A flashing LED stop light

I wanted to improve active safety when braking & increase the visibility of an additional LED stop-light : making the stop-light blinking at about 10 times per second makes it more eye-catching than a fixed light. This is not the usual slow-blinking light, the effect is more stroboscopic-like.
The LED stop-light is controlled by a simple electronic oscillator whose frequency may be choosen between 5Hz and 12Hz.

LED Pros :
• Monochromatic light, no need to use a color filter.
• Instant light-on / light-off : great for a stop light.
• No light-on intensity peak, like with a regular bulb.
• Very long lifetime : more than 100 000h (about 10 years !).
• Vibration-proof.
• High light efficiency, power consumption reduced by 10.
• Few heat is produced.
• May be directly controlled by an electronic circuit.

LEDs Cons :
• Lower light flux but improving fast.
• More expensive than bulbs.
• Require electronic control.
• Sensitive to over-voltage.

Schematic & board

This a very classic astable multi-vibrator based on the well-known NE555 (click image to enlarge).


Connectors on right side on the main schematic are connection to :
• Signal from stop light at rear connector (Stop)
• LEDs +
• LEDs -
• Neutral switch (under tank)
• Ground

On the optional part : a three position switch SW1 (ON/OFF/ON) to choose between :
• disconnected (middle position).
• strobe stoplight.
• standard stoplight.

Components :

IC1 : NE555 integrated circuit or CMOS equivalent 7555.
T1 : 2N1711 NPN transistor or equivalent (min 500mA/50v).
D1 : 1N4001 silicium rectifier 1A/50V or more.
C1 : 1µF polarized capacitor.
C2 : 10nF polyester capacitor.
VR1 : 47K variable resistor. Allow variable blinking frequency from 5 to 12Hz (ap).
R1 : 47K resistors (5%, 1/4w or more).
R2 : 1K resistors (5%, 1/4w or more).
R4 : 10K resistors (5%, 1/4w or more).

All components are standard and may be purchased at any component shop.
Active components have many equivalents.

Notes :
• Optional diode D1 allows disabling of the flashing mode when in neutral gear (generaly at a stoplight) to prevent the cager behind you getting nervous/blind/epileptic.
• No heat sink is required for the transistor as it works only in blocked/saturated mode.
• Frequency may be calculated like this : F(Hz) = 1.44 / ( C1 * ( R1 + 2 * ( R2 + VR1 ))
• With values given above the output signal cyclic ratio is 50%.
• All components are soldered on a 35x20mm PCB.
• Use a small plastic casing to preserve from moisture/vibrations.
• The harness connector for the rear light is located under the seat, on the left side.
• The harness connector for the neutral-gear is located under the tank (sky-blue wire connector).
• Overall cost : less than 3€ for the components, variable for the LED stoplight.

LEDs Stop Light

I bought a LED stoplight from a shop for about 6€. It's made with six clusters of four high-intensity red LEDs.
The supply voltage is 12v, peak current is about 200mA (a regular stop light bulb draws 2.0A). Actually average current will be only 100mA (it blinks !).
I've placed the light under the luggage carrier : there are two screw prints under it.
Two inox hex-screws will do the job.

Use only a true LED stoplight, not a scrapped car third stoplight, they are often made with small standard bulbs.
Such stoplights will burn the output transistor because of the high intensity they require (ten times more).


(image) (image)
(image) (image)


This device will likely NOT pass MOT, use it under your own responsibility. You may get trouble with the police using it - I didn't so far - that's why I recommend using it as an additional light.

Anyway, I've seen some high-end german cars using the same device - it also exists for some BMW bikes as optional part (very expensive).


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