Looking for a pulse switch? Does such a thing exist?

Hi all,

I have a battery-based application powering a 12V light. I want to pulse the light on and off automatically (similar to a strobe). The duty cycle will probably be around 10%, but I'd accept just about anything if the solution is simple enough.

I want to avoid use of PIC or other microcontrollers, as that would likely require power regulators...recall, I'm using a 12V "dirty" source. Not only that, but the intended use is in a somewhat harsh environment (severe vibration, moisture) and I have very little space (the 'switch' must be attached to the light itself).

So, does anyone know if there any ready made products (or simple solutions) to pulse a light on/off?

Cheers, Dave

Reply to
dave_bonnell
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An LM555 from national semiconductor, sounds ideal for this sort of thing

martin

Reply to
martin griffith

go to your auto part store and ask for a bimetal flasher. you will need to know the approximate current your lamp/load is. they also have flasher modules for 12 operations.

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Reply to
Jamie

If you use the 555, you'll probably need to drive an FET with it. You did not mention the current requirement of your light, but the 555 is limited to no more than 200 mA.

If you just need something that blinks, not that particular light, there are blinking LEDs you could use.

Ed

Reply to
ehsjr

OP here,

I had a look at an LM555 datasheet, and it indicated that the max. output current was around 200 mA. The light I'm driving is an off-the-shelf LED automotive taillight which *may* come under 200 mA, but I haven't tested it (there were no ratings on it). Since the light isn't at my desk I can't check right now (I wouldn't be surprised if it was up in the 0.5 A - 0.8 A range). In all likelihood I would end up using a FET of some sort; for my application, it needs to be on the high side of the load.

I have tried the LED blinking lights (generally they run on 3V = 2 x

1.5V batteries). However, IMO they just aren't bright enough for my application...they have enough intensity from directly behind the light, but trail off dramatically when you move to the side.

The application is a bicycle taillight. I found a 12V LED light that has excellent visibility from all relevant angles. Traffic in my area is generally ignorant of cyclists, so anything to grab their attention is a good thing (ignorant != malicious). I already run a 20W halogen up front, so powering the light is not an issue. The taillight has a low- and high-intensity mode...so I want to be able to run the light constantly 'low' and strobe the high-intensity light for approx 0.1 second at a frequency of around 2 Hz. This would hopefully serve to make me more noticeable, drain less power, and keep me legal (the light is constantly on).

Someone mentioned the use of a bimetal flasher...this sounds interesting, I will check it out. I may not find one with the flash frequency/duration that I am looking for, and even if I do the size and/or cost of the device may be prohibitive.

Cheers, Dave

Reply to
dave_bonnell

Hi Dave, I must buy a new bike. But have a look at this

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its all about led drivers, and efficiency, also look at luxeon LEDs. I have my doubts about a bimetal thingy, it will waste power heating up, which is bad news for a bike application

martin

Reply to
martin griffith

What is your power source for the 20W halogen? The LED tail light may not be so spectacular at marginal voltages under 13V. The 12V rating is more of a descriptive indicating 12V automotive system and really means the best performance is at 13.5V or above. Unlike the halogen technology, the LED output will fade fast below a certain threshold. This means that a small switching boost regulator to maintain 13.5V may be required. I take it the LED assembly is a two wire with case grounded to the frame. There are any number of ways to strobe the high intensity cheaply, but the question is can both the high intensity and low intensity leads receive voltage simultaneously or does the lamp require that only one at a time be used. Your most readily available timing source is probably an existing reed switch pickup on a rear wheel spoke or pedal.

Reply to
Fred Bloggs

I'm guessing that there's one ground on your light, and two leads which are "low beam 1" and "low beam 2", where both "low"s to +12 is high beam, right?

I'd do a 555 with your duty cycle, but upside-down, i.e., low for the .1 second and high for 1.9. Put about a 4.7K pullup to +12V, and get some P-channel power MOSFET; put its source to +12, its gate to the output of the 555, and its drain to the section of the light that's not always on.

Good Luck! Rich

Reply to
Rich Grise

The source is currently a 12V SLA battery. Effectively equivalent to a (really) small car battery. I may migrate to a NiMH battery pack, but it's not in the near future due to cost.

