any difference. I may try some heavier duty transistors such as the SS8050 or maybe the ZTX1048A that I purchased recently. I have a few other ideas that I may try to implement. One of these is to switch R4 to a 1N5817 Schottky diode. The one problem I see is that with resistors, I could change the battery to two cells, especially two 1.2V Ni-MH rechargeable cells. But if the diode is there, the higher voltage would cause the LED and diode to conduct heavy current with overheating and possible damage.
Another flasher
I found a similar flasher on TE, but it uses only two transistors. Scroll down to the figure that says “1.5V LED Flasher circuit-2”. This gives similar performance to the Bowden’s Flasher, but it uses only two transistors. Fewer transistors means less gain and better stability. I’ve built a few of these and the flash is very bright, and there is no problem with the stability. I measured the average current and it was about a half milliamp, which is ten times higher than the Pigeonsnest.co.uk flasher, but a bit less than the Bowden flasher. This is a point where there is a good compromise between the flash brightness and the battery lifetime, which should be many months. After comparing this circuit with the flasher from Bowden, I would say that since this one is simpler and shows no sign of instability, I would recommend it over the Bowden flasher to the experimenter who wants something to work right off the bench.
Update Apr 13 – I checked the schematic on Bowden’s Hobby Circuits website and it still hasn’t been changed. I still haven’t heard from Mr. Bowden and it’s been more than a month.
Back to experimenting…
