Simple Headlight Modulator

I got around to building up the simple headlight modulator that I designed. The objective was to have it work using only the headlight ground wire, so you can select either HIGH or LOW beam to modualte.

As required by NHTSA, it oscillates at 4 Hz. However, it does not have a darkness sensor to automatically disable the flasher. You have to manually throw the switch to bypass it.

This one is not strictly legal, but it is simple and requires no 12V feed line. Also, if something fails, you can restore stock wiring by unplugging the unit and connecting the two wires coming from the headlight together (that restores the original ground lead).

It uses a low resistance N-FET as the solid state switch, a 555 timer runs the gate and I chose to have a red lED in series to show when it is pulsing. A 25 Ohm, 3W resistor keeps the light current at about 20% of maximum during the FET off time (as required by spec, but also keeps the halogen from going completely cold during the off time so it will minimize wear and tear on it).

I tested this one on a 55W halogen light and the N-FET shown does not even get warm. A FET with a little higher resistance could be used.

I'm probably the only one here who does not know this but what is the objective of modulating a 4 KHz at 80% on the head light beam?

I understand that anything in the range of about 24 Hz (like film) and up is not considered visible to the human eye as any kind of flicker.

In the mid 1970's I worked on a stadium scoreboard system and all the thousands of light bulbs were left partially and continuously on to reduce the inductive load to the power supply at bulb turn on time... and not to protect the bulb filament. Just my 2 cents worth.

Neat project.

Best of success,
Bill

Will this work with a LED driving light? If not, what would it take to make it work with LEDs?

bill_wilcox100 wrote:

I'm probably the only one here who does not know this but what is the objective of modulating a 4 KHz at 80% on the head light beam?

I understand that anything in the range of about 24 Hz (like film) and up is not considered visible to the human eye as any kind of flicker.

You want 4Hz, not 4kHz. 4Hz definitely has a visible flicker, that's the whole objective. You use it in daylight to make the headlight more noticeable (some would say more annoying) to increase visibility, especially coming up from behind. You don't use it at night, only daytime.

bill_wilcox100 wrote:

In the mid 1970's I worked on a stadium scoreboard system and all the thousands of light bulbs were left partially and continuously on to reduce the inductive load to the power supply at bulb turn on time... and not to protect the bulb filament. Just my 2 cents worth.

Neat project.

Best of success,
Bill

I understand what you mean about reducing the power supply loading effect. We used to take a power supply and put a standard resistive load on it and then turn it on and off once a second and let it run over the weekend. Amazing how many we blew up that way. Turning them on causes a surge current, even in well designed ones that control the inrush. Bulbs do have s shorter life if you turn them on cold-to-hot more, keeping them a little warm in the off state should reduce that effect.

otakar wrote:

Will this work with a LED driving light? If not, what would it take to make it work with LEDs?

Depends on the impedance. This circuit works because the 470uF cap can charge up to 12V through the cold filament impedance before it starts flashing. That filament looks like a resistance of about four Ohms. On each "off" cycle of the halogen, it refreshes the charge on the cap. An "off" LED isn't a filament and I think it's resistance would be too high to work correctly. An easy way to get around that is just to run a 12V line into the circuit to feed the 555 timer circuit. That would take an extra wire, but you could reduce the size of the 470uF cap to maybe 4.7uF since it doesn't have to store charge to run on every time.

Take out CR1 and run 12V straight into the top of R3 to feed pin 8 of the 555. You would also need to remove R5.

Very cool! I run HID on my headlights so this would not work there, but I also have 2 driving lights I always run durring the day. I like having the extra lights as it helps give perspective so other drivers can better judge that I'm moving. This modulator would work on them. The driving lights are 2x 55watts.

Three questions: 1)is that much load (110 watts) OK on this circuit? 2) Do you think a heat sink would be required if 110 Watts instead of 55? 3)Do you have a part number for the 555 timer?

Crazy busy this summer but this would be a great "winter" project!

Thanks!
Andy

Just look up NE/SE555 on ebay. There's hundreds for sale.

Radio Shack has them too.

1shot wrote:

Very cool! I run HID on my headlights so this would not work there, but I also have 2 driving lights I always run durring the day. I like having the extra lights as it helps give perspective so other drivers can better judge that I'm moving. This modulator would work on them. The driving lights are 2x 55watts.

Three questions: 1)is that much load (110 watts) OK on this circuit? 2) Do you think a heat sink would be required if 110 Watts instead of 55? 3)Do you have a part number for the 555 timer?

Crazy busy this summer but this would be a great "winter" project!

Thanks!
Andy

If you use the FEt shown (NDP706AL) I believe it can handle 110W of loading without a heatsink. That corresponds to maybe 8.5A. Power dissipation is the current squared times the ON resistance of the FET, which is about .018 Ohms for that FET, also multiplied times 0.8 since the duty cycle is about 80%. That gives you about 1W of power in the FET. If you use a TO-220 package FET, that can easily handle 1W without a heatsink. It will get up to about 80C temp which is no problem for the FET, a shade warmer than you would want to leave your finger on for a long time. As long as the ON resitance of the FET is less than about .020 Ohms, should be OK with 110W no heatsink. I believe you can get those FETs from Digi-Key.

The part number for the 555 timer is typically LM555 or MC555 or uA555 or whatever the manufacturer's letter prefix is put before "555". That part is so common, Fry's or radio Shack or any electronics store should have them. Just make sure you don't get the CMOS version of the 555 (which is usually tagged LMC555 or similar) as I don't think those are robust enough for use in a bike electrical system. Just use the standard 555.

Excellent! thx