Help with Alternating 12 volt DC Headlight flasher "WIGWAG"

[gmcjetpilot]

Help with Alternating 12 volt DC Headlight flasher "WIGWAG"

The off the shelf versions for emergency vehicles are expensive and heavy, often in heavy potting. I want solid state and lighter design.

Req:
12-14 volt
two 60 watt (about 4.5 amp each) halogen lights....
Flash about 50-100 fpm (flash per minute).... adjustable is good.

I am thinking of a 555 circuit may be? No chip would be fine or even better.
What transistor for switching? NPN, PNP, FET, MOSFET?
SCR seems good but once on switching off is a problem?

 

[(*steve*)]

I'd be thinking a 10W LED -- it's likely to be brighter but make the design somewhat more complex -- but contribute to a much lighter design. If you need an omnidirectional source of light, than maybe a set of 3W LEDs mounted appropriately.

50 to 100 flashes per second? are you sure -- your eye will see that as continuously on.

A 555 may be an appropriate device to generate the timing. You could use that to switch a (or a series of) LED drivers, or use a simple constant current source to power them (I'd tend to advise against resistors).

Your switching element may be anything, or may be incorporated into the driver for the LEDs.

However I suspect you need to revise that flash rate first...

 

[gmcjetpilot]

I'd be thinking a 10W LED -- it's likely to be brighter but make the design somewhat more complex -- but contribute to a much lighter design. If you need an omnidirectional source of light, than maybe a set of 3W LEDs mounted appropriately.

Appreciate the input but going with the bulbs I have. Not enough light from most LED lights, at least at this time.

50 to 100 flashes per second? are you sure -- your eye will see that as continuously on.

TYPO it is 50 to 100 flashes per minute. Thanks!

A 555 may be an appropriate device to generate the timing. You could use that to switch a (or a series of) LED drivers, or use a simple constant current source to power them (I'd tend to advise against resistors).

Your switching element may be anything, or may be incorporated into the driver for the LEDs.

However I suspect you need to revise that flash rate first...

Thanks again. I have some ideas I cobbled together. I'll draw it up and post it, see what you all think.

 

[(*steve*)]

If you use filament type bulbs you are best advised to maintain a current through them when they are "off" to keep them hot and to minimise the shocks to the bulb each time power is applied.

Sorry, I didn't read "headlight"

 

[gmcjetpilot]

If you use filament type bulbs you are best advised to maintain a current through them when they are "off" to keep them hot and to minimise the shocks to the bulb each time power is applied.

Sorry, I didn't read "headlight"

Excellent Steve... How do I do that, capacitor?

With a 1.5 Hz, or 90 FPM, that is .6667 seconds. Not sure what the cool down time is on the filament is? I can speed it up a tad. I don't want to go over say 1.7 Hz or 0.588 sec. They are halogen, MR16's, not that expensive. So if they burn out in 500 hours verses 2000 hours, they burn out. I suppose I could put a capacitor on the light and keep the filament

Suggestion on how to keep the trickle current on bulb? Capacitor across the 12V halogen? The light does NOT have to go off completely, only dim to say 70% or more from full bright.

I got a circuit with some help.... 555 timer, 50% duty and a square wave, turning two MOSFET (P-Chan) ON and OFF alternately. The 555 will trigger one light when the 555 output goes low and turns ON the MOSFET-P. When the 555 triggers high it will turn this light OFF, but the other will light will be on, with the 2N2222 transistor inverting, high at it's base is ON, which draws the other light's MOSFET gate low, for ON.

Thanks

 

[(*steve*)]

No, the simple way would be with a resistor, but that would probably get very hot.

Alternatives would be to use PWM and vary the duty cycle from low (maybe 30%) to high (99%) or perhaps doing the same thing with an inductor in series with the bulb to produce a crude SMPS.

