Use in my motorcycle:
Several years ago these flashers were introduced in the automotive industry as part of the third brake light and
contained strobe lights (and strobing continuously for the duration of pressing the break pedal) but got abandoned
almost immediately sometime later because of the 'strobe' effect it has on some people.
This circuit has become obsolet since much better circuits are available these days without the 'lag' in between
brakes caused by R4 and C3. One of the circuits is listed below.
For a good brakelight flasher for your motorcycle use this one: [Motorcycle Brakeligh Flasher]
Q1 is a PNP Silicon 'Audio Power Out/Medium Power Switch Transistor', 7A, with a TO-220 case. As long as you have a
transistor which is close it will work fine. The SCR is a 100vrm, 0.8A, sensitive gate with a TO-92 case.
IC1,IC2 = LM555 or NE555 Timer/oscillator
SCR1 = NTE5402, ECG5402, RS #276-1067, EC103A, MCR104, etc.
Q1 = NTE197, ECG197, SK3083, TIP125, NTE262, or equivalent
D1,D2,D3 = 1N4148, 1N914, NTE519, ECG519, RS #276-1122
D4,D5 = 1N5400, NTE5850, ECG5850, RS #276-1141, or equivalent
All resistors are 1/4W, 5%, unless otherwise indicated
R1 = 18K
R2 = 330 ohm
R3 = 270K
R4 = 82K
R5,R6 = 1K2 (1.2K)
R7 = 100 ohm
P1,P2 = 50K Bourns trimmer (see text)
C1 = 100uF, 25V
C2 = 22uF, 25V
C3 = 220uF, 25V
C4 = 10uF, 25V
Diodes D1, D2 and D3 are standard small signal diodes.
Power diodes D4 and D5 are the 6A, 50prv types, cathode case. The 60vrm type will work as well.
I used for IC1 & IC2 the LM555 type timer/oscillator.
P1 controls the 'on' and pulse-duration, P2 controls the pulse-timing. You can use whatever you have for these two
trimmer pots. I used 10-turn types which makes it a bit easier to do the adjustments, but regular trimmer pots will
For the electrolytic capacitors: the rule of thumb is to select a capacitor working voltage which is at least twice the
supply voltage. With a running engine, the supply voltage is a standard 13.8V so twice that is 27.6V. The closest
match is 35V. A 25V type will work but is right on the edge.
Applying the Brakes: When you first press the brakes, this circuit will turn on your 3rd brake light via the
main brake lights. After about a second a series of short pulses occur. The number of pulses can range from
approximately 1 to 10, depending on the setting of P1/P2 and when the brake pedal was applied last. After the pulses
have been applied the third brake light assumes normal operation. The prototype was set for five flashes which seemed
more than enough. Two days later I re-adjusted the trimmer potentiometers for 4 flashes. Looks pretty cool!
Circuit Description: The schematic consists of two LM555 timer/oscillators in a dual timer configuration both
setup in astable mode. When power is applied via the brake pedal, the brake light driver Q1 is switched on via the
low-output pin 3 of IC2, and timer IC1 begins its timing cycle. With the output on pin 3 going high, inhibiting IC2's
pin 2 (trigger) via D2, charge current begins to move through R3, R4 and C2.
When IC1's output goes low, the inhibiting bias on pin 2 of IC2 is removed and IC2 begins to oscillate, pulsing the
third brake light via the emitter of Q1, at the rate determined by P2, R6, and C4. That oscillation continues until
the gate-threshold voltage of SCR1 is reached, causing it to fire and pull IC1's trigger (pin 2) low. With its
trigger low, IC1's output is forced high, disabling IC2's trigger. With triggering disabled, IC2's output switches
to a low state, which makes Q1 conduct turning on the 3rd Brake Light until the brakes are released. Obviously,
removing the power from the circuit at any time will reset the Silicon
Controlled Rectifier SCR1, but the RC
network consisting of R4 and C2 will not discharge immediately and will trigger SCR1 earlier. So, frequent brake
use means fewer flashes or no flashes at all. But I think that's okay. You already have the attention from the
driver behind you when you used your brakes seconds before that.
