Daylight Visible Bike Light - Front/Rear Combo - 100 Lumens on 2 AA's




Introduction: Daylight Visible Bike Light - Front/Rear Combo - 100 Lumens on 2 AA's

About: Dan Goldwater is a co-founder of Instructables. Currently he operates MonkeyLectric where he develops revolutionary bike lighting products.
The Ultimate Combo Bike Safety Light - It's a front/rear combo light with two 3-watt high-power LED's, all powered by just two rechargeable AA's. No heavier than your old setup but 10x brighter! All for only $20, and you can re-use the case from your oldest crappiest bike blinker to make this one!

- Daylight Visible!!
- Combo front headlight & rear blinker, or front/rear blinkers - 100 lumen total
- Full 360 degree safety light visibility
- Full brightness with rechargeable batteries (with option for alkaline use)
- Just 2 AA batteries power both lights (with option for AAA's)
- 8 hour runtime (with option for longer runtimes)
- fully waterproof and durable
- Total parts cost: $20
- Option for rear blinker only
- Simple electronics project using a proto-board and easy-to-solder parts.

See it in action! Side-by-side comparison videos below showing (on the right side) a top of the line commercial tail-light with 10 LED's, the Cateye TL-LD1000, and on the left side - the ultimate bike blinker front/rear combo.

close-up walkaround:


Step 1: What You Need

you will need the following stuffs to make the ultimate rear-only "AA battery" blinker:
(parts appearing in the circuit schematic are noted)

- old bike light, providing a good waterproof battery holder and bike mount
- LED1: Luxeon 3-watt red/orange high power led (part# LXHL-LH3C)
- L2optics 5x20 lens (OP-520)
- L2optics lens mount (OH-S35)
- prototyping board (such as: schmartboard 201 or vector V2018)
- SW1: waterproof on-off switch (such as: E-Switch 100AWSP1)
- cmos 555 timer chip (such as: TI TLC555CP)
- Q1: ultra low threshold PFET (such as: Fairchild NDP6020P)
- R1, R2: 2 x 6800 ohm resistors (such as: Xicon 291 series)
- C1: 10uF capacitor (such as: Xicon 140-SRL series)
- 18-22 gauge stranded wire, 2 feet
- 22 gauge solid wire, 2 feet

additional parts for the front light option:
- LED2: Luxeon 3-watt yellow high power led (LXHL-LL3C)
- L2optics lens (OP-520)
- L2optics lens mount (OH-S35)
- 2 pairs of spade-lug crimp connectors (or other connector)
- R4: 1-ohm, 1-watt resistor (such as: Xicon 294 series)
- handlebar mount from old bike blinker, or hack something together.

additional parts for "AAA battery" option:
- R3, R4: 1-ohm and 2.2ohm, 1-watt resistors (such as: Xicon 294 series)

additional parts for alkaline battery option:
- D1: 1-amp standard diode (such as: 1N4001 with a DO-41 package)

additional parts for both alkaline & rechargeable battery compatibility option:
- D1: 1-amp standard diode (such as: 1N4001 with a DO-41 package)
- exchange the SW1 on-off switch for an "on-off-on" SPDT toggle switch (E-Switch 100AWSP3)

where to get the parts:

LED's & lenses: the cheapest place to get these ($3.50 per LED and $1 for lens and lens mount) is from future electronics also see here - the 2nd link is a direct search for the LED's and buckpucks. for the lenses, here is the direct search to find them. You can also get the LED's from or in Europe, dunno if they have lenses though.

All the other electronics are available from
normally I use digikey but mouser was one of the only places stocking the Q1 transistor needed, so all the other part numbers above are available at mouser.

Step 2: How Bright Is It? How About the Side Visibility?

it's wicked bright!

we run the 3-watt led's at a conservative 1.5W (see later steps regarding the why's of this).

the red/orange led is rated 190 lumens at 4.5W, so we get about 60 lumens out of it. the yellow led is rated 110 lumens at 4.5W, so we get about 40 lumens out of it. actually we get a bit more than that since the led's are more efficient at lower power draws. that's a lot of light!

total for the front+rear combo: 100 lumens minimum! that is probably 5x more than any other bike safety light on the market.

on top of the LED we use a quality 5 degree x 20 degree lens that focuses all the light at car level instead of at the ground or up in the air. the side-visibility even with this lens is still much brighter than anything else out there. just the 5% of light scattered to the sides by the lens is enough to give great side-on visibility. and keep in mind that a '20 degree' lens still has plenty of light falling outside the 20-degree range, that is actually just where the light is 50% as bright as the center. take a look at the videos to judge for yourself! the comparison Cat-eye light has 6 rear-facing led's and 4 side-facing led's - most bike lights don't have ANY side-facing led's, this comparison is against the top-of-the-line. and crushes it, of course.

