I decided to see if the Planet Bike Blaze 1W Headlight had some PWM going on behind the scenes. It seemed to me that the logic analyzer would be perfect for this task. I was actually surprised by the results. One thing that I did decide on was using different test equipment in succession so nothing got smoked.
First test was using the voltmeter. It could handle it, since the headlight was powered by 2 AA batteries in series, aka 3vdc. The voltmeter read 2.78vdc at the battery terminals of the source pack. Next I tested the low beam voltage, it read 2.65vdc. The high beam was 2.98vdc, interesting since the source pack was lower. Finally, the strobe beam measured in at 2.3vdc. Nothing fantastic here.
Next up was my Propscope (USB Oscilloscope made by Parallax). It confirmed that the off voltage was 0vdc and the high beam was 2.98vdc. However, the Low and Strobe beams had more interesting wave forms.
The low beam was a 164hz square wave 50% duty cycle. The artifacts in the square wave show some RC circuit in the mix. One other thing to note is the off portion of the duty isn’t really off, it hovers just above 2vdc.
The strobe beam revealed some interesting on off blocks. There are 2 long strobes followed by 4 short strobes. The pulse width following each short strobe was 100ms. The pulse width between the rising edge of the first long strobe and the falling edge of the second long strobe was 200ms. It total time for each strobe loop measured 715ms. There is considerable amounts of noise that appears in the off state. Also, the short strobes are not even.
Zooming in to the short strobe showed that they are actually comprised of 3 pulses measuring 1.93ms each that are gaped with two 787us of off cycle. The total on time of the short strobe is 7.36ms. The low levels show a sawtooth pattern typical of a RC circuit. I’m assuming that this is some kind of charge pump to maintain the voltage level on the LED.
It was a let down to find nothing detected on the logic probe. I had to cycle the headlight on and off just to get this measurement. I assumed that it would at least get the pulses from the strobe. The only thing I can conclude is that the logic probe needed a true low, not something hovering around 2vdc. It didn’t seem practical to add a bias just to get the logic probe to show the same thing the oscilloscope already had.