SCR1 is a so-called "sensitive gate" type. The NTE5402 is 100V but the 5401 (60V) or the 5403(150V) also works.
Rms-on is 0.8A, Peak Surge on-state is 8A. Save the pdf file for your records.
The collector/emitter voltage drop across Q1 together with the loss over the series fed diodes D4/D5, will reduce
the maximum available light output, but if your car's electrical system is functioning normally in the 13 - 14volt
range, these losses are not noticeable.
Building Tips: You can easily build this circuit on perfboard or on one of RS/Tandy's experimentors boards
(#276-150), or use the associated printed circuit board listed here.
Keep in mind that Q1 will draw most likely 2 or 3 amps and mounting this device on a heat sink is highly recommended.
Verify that the scr is the 'sensitive gate' type. In incandescent bulbs, there is a time lag between the introduction
of current and peak brightness. The lag is quite noticeable in an automotive bulb, so the duration of a square wave
driving such a bulb should be set long enough to permit full illumination. For that reason, and because lamps and
car electrical systems vary, adjustment via P1 and P2 is necessary to provide the most effective pulse timing for
your particular vehicle.
The reason that the third light is connected to both brake lights is to eliminate the possibility of a very confusing
display when you use your turn signal with the brakes applied.
The cathode of D4 and D5 are tied together and go to point 'B' of the third brake light in the component layout
diagram. Point 'A' goes to the other leg of the third brake light. Most if not all third brake lights in Canada &
USA have two wires, the metal ones also have a ground wire which obviously goes to ground. I don't know the wiring
scheme for Australian and European third brake lights.
Don't forget the three jumpers on the pcb; two jumpers underneath IC1/IC2 between pin 4/8 and the one near Q1/R6.
If you use a metal case, don't forget to insulate the D4/D5 diodes. (For motorcycle you can eliminate D5).
Some 90's cars, like a Mercury Sable, have two bulbs inside the third brake light, each bulb is hooked up
separately to the left and right brake light for reasons only Ford knows.
Click here for a possible 2-bulb hookup. It shows how I modified mine to get it working; and that was easier
than I expected. Current draw with the two bulbs was measured at 1.85Amps (1850mA). Even with double the current
none of the circuit components were getting hot. I had to re-adjust the two pots to make it flash since the bench
testing was done with one bulb.
The pcb measures 2 x 2.5
inch (5 x 6.4cm or 170 x 200 pixels) at 2 colors and is shown smaller when you print these pages. If you need a
direct, full size copy of the pcb I suggest to load the gif file into a program like Paint Shop Pro or one of the
many gif viewers available. The layout is enlarged a bit for a better component view. Note that Q1 is drawn soldered
on the pcb but if you have a metal case you can put it anywhere on the metal case (as a coolrib) and use heavy duty
wiring between Q1 and the PCB.
Bench Testing: I tested different semiconductors like the 1N5401/1N5404, NTE153, and 4A type power diodes for
D4/D5. All worked very well. As expected, Q1 is getting very hot. Current draw was measured between 680 -
735mA with a regular automotive 'headlight' bulb, extra heavy duty to make sure the circuit was safe. I tested
several other power transistors including some darlingtons like the TIP125 and the TIP147. I eventually settled for
the TIP125 myself because I had it available but anything with 5A or more will do fine.
The actual third brake bulb is a lot smaller. Adjusting the trimpots (P1/P2) may take a bit of patience but really
fine-tunes the circuit well.
The circuit I have installed on the brake light of my motorcycle works fine for more than 6 years. There are NO
delays for the light to come on or delay between flashes and solid on. When the break pedal is pressed, the light
should start to flash immediately; every time!
If you wish to go 'LED', check out the modified circuit below. I'm working on a enhanced version with SuperBright
Leds and will post the results when it becomes available.
Rx is calculated depending on the amount of Led's you use and their current consumption. Q1 is a TIP125, PNP
Darlington in a TO-220 case.
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Page copyright © 2000, Tony van Roon