Step 3: Building Overview & Options

this is a fairly simple electronics project, but probably a bit much to be your first electronics project. i am assuming you already know how to solder, and that you understand how to use a proto-board, that you know how to identify electronic components. if not, there are tutorials on the web about how to use proto-boards and such.

the simplest possible build is a rear-only blinker, but once you've got that far it is very easy to add the front light or other options also.

simplest: ultimate rear safety blinker with 3-watt blinking led, AA rechargeable batteries

- option 1: add 3-watt front headlight (two bike lights in one!)
- option 2: front blinker instead of headlight, or front blinker/headlight toggle
- option 3: change to alkaline batteries
- option 4: support both alkaline and rechargeable batteries
- option 5: change to AAA batteries

you can pick and choose any options you want!

i built mine with a front blinker and support for both rechargeable and alkaline batteries, and i'll point out the places where other options differ in the steps.

the parts list tells you which parts you need for each option, and refer to the schematic as well, eliminating any parts that you don't need for the light you are building. in the schematic, D1, LED2, R3 and R4 may not be needed depending on your options.

front headlight or front blinker?

this is personal preference. blinkers are a lot more effective for getting you noticed, on the other hand a headlight has obvious advantage if you want to bike somewhere without streetlights. it is a trivial circuit change to have either one, or to add a switch to have both. in the circuit diagram, you wire point A to C to have a headlight, and A to B to have a front blinker. use an SPDT on-off-on switch and then you can have either one and turn off the front too. i built the the front blinker but i may try switching to see how i like them - the front blinker is so bright everything reflective in front of you for 100 yards will blink back at you, you may find that annoying.

the REAL simplest project: if you get stuck, maybe just scrap the electronics and have a super-bright non-blinking light? all you need then is a switch and battery, and use maybe a 1-ohm resistor to save battery life.

Step 4: Circuit Overview & Parts Selection

the design goals of the circuit are:

- 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.

resistor selection

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).

Step 5: Picking a Re-used Old Crappy Bike Light & Removing Its Guts!

your re-used old bike light housing is going to provide a nice waterproof home for your batteries, and a convenient frame mount for the light. the main thing when choosing your old bike light is whether it holds AA or AAA batteries, and how long you want your batteries to last.

i recommend using AA batteries so that you can have full brightness and still get decent battery life, however it is getting hard to find AA bike light cases these days. if you use AAA, you will get 4-5 hours for a rear-only light, 2 to 2.5 hours for the dual light. so the AAA build-option includes resistors to reduce the brightness a bit and increase battery life, but if you are ok with the short life you can still get full brightness with AAA's.

you will need a bike light that has an easy pop-off cover exposing the battery connections - virtually all of them are like this.

start by opening it up and removing the guts - including desoldering the battery wires.

now you can see how much space is inside the case and figure out where you will be mounting the control circuit you will be building. all my cases were too small to fit the circuit, so probably yours will be too. that doesn't make it any harder to build, it just means we'll be gluing the circuit to the outside of the case where you can show it to your friends more easily.

Step 6: Cut the Proto Board

cut with hacksaw into about 1" x 1" square for this project. or maybe a bit more if you dont want to be crammed (take a look at the final assembly i did on the 1x1 board and if you want a bit more legroom now is your chance)

or if you've figured out how you might fit everything inside your case, cut your proto board accordingly.

Step 7: Start Adding Parts

first attach the 555 chip. pin 1 is in lower left corner. then the two resistors

on the proto-board i used it is really hard to tell where the rows & columns of connected pads are! i will try to use a much more obvious proto-board next time. with this one all the columns of pads are connected, with a break at the row underneath the 555. anyway be careful that you put the 555 straddling the break in the columns so its legs are not shorted together.

  • if you have never used a proto-board before, first get the basic circuit working on your desk with a solder-less proto board and then switch to the soldered one. basic instructions on proto-boards are on the web, such as here:

Step 8: Add More Parts

add C1 first.

before adding Q1, glue a little plastic insulator onto the proto-board where the tab of Q1 will go. Q1's tab is live, you don't want it accidentally touching something.

then attach Q1, and glue the tab down to the plastic insulator.

if you are using the dimming resistors R3 & R4 you can add those also to pin 2 of Q1.