A simple approach I've seen is using a power transistor with limited base current. This acts to limit the available current to the lamp causing it to turn on more slowly. This has the additional benefit of limiting the cold current without a lot of components. It does reduce the voltage to the lamp slightly, but not a lot (0.2V perhaps)

If these are the same lamps that are used for the headlights, then you need to ensure that the circuit operates correctly whether they are turned on or off. Are they? and if so, what is the desired behaviour?

1.5Hz is (with 50% duty cycle) 0.3333 seconds on, 0.3333 seconds off. The entire cycle takes 0.66667 seconds.

If the bulb filament stays red hot for at least this time, then you should be fine with no additional steps.

Turning 2 mosfets on is a good idea, but using a P and an N channel is probably not going to work unless you can switch the +ve lead to one lamp, an d the -ve to the other.

If you can only switch the +be lead, then you will need to use P channel mosfets for both, and this will require that you employ an inverter to switch the other P channel mosfet. You specify using a 2222 as an inverter which sounds right, but the rest of the explanation is odd. Show me the circuit.

 

[gmcjetpilot]

No, the simple way would be with a resistor, but that would probably get very hot.

Alternatives would be to use PWM and vary the duty cycle from low (maybe 30%) to high (99%) or perhaps doing the same thing with an inductor in series with the bulb to produce a crude SMPS.

Yea that resistor is not going to happen. May be some in-rush limiter, thermistor, but I am not going to worry about it until I prototype this. There are 1000's of these on police cars and aircraft... but it is something to think about. A capacitor across the bulb or resistor + capacitor (RC) will smooth out the pulse. The bigger the capacitor the "smoother" the square wave. The issue is weight and complexity. I have made PWM dimmers with 555 timers, work like a charm! However I am trying to KISS. This is just a simple astable multivibrator with transistor switching. I don't want to re-invent the wheel... If the bulb life is reduced so be it.

A simple approach I've seen is using a power transistor with limited base current. This acts to limit the available current to the lamp causing it to turn on more slowly. This has the additional benefit of limiting the cold current without a lot of components. It does reduce the voltage to the lamp slightly, but not a lot (0.2V perhaps)

I will have to play with that, but I do want FULL voltage/current for max brightness! I will have to research the affect cycling has on halogen. If there is no data I will test it my self. The life of a MR16 is in 1000's of hours at steady. I'm not that worried about the halogen bulb. I am already running them at higher volts, so I have halved the life. Aircraft do not put on hours like cars or lamps in homes. 500 hours is several years of use. I have a commercial whelen SSF-2150 flasher. It's a pure DC pulse, on and off, about 100 times/min or about 1.7 Hz. There are 1000's of these commercial wigwags flashing halogen car headlights..

If these are the same lamps that are used for the headlights, then you need to ensure that the circuit operates correctly whether they are turned on or off. Are they? and if so, what is the desired behaviour?

This is actually an experimental aircraft. The two lights out in the wing tip, normal use is for landing and taxi at night. In the air, day time they will be used for "recognition lights" to be more visible, reducing mid air collision risk. Strobes are required for night, but useless in daytime, due to low lumen's. Strobes don't need much light at night to be visible. Landing lights make more light, (albeit focused in a narrow beam), by a factor of at least 10 or more, much more. Thus the "recognition lights" more visible in day time. Flashing landing/taxi lights in the wing tips makes RELATIVE motion back and forth, so people can see you easier form head-on or some converging frontal angle, during the day. The rule of the air is SEE and BE SEEN. So one must look out the windscreen, but also being visible is a plus.

1.5Hz is (with 50% duty cycle) 0.3333 seconds on, 0.3333 seconds off. The entire cycle takes 0.66667 seconds. If the bulb filament stays red hot for at least this time, then you should be fine with no additional steps.

