A tea break challenge to you

 For the purposes of clarity I have to reveal at this point I occasionally make dynamo-powered bicycle lights, and that the last two An Engineer in Wonderland entries are parts of this winter’s planned front light: the Mark V

bikelightmt470.jpg For the purposes of clarity I have to reveal at this point I occasionally make dynamo-powered bicycle lights, and that the last two An Engineer in Wonderland entries are parts of this winter’s planned front light: the Mark V

Safety note: Do not charge Li-ion cells below 0°C, as they can be damaged, and sometimes behave dangerously, unless you are sure the specific type you are working with allows this.

Eventually the whole thing will be linked up by a microcontroller, but as I have yet to finish testing a surprisingly large amount of code – I finally rewrote the whole thing using a state-machine rather than the rat’s nest of flags that graced the MkII.

So I need a nice simple hardware controller to road test the analogue bits now.

The criteria for input and output of the mystery box are:

Moving – led power comes separately from dynamo
Pulses at the input
Output low to hold off current source.

Stopped – led power needs to come from current source
Pulses stop, input stays high.
Output high to make current source feed led.

Parked – button push initiates micro-power shut down.
Input high
Output low

My solution is below.

This blank bit is in the interest of not spoiling your fun if you want to have a go without mental contamination.

If you can’t wait, and still have some curiosity, scroll down now.

Here is my solution – as yet untested – it uses half a hex Schmidt inverter chip and a whole lot of discretes.

bikelightfull470.jpg   I also have a version of the above using many discrete NPNs and PNPs, but my feeling is that there must be a much simpler answer, likely using a programmable unijunction transistor or some other wonder that most folks have forgotten about.
If not that, at least a circuit with fewer components.

If I have got it right, the circuit will work like this:

Schmidt S1 and the RC on its input acts as a movement detector.
S1 output goes immediately high when the NPN receives pulses indicating movement and discharges the capacitor through the diode.

Through the output capacitor, S1 flips the state of the flip-flop formed by the other two gates turning off the current source.

The RC time constant is long enough to keep S1’s input below its 66% switching threshold until pulses stop, a time constant of half a second should be fine.

When it does time-out, S1 output through the capacitor flops the flip-flop and turns the current on again.

The other diode is a belt-and-braces way to keep the flip-flop holding off the current source during motion, and the push-button acts on the same point to turn off the current source when parked.

A benefit of the circuit, I claim, is that nothing interesting happens if the button is pushed when the bike is parked so kids will get bored if they mess with it.



  1. I did something similar some years ago when I had a 6v dynamo powering light bulbs (remember those?). Here I wanted the rear light to stay on whenever I stopped anywhere. The solution had a bridge rectifier on the dynamo output with a smoothing capacitor of a few thousand microfarads. The rear bulb was connected across this capacitor. A 6v battery was also connected across the bulb, via a diode. When stationary the battery powered the bulb. When moving, the dynamo would contribute to the bulb current and when running fast enough would create enough voltage to cut off the battery. The smoothing capacitor is important here; if missing then the battery would conduct during the dips of each alternator cycle. The front light was connected directly across the dynamo as usual so this still switched off when the bike was stationary. The whole thing is not exactly efficient, but was simple.

  2. howdy “Alice”,
    Looking at the circuit and thinking about the ultimate goal (“don’t discharge the battery while the dynamo is running”), I had an idea that takes a different approach.
    First, I have to make some assumptions:
    #1. the dynamo is sourcing 0.5A, and ….
    #2. the battery is set up to source less than 0.5A. It’s #2 that may be mistaken….
    If the standlight is intended to run below 0.5A, is it possible that the LM334 will keep the battery from discharging as long as the dynamo is sourcing 0.5A? Of course, the power from the dynamo is rectified AC, so there is a loss of power every half cycle of the AC. A large cap might have to be connected across the LED to keep current flowing throughout the AC cycle.
    Let me know if my assumptions are wrong. In the meantime, I’ll see if I can think of a more conventional shut-off circuit.

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