12VDC LED Flasher

[Lance Cory]

The title is a bit misleading, allow me to explain. I drive a very small and lightweight car, and feel very vulnerable in it, especially to rear end collisions. I have a number of very bright LEDs with a forward voltage of 2 volts, and a design current rating of 50 ma. I would like a circuit which would power an array of these LEDs, perhaps up to 80, but at least 50, and I would like it to behave in this manner:

Closing a switch (brake switch) supplies positive positive voltage which may vary between 11 and 15 VDC (natural variation in car's electrical system) to the circuit. At the presence of this voltage, the the LED array would flash 3 or 4 times befor lighting steady. They would remain steady on until the switch is opened again. Whether the LED flashes are limited by a counter or a timer and flash rate is irrelevant. Also, it is acceptable for the circuit to drive a relay which turns on another circuit containing the LED and their current limiting devices (as opposed to this circuit limiting the LED array current directly).

I can follow schematics etc, but a designer I am not. Any ideas would be appreciated. Thanks

 

[Nigel Goodwin]

The easiest answer would be a micro-controller, you could then easily alter the operation after construction - it would also be a much simpler circuit.

You don't mention where you are, are you allowed to do this?, or are you going to be pulled up and prosecuted?.

 

[stevez]

I have to agree with Nigel on the concern for legal issues. If the legal issues become an impediment consider this: I've seen LED flashers, the kind on bicycles or that runners wear. They are just LEDs but they really get your attention. I came to understand that the LED is given a much higher voltage for a few milliseconds - a level that could be tolerated by the LED - then the voltage delivered settles to a more normal value. The resulting "twinkle" adds greatly to grabbing one's attention. I think the old LM3903 was designed to do just this. I wonder if you couldn't light your LEDs similarly - possibly staggering or delaying groups of them for a more profound twinkle. Employing a PIC certainly sounds like it might allow for some experimentation. This might also help with the flashing issue - since it won't really be flashing.

Several years ago some traffic lights in the area were modified to include a bright stobe that was contained within the RED or stop signal. When the light went red the stobe would flash periodically to get attention. The flash was such that most people didn't notice what it actually was - all they know is something caused them to look.

 

[k7elp60]

Led flasher

Here is a circuit that uses 2 IC's and a few other components. The LED's in series have the current limited by the 68 ohm 1/2 watt resistor. The other strings of LED's can be put in parallel. Once power is applied the LED's will flash 4 times and then stay on steady.
I forgot to include a 1M resistor from the gate of the Mosfet to ground.
A new schematic is posted

 

[Lance Cory]

On the legal front, I live in California. This is from our Vehicle Code:

25251.5. (a) Any motor vehicle may also be equipped with a system in which an amber light is center mounted on the rear of a vehicle to communicate a component of deceleration of the vehicle, and which light pulses in a controlled fashion at a rate which varies exponentially with a component of deceleration.

(b) Any motor vehicle may be equipped with two amber lamps on the rear of the vehicle which operate simultaneously with not more than four flashes within four seconds after the accelerator pedal is in the deceleration position and which are not lighted at any other time. The lamps shall be mounted at the same height, with one lamp located on each side of the vertical centerline of the vehicle, not higher than the bottom of the rear window, or if the vehicle has no rear window, not higher than 60 inches. The light output from each of the lamps shall not exceed 200 candlepower at any angle horizontal or above. The amber lamps may be used either separately or in combination with another lamp.

(c) Any stoplamp or supplemental stoplamp required or permitted by Section 24603 may be equipped so as to flash not more than four times within the first four seconds after actuation by application of the brakes.

So, it looks as though this can be set up to operate within the law. The micro controller sounds fine, but as I said, I am no designer, and actual circuit is what I need, I am too ignorant to make use of generalitys. The circuit above looks simple enough, I presume flash rate can be altered by selecting a resistor value near the 555 timer or something. Thanks for the responses guys. I'll try this circuit out within a week or two, but I am also open to other suggestions if anyone has them.
I have a question about this circuit, did you design that that quickly, or was this an existing circuit out there somewhere that I was unable to find? Either way, I really appreciate it, thank you very much.

 

[k7elp60]

12V LED flasher

Lance,
The circuit was designed and tested recently. I have been building circuit to flash LED's for quite a while. The resistors connected to pins 6 & 7 plus the 4.7 uF capacitor affect the flash rate of the LED's. The 4017 determines how many flashes occur, so it can be changed also. Glad to
help.

 

[k7elp60]

12V LED flasher

The circuit I posted was missing a 1M from the gate of the Mosfet to ground. Sorry for the error look at the post for a new schematic.

 

[Lance Cory]

Thanks for the help

I wanted to acknowledge that I saw the updated schematic, thanks.

I really appreciate what you have done for me here. If you might indulge me one more time (I think), not being the electronic whiz. There is no part number for the mosfet, before tonight I could not have told you what a mosfet was. I gather they are somewhat universal, as long as specifications match intended use. Could you verify that this ( https://www.newark.com/product-details/text/CD121/65870.html ) is what I want or alternatively, steer me in the right direction with a recommended part number and source. Thanks again for the help, you've been great!

Lance

 

[k7elp60]

12 Volt LED flasher

The NDP6060 should work. And would reduce the 1M resistor from the
gate to ground to 47K ohms.

 

[Lance Cory]

Thanks so much

Thanks so much, this has been more help than I expected. I really appreciate this, going to order up some parts and build it. I'll let you know that it was a success (optimism). Thanks again, you have been great. It must be neat to know stuff! 8)