Step 9: Add Some Connections

the first image shows 3 connections all circuits need.

the 2nd image shows stuff for various options.

the 3rd image shows some more connections needed by all circuits

R3, R4 connections: if you are doing the front headlamp, connect one end of R4 to pin 8 of the 555 (next to the diode and wire shown). if you are doing the front blinker, connect R4 if used to pin 2 of the PFET. R3 always goes to pin 2 of the PFET if used.

Step 10: Finish Up the Control Circuit

glue the switch to the top of the 555 chip to keep it in place.

solder the power input wires to the switch as shown, or only one of them if you are not doing the multi-battery-type option. with the multi-battery option we will just toggle the switch one way to turn it on with rechargeables and the other way to turn it on with alkalines. very much a hack.

add wires for the common ground and the led positive output (1 wire for blinking output and 1 wire for non-blinking output). if you are using the R3 and R4 dimming resistors, the led positive outputs are at the other leg of those resistors.

i initially built with only blinking led's so i 've only got the 2 wires shown.

Step 11: Build Up the Led & Lens Assembly

- first glue the led to the center middle of the housing. if you are using the 5 degree x 20 degree lens, the orientation of the led is critical because the lens mount only attaches in one orientation.and you need the wide beam of the lens parallel to the ground. the led should be as shown with the led legs oriented vertically.

- drill a hole next to the led to route the wires through, use 18-22 gauge durable flexible wires 6-8 inches long. these wires will flex every time you change the battery.

- solder flexible stranded wires to the led, then check that the lens mount still fits properly on the led, if your solder blobs are too large the mount will not sit flat

- route cable through the hole, then simultaneously glue down the wires and glue on the lens mount (without lens inserted)

- probably want to leave the lens out of the mount until you are all done.

Step 12: Assemble Front Led Assembly

if you are making the front/rear combo light:

there's plenty of easy ways you can mount the front light - i used just the handlebar mount from an old headlight and glued the led straight to it.

otherwise wiring and lens mount similar to the rear light.

NOTE! in the 1st to images i have the led oriented 90 degrees incorrectly for the 5x20 lens, don't do it that way or you'll end up re-doing it like i had to.

use about 5 feet of 22 gauge flexible stranded wire, you can route this along your brake cable to the frame, then around the top tube until you get to the seat.

i used a couple of cheap automotive spade connectors on the end of the front led wires to make my front/rear detachable, you probably want to do that so your seat is removable more easily.

Step 13: Wire Routing

solder 4-8" wires to the battery terminals, depending on how long is needed to get to where the circuit wil be. glue down the battery wires so prevent them pulling out.

route the battery wires and the rear led wires to the place on the case where you will be attaching the circuit. i'm going to glue the circuit to the back of the case. drill a hole in the case if needed.

depending on where you mount the circuit, plan on having an extra 3" to 4" section of the rear led wires inside the case, this will make it so you can open the case and switch batteries easily.

then i measured and glued down the wires on the outside of the case, making 3 attachment points where the circuit will solder onto. i call this method "the big ugly mess"

Step 14: Attach Circuit and Encaplusate

place the circuit in approximately final position but don't glue yet.

solder the input/output wires from the circuit to the proper input/output wires from the case, and the circuit outputs to the led's.

add batteries and make sure it works! after this it will be very hard to fix anything.

glue down and fully encapsulate the circuit with glue to protect it. hot-melt glue or silicone are good for this. be careful to keep glue out of the switch! also i positioned my switch pointing downward so that it does not collect water in the knob - even though it is a waterproof switch.

when encapsulating with glue, try to inject the glue into all the tiny crevices to get rid of as much air bubbles as you can, in case you take your bike to another altitude.

Step 15: Its Done!

now put the batteries in, attach to bike and go make the cars feel your presence!

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    66 Discussions

    andrea biffi
    andrea biffi

    7 years ago on Introduction

    that's great, very clean circuit too. Maybe you wish to read my BBB instructable ;-) (


    11 years ago on Introduction

    In many places, you can get heavily fined, because from a distance the lights resemble an emergency vehicle.


    Reply 11 years ago on Introduction

    True, but police would be more likely to encourage than discourage their use. It's much better than riding at night without lights, which many bike riders do and don't get stopped for.


    Reply 8 years ago on Introduction

    Ha, then you do not live in my country. 3rd brake lights for cars were forbidden, but as the general undesatnding was that these would increase safety, a date was set for them to become legal.
    Eventhough everybody agreed that these increased safety and eventhough the law was written to make them legal, police were fining people who had these lights till midnight of the day before the law came into practice, After all, 'rules are rules'
    These lights on my bike would no doubt get me shot.

