Hi there folks,

Im new to this forum and rather new to electronics in general.
so far I have constructed a few FM transmitters, Electret Amp circuits, and a couple of Rain Alarms with good success. All this in return has given me a firm obsession with this new hobby and I plan to progress it much further so I am indeed glad to have discovered this helpful forum.

now down to the technical stuff. I have an interest in weather sensing circuits, I have found schematics for most types such as rain alarm, lightning detector, frost alarm etc. One which interests me is a Fog Horn type circuit. I have found a Schematic on the Talking Electronics site

I had studied how Real Fog Horns work by using a Laser which is beamed out to sea, then if the laser is reflected back or broken due