Step 4: Circuit overview & parts selection
- up to 2 high-power leds for front headlight and rear blinker
- run on 2 rechargeable batteries with full light output
- thru-hole parts only
- simplest possible circuit with no microcontroller or programming
perhaps the most surprising thing you will notice here, particularly if you've looked at my other power led projects is that this circuit runs the led's directly off the battery, with no regulator or current limit at all! not even resistors! (unless you add the dimming resistors to save power).
how is that possible?! here's why: normally we strongly recommend against running led's without any current regulation because the led's resistance changes when it heats up. normally we run led's near their maximum power rating for obvious reason - to get the most light out of them. in that situation we are right at the edge of failure if the led heats up a bit and we need a regulator for safety.
this circuit is different! it intentionally under-powers the led, keeping it well within a safe operating range even if it heats up a bit. doesn't that mean we aren't getting the full brightnes potential from the led? yes! in fact that is better for the bike blinker because at full power your battery would last only an hour or two with a 3-watt led. it pretty much boils down to this: the cost of a luxeon 3-watt led ($4) running under-powered at about 1-watt is much less than the cost and complexity of a regulator circuit to go with a 1-watt led. so we just use a 3-watt led at way less than its spec, it still makes tons of light (in fact, it is more efficient than a 1-watt led), and no regulator to deal with.
this no-regulator method is a bit of a hack, in that it is fairly specific to the Luxeon 3-watt LED and the 2.4V from the 2 rechargeable batteries - which just happens to be the right voltage to power the led's at about 1-watt. if you use a different type of led's you will need to check their power use at 2.4V but otherwise this simple method works really well. the led brightness will vary a bit as the battery goes down, but then all the cheap commercial bike lights do that anyway.
also, because we are running right off the battery we can't use white led's in front since they need 3.2V at least. yellow is good enough for me!
there are not a lot of choices for through-hole parts that run on less than 2.4V. if you want to make the same circuit with surface-mount parts, you've got plenty of choices.
the 555 timer is a classic component that can produce a square wave output with any frequency and duty cycle. for a blinking safety light, the desired blink rate is 7 Hz, with about a 33% duty cycle (the light blinks 7 times per second, and is on 1/3 of the time and off 2/3 of the time). these numbers are optimized for the response of the human visual processing system to produce maximum awareness with the least light / battery power.
if you have a 555 already you need to check that it is a low-voltage 555C type - many 555's won't work at 2.4V.
there are many good 555 tutorials on the web, the linked one includes a calculator for how to choose R1, R2 and C1, which together set the blink rate and duty cycle of the 555 output. you can use quite a variety of values for R1, R2 and C1 other than the values i used, as long as the ratios between them are still correct. for example, you can use two 680k-ohm resistors with a 1uF capacitor instead.
the Q1 transistor is an ultra-low-threshold PFET, able to switch the full led power on and off with as little as 2.0V drive.
the D1 diode is used to drop the 3.0V from 2 x alkaline batteries to match the 2.4V from rechargeables. to support both rechargeables and alkalines at once, we can use an on-off-on toggle switch for SW1 to choose between diode or no-diode. it's a simple hack, as long as you don't leave it running in the wrong position no problem - quickly testing both settings when you turn it on is no danger. or you can have a super mega-bright blinker with alkalines and no diode, although the led's may need a heatsink then and the batteries will last a very short time.
the R3 & R4 resistors are used if you want to have longer battery life - such as with AAA batteries, or with a front headlight that doesn't blink (and therefore uses 3x the power of a blinking light). typical current for the luxeon 3-watt led's direct from 2.4V is about 500mA for the red led and 400mA for the yellow.
for blinking LED we blink with 1/3 duty cycle: 1/3 the current. new rechargeable AA's have about 2500mAh capacity, for about 15 hours with 1 blinking LED with no resistor - no problem. AAA's have only about 900mAh, so with no resistor you'd be at 6 hours with 1 blinking LED.
for the non-blinking headlight you need a resistor to get decent battery life.
use a 0.5 ohm resistor to reduce current to about 300mA, 1.0 ohm for about 200mA, 2.2 ohm for about 125mA.
The circuit is drawn with all options, just ignore the bits you aren't using. if you want a front headlight connect point A to point C. for a front blinker connect point A to point B. or use a switch and you can have both (in which case, you may want 2 R4's for headlight and blinking modes).