Simple bicycle parking light

DISCLAIMER

As usually, we (especially I) will not be responsible for any damage to you, your ego, your stuff, tools, house or anything else you own (or not own).
The things decsribed on this page may be potentially harmful to you and/or your environment (including other people and/or animals).
It may be illegal to use the described circuit with your bike. This may depend on where you live and your local laws. You should know yourself (or at least find out) if you are allowed to use this circuit.
Thus, I'm not responsible for anything. YOU HAVE BEEN WARNED.

The problem

My "everyday bicycle" which I use to get to the train station and back home has a bottle dynamo. The problem is that whenever the lighting is needed most (e.g. on rainy, foggy or snowy days) it fails because the tyres get wet and the dynamo starts to slip.

The best way to solve this problem would be a hub dynamo. But they are quite expensive and difficult to install. A battery powered lighting system would have been the second alternative. I didn't want to use it, because the dynamo works fine 90% of the time.

The solution

I decided to use a battery pack and a small circuit which switches from battery power to dynamo power depending on the dynamo voltage. Additionally it opens the opportunity to add a parking light which I like for safety reasons because it improves your visibility if you have to wait at the crossroads.

The circuit

At the beginning I thought it would be nice to have a microcontroller controlled parking light with step-up and step-down converters which can charge a small lead battery and so on. Something like a full blown lighting system, like a car has. I didn't have the time to design and build it and there are not many advantages over a simple system. Because of the low efficiency of the bottle dynamo I wouldn't have used it to charge the battery anyway. I'd rather have taken it out and charged it in a normal battery charger. Therefore I decided to keep it simple:

The circuit is quite simple. D1 through D4 and the (external) capacitor rectify the AC voltage from the dynamo. You should use schottky rectifier diodes if possible, to reduce the voltage drop. A small relay (with a 4.5V DC coil) is used to switch between dynamo and battery powered. The NC contacts connect the output to the dynamo generated voltage. As long as the dynamo delivers power, C3 is charged via D6 and C2 via D5 and R5 delivers base current to Q2 which is robbed by Q1 because Q1 gets base current via R11. Once you stop, the dynamo voltage collapses and after about a quarter of a second C2 is empty, the current through R5 can flow into the base of Q2 which turns on Q4 and which in turn energizes the relay coil. The relay switches to battery mode. After about one or two minutes C3 has drained too and the relay switches back to dynamo mode (or off in this case, because we've stopped). An additional switch can be connected to the pinhead which is marked "REED". If this switch is closed, the circuit stays in battery mode for infinity time. I use this mode if the dynamo doesn't work at all. The resistors R2 and R6 set the dynamo voltage which is necessary to switch back to dynamo mode. They should be in the lower 10kOhm range. R4 should be in this range, too but it isn't critical. Adjust them to set the bike speed at which the circuit switches from battery to danymo mode. Concerning the storage capacitor, you should spend a large enough capacitor, because if you drive slow, the dynamo's output frequency will be quite low, about 20Hz or lower. Additionally a typical 3W front light will draw about 0.5A. Let's assume that we accept a voltage dropt of about 2V per half period (1/40s=25ms) we get C=I*t/deltaU = 0.5A*25ms/2V = 6250uF. I've put in just 3000uF and it works sufficiently well. That's the reason why the capacitor is "external". It would have enlarged the circuit board a lot.

I've mounted the circuit and the battery pack water proof in a gherkin jar which in turn is mounted on my bicycle's carrier. There are, of course, drawbacks of this simple circuit: Firstly, if you drive near the thereshold speed, the relay switches back and forth because the circuit has no hystereses. Secondly the light on dynamo power is a bit dimmer with the device installed, because of the approx. 1.4V diode loss of the rectifier. You should use shottky diodes like mentioned above, to minimize the effect. If you only need the light to be seen (and not to see yourself) like I, it isn't important if it's a bit brigther or darker. (In fact it has the hysteresis of the relay, but it isn't big enough.) This will of course shorten the lifetime of the relay, but I'll just try how long it keeps working. The battery pack is made of a piece of PVC pipe, wood and some custom made battery springs I made out of a piece of steel wire. I've stuffed some rags into it to prevent it from falling around in the jar.

As in almost any case you insert a bridge rectifier after the dynamo, you have to mount the dynamo isolated from the bicycle frame, because one terminal is usually connected to the screw. The way you do that is left to your own creativity, but I wanted to note this to prevent you from missing this subtle detail.

	The jar mounted in the back. The LED and the connectors are hot-glue into the lid and hopefully air (and water) tight.
	Battery springs. They are hand-made and required some nasty fumbling with stiff (and rusty) steel wire.
	Closeup shot of the jar without rags. You can see the PCB in the from and the "external" capacitors in the background.

As you can see, everything is more thrown than carfully mounted together. I'm curious how long it will last.

Addition 27.09.2009: The device has been mounted for about three quarters of a year and had no problems so far. I've used NiMH accumulators so far, but they are empty due to self discharge after some weeks because current is only drawn during stops which is about 1 minute per day in my case. For this reason I switched over to conventional non rechargeable batteries now. Let's see how long they last.

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Copyright (C) 2008, 2009 by Wiesner Thomas

Last change: Septemper 27th 2009