The LED assembly is DOT approved and intended for use in automotive applications, i.e. running off a 12V lead acid battery. When I first tried plugging the light in, I used a spare battery that I'd estimate was at 60%...and the light output was pretty damn bright. I don't have access to the internals of the light (it's a sealed unit) but I expect that the circuit regulates the input appropriately.

The assembly has three terminals: ground and two +12V lines (lo and hi). Hi and Lo can be simultaneously connected to +12. So for flashing operation, the lo line will be always connected and the hi line will simply be pulsed at the required frequency/duration.

I will also be requiring the off, steady-low, and steady-high modes. One of the reasons the pulsing needs to be done on the positive side of the load.

Cheers, Dave

Reply to
dave_bonnell

On Thu, 01 Jun 2006 13:32:53 -0700, dave snipped-for-privacy@hotmail.com top-posted:

I answered this for you about an hour ago. A 555 and a P-channel high-side driver. With the new "steady on" requirement, a rotary switch.

Good Luck! Rich

Reply to
Rich Grise

If you're concerned about size and durability, then a ready-made product is the way to go. There are some suggestions here:

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which advocates the xenon strobe as the best light to use. These products give you the light source and electronics all in one. You could make something from Radio Shack parts, but it would fairly big, and run $20 or more before you're finished with it. Was that AA battery powered job something like this CatEye TL-LD1000 from Agee's. They say

50 hours continuous so that at approximately 3000 mA-Hour capacity, and the batteries probably in series for net 3V, the LED light draws about 60mA. It would be far simpler to make a small regulator to adapt this product to a 12V system. That would be a LM317, series power resistor, some filter caps, and small voltage setting resistors- all available from Radio Shack. You can find out how to use the LM317 by going to
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and entering -LM317- in the part number search box.
Reply to
Fred Bloggs

Rich,

Yes, you did answer my question, for which I am thankful (sorry if I didn't acknowledge). I'll probably avoid the rotary switch and opt for a dual micro switch...one to turn the light on (low), the other on-off-on switch to control high/strobe/low modes. Perhaps the most compelling reason is the size of the project box (actually just a spare anti-static container from Maxim, which was smaller than any 'real' project box I could find). Rotary switches aren't terribly common in my area. I only found a couple locally, and neither was well suited to the application.

BTW, the 3V LED I was using was indeed a Cateye LD1000. I was very disappointed with the light output from this thing (especially at $30+). Again, very bright from directly behind, but light output was miserable once you move a few degrees either side (even with the dedicated side-facing LEDs). I have a number of similar CatEye products which seem to perform just as well, but only cost $8...so I'll continue to use these as backup.

Referring to a post about reduced voltage levels, I hooked up a nearly discharged battery, and only the central bank of LED's lit up in low mode (while high mode managed to illuminate all of the LED's).

I have a prototype built, and will be field-testing shortly. Thanks all!

Reply to
dave_bonnell

Just on a lark, I did a google search on "DP3T slide switch" and got a lot of hits - that could solve your space problem. :-) This was the first hit:

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Good Luck! Rich

Reply to
Rich Grise

Don't need a rotary switch - a double pole double throw center off switch will do:

source ----------+-+---MOSFET---[LED]--- Gnd | | | o | o | / | \\ |

555 pin 8 ---o | o---+ | o-+ o | 555 pin 3-----------+

Ed

Reply to
ehsjr

Yup, I'm using a SPDT right now (on-off-on), which doesn't do everything I want it to do(yet). Not sure about the ascii diagram, as the characters aren't lines up correctly in my browser/usenet client (google groups). Give me a couple of minutes to figure it out :)

Cheers, Dave

Reply to
dave_bonnell

One side of the DPDT connects the 555 to + (It's the on/off for the 555). The other side connects the mosfet gate to either V+ or to the 555 output pin, pin 3. The focus was just showing the use of the switch - it wasn't meant to be a detailed schematic. For example not drawn, but assumed, is a resistance between pin 3 and the mosfet gate, say 330 ohm. And you can put a .05 across that reststor. The 555 ciruit is also assumed.

Ed

Reply to
ehsjr

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