    But it sure is a good instructable


    Reply 11 years ago on Introduction

    but you might get shot by a drug dealer :s


    10 years ago on Introduction

    the idea and the instructable is nice but can it be that i'm the only one who thinks that this amount of light is kind of dangerous? i mean if i would drive behind you, i would at least be distracted or slightly blindet (only seeing cyan dots all over the place). even my flashlight replacement (5red leds each 6000mcd 20°) (thats about 0,5 lumen per led) has this effect if someone looks directly at it. i don't even want to imagine how it is with a 100 lumen led at eyelevel on a dark road.


    Reply 8 years ago on Introduction

    I can only hope that yes, you are the only one who thinks this amount of light is dangerous. How bright is 100 lumens compared to car tail lights? Or car headlights? Probably 30% as bright as tail lights and 5% as bright as headlights. Those also are at eye level, and are much bigger and brighter than any bike light you can buy or make. My reasoning goes like this: Cars and trucks are big hunks of metal that are hard to miss, yet evidently they need a lot of bright lights at night to make them visible to others. So what on earth makes anyone think that a cyclist (which is easy to miss even in the daytime) should just ride along with the usual piddly 20 lumen flasher? Furthermore, cars and trucks are relatively safe places to be in a crash, and yet they still have lights galore. Bikes, on the other hand, are very dangerous in crashes, and yet some people think cyclists should be apologetic about having bright lights that announce "I'm here, don't hit me!"


    Reply 8 years ago on Introduction

    I didn't expect an answer after that time but okay ^^

    My problem isn't really the amount of light that's shining in my eyes, it's the blinking. I quote myself here: "only seeing cyan dots all over the place".
    The other problem is the time when the LED is off, maybe it's not much maybe only about 1/10 of a second, but with 100 km/h (that is legal standard outside of towns in Germany and trust me on that: no one is driving that slow if no one is around) that are still 2,8 meters where the bike is invisible.
    For the sake of argument...lets make that 1/100 of a second, how close was it the last time you where passed? Yes, that's the moment where you should think of the guy that almost killed you. Was it more than 28cm? Even if it's blinking faster as long there is no light while someone passes you IT IS DANGEROUS! If the cyclist makes any sudden sidemovements (maybe because of a dead rabbit or because he's drunk, doesn't matter) no light at the wrong moment can and end up someone getting hurt.

    2 of this Lights max 10 cm apart, one lit up while the other is off would be awesome. Very visible from long range because of blinking and not too distracting while being close. Maybe connecting one led with a pnp and on with a npn transistor/mosfet after the 555 timer could do that.


    9 years ago on Introduction

    Must the transistor be able to handle 24amps? It's alot easier for me to get a p-channel that handles 100V 10A, sorry but haven't used one of these in years.


    9 years ago on Step 4

    What is the value of Q1 transistor ultra-low-threshold PFET ????


    10 years ago on Step 15

    Nice light, I am a bike paramedic and would like to use a PIC (16F628A) in place of the 555 to provide 2 alternating flashing lights. would the same moset placement work for me?

    Thanks in advance


    11 years ago on Introduction

    Can I forgo the circuit board modification and just replace one of the existing led with the high powered led? someone please tell me please cos I really really wanna do that...


    11 years ago on Introduction

    m new to electronics can u tell me the use of the circuit you build?


    11 years ago on Introduction

    I just finished mine! I tried to make the blinkers and circuitry easily detachable from the bike so I can take it all inside when I park. I also put a 5k pot as R2; dialing it to zero gives a solid light. And damn, it is bright. Thanks, Dan.


    11 years ago on Step 1

    The lumiledsfuture link is a parked link. You should update that section. How about using one of the leds from They have 1 watt lights, would it be better to have 3x1watts vs. 1x3watt light? Perhaps you can spread the light out better.


    11 years ago on Step 15

    Can I use the same components and run the circuit on 4AA 2700mha Rechargeable and thus have more power from the 3W luxeons? IE will the extra power available simply make the LED brighter with no ill effect on components, obviously the led will need heat sink but what about the FET? or should it be heatsinked anyway for safety Basically I just want a single 3W led to blink at as high a brightness as possible from 4AA batteries


    12 years ago on Step 15

    I think I am going to build your light. What are the possibilities of using one the radio sh*ck 7.2v or 9.6v NiCad rechargeables? Would it overheat the LEDs or have other negative results?


    12 years ago on Step 15

    This is good and you could also use a bike light generator th keep batteries charged as you drive. You would only need voltage reducer from 6 volts to 3.