That was a typo. Each cycle is 0.6667 or time = 1/f, where f = 1.5 Hz ... Saying 1.5 Hz at 50% duty implies divide by two. Yes off period is half of period or 0.333 sec. With out test data, I can only guess if that's enough to keep the filament hot (not cooling between cycles). It's more pulsed than On/Off. Not able to eyeball it directly there is still a glow between on periods, using the commercial Whelen flasher on the oscilloscope. There is no big overshoot in current Again the Whelen is on & off and there's some glow between on period. It is too bright to eyeball direct, but in a dark room the bulb is still HOT (making light). I have done some home work my friend. It would be nice to keep some current going through it, I agree, but I'm not going to belabor this aspect. The benefit to complexity does not seem warranted. Halogen is be more forgiving and run hotter then hell, so keeping them warm is not an issue.

I rejected using HID lights for this reason, you can't flash them or it's more difficult. The flashing circuit for a HID is more critical. HID lights must also be warmed up before flashing. HID is expensive, so reducing their life is not cost wise. A MR16 bulb is a buck or two. Bright white LED technology is catching up and making the scene in aviation, but it's not up to making as much lumen's as the halogen in this small form factor and cost. Flashing LED's would be nice, but I have enough electrical power to do the job.

Turning 2 mosfets on is a good idea, but using a P and an N channel is probably not going to work unless you can switch the +ve lead to one lamp, an d the -ve to the other.

If you can only switch the +be lead, then you will need to use P channel mosfets for both, and this will require that you employ an inverter to switch the other P channel mosfet. You specify using a 2222 as an inverter which sounds right, but the rest of the explanation is odd.

No that's not what I am doing. I'm using only MOSFET P-channels, two for wigwag and two more for steady switching, allowing use of a low current rated switch. The 2N2222 is the inverting transistor for one light, as I said. Not sure what is "odd"? Could you be more specific.

Show me the circuit.

Since you demanded it (?) this is the circuit. Thanks.

 

[(*steve*)]

Yeah, that circuit looks fine. One schematic speaks 1000 words. I misunderstood your explanation.

It seems you've done enough to show that there won't be significant power on surges between flashes. Just make sure the mosfets can handle the initial switch on surge and I think you're fine to go.

Actually I might add some small amount of resistance in the "steady" lead. And you may wish to consider a fuse in a strategic location. If the power supply is at all noisy, you may wish to decouple the power to the 555 with a small resistor and possibly a zener to allow for transients.

 

[WA6DZS]

I'd switch out the pulse (+/-) to steady ON, and eliminate the other two FETs, for your Steady Recognition mode. That'd be a DPDT switch, wired into the control circuits, and not more wire/weight. Some (power) resistance across each FET would do your keep-warm function, as long as you could turn OFF elsewhere. Cheers & 73, Phil

 

[duke37]

A 4047 CMOS chip will give two outputs in antiphase and will drive two mosfets directly. This will make a much simpler circuit.

 

[gmcjetpilot]

I'd switch out the pulse (+/-) to steady ON, and eliminate the other two FETs, for your Steady Recognition mode. That'd be a DPDT switch, wired into the control circuits, and not more wire/weight. Some (power) resistance across each FET would do your keep-warm function, as long as you could turn OFF elsewhere. Cheers & 73, Phil

True and I drew it out a few weeks ago. It's quite the triple pole, double throw (really three throws one being off). This is what I was trying to avoid. I want one light switch or even a rotary switch. Those switches get expensive in "aerospace" quality, and they are not that common, especially in high amps.... I drew it out. I was able to do it with two MOSFIT's Thanks for the tip!

A 4047 CMOS chip will give two outputs in antiphase and will drive two mosfets directly. This will make a much simpler circuit.

I don't know how much simpler, but I like the idea of having balanced output to the MOSFIT's, because the signals will be different between left and right as I have it, one triggered direct from the 555 the other through the NPN.

However to make it even simpler, , I'm going to use a micro processor, so I can set alternate patterns and just program the rate of flash. It does not really make the circuit more simple, but cleaner. The cost of a 8-dip PIC processor IC is $3.50 verses $0.50 for a 555 (plus associated parts)..... However it will give me more flexibility. The end results will be different switch logic and switch flexibility, two mosfits (going with better ones than these so they get more expensive), and the switches can be milliamp rated. The drawing above and before will not work. I had to add a diode and extra pot to trim it, to get 50% duty. I succeeded but it was a pain and more parts.

Don't laugh I used my Science Fair 200 in 1 radio shack deal to make a basic RC circuit. This would be better than a 555, simple circuit. The standard RC + transistors astable vibrator will do the trick. It does not make as clean square wave, will change a little with load. The circuit can be inverted to trigger high or low side.

PS: I did the heat sink calcs on the MOSFIT's for slow switching (really just ON and OFF, not high-speed) and just ON steady. It will be cool with a small aluminum project box and min heat sink, but it will not be just in free air.... I still do have a watt or two to dissipate.

 

[daddles]

Another design to consider is to shine the light on a rotating reflector. Simplifies the electronics, but now you have a mechanical part that can fail and probably consumes more space. The reflector could collimate things for a brighter flash (e.g., with an integral lens).

Another idea would be to have a rotary array (say, over 180 degrees for each side of the aircraft) of fixed lamps that each get turned on and off (e.g think projector bulbs). Some bright LEDs with lenses could also do this type of duty and have the advantage of sharp on/off transitions, which tend to catch the eye more than the more sluggish incandescent bulb's transitions. Just some fodder for thought...

 

[gmcjetpilot]

Another design to consider is to shine the light on a rotating reflector. Simplifies the electronics, but now you have a mechanical part that can fail and probably consumes more space. The reflector could collimate things for a brighter flash (e.g., with an integral lens).

Another idea would be to have a rotary array (say, over 180 degrees for each side of the aircraft) of fixed lamps that each get turned on and off (e.g think projector bulbs). Some bright LEDs with lenses could also do this type of duty and have the advantage of sharp on/off transitions, which tend to catch the eye more than the more sluggish incandescent bulb's transitions. Just some fodder for thought...

Great creative ideas! but.....

OH NO~! Yes mechanical is way too heavy, complicated and will not work for Landing and Taxi lights on a small plane.... You did not know that part... Light, simple is key. I would rather make a few transistors work hard that weigh way less than a pound...

In the old days the "anti collision" or "rotating beacon", a red light on the vertical tail would flash, red on and off. It was NOT used to illuminate your landing or taxi, just a becon on the tail saying live plane, going to move or moving. These had motors and a rotating plate that made the light appear to flash to observers. These of course are not used any more, went to electronic flashing or strobes for this use decades ago. There are still some out there.

The technology is now LED for almost all aircraft lighting, nav, anti-collision (red flashing) and even strobes and landing/taxi lights .... The latter two I don't think are up to the traditional lights, strobe tubes and incandescent/halogen/HID yet .... but they are getting there.

The LED high powered landing / taxi and recognition lights..... the best ones that even come close to the same illumination as traditional lights; they are VERY expensive but easy on the current (power). Also they should last for a long time. They still are not up to the cost (cheapness) of a traditional incandescent/halogen lights. HID lights are also used on small planes like high end cars as an option. HID lights does not really loving being flashed, but it can been done. For what I want and need the Halogens that come in 60-75 watts is more than enough light..... this is a small two seat experimental plane not the B757 I drive for work. Most general aviation planes use round 100 watt incandescent lights, sealed beams very much like cars up to the 1970's or 80's did, before they went to halogen and stylized light lenses.

 

[Sunnysky]

As indicators, dual Xenon flash is best with small dual HV supply and capacitor discharge and remote controlled timer relay for 10km visibility on wing tips. EMI must be controlled well as these are noisy.

Otherwise 2cm 100W LED array can be just as effective with 200W short pulses but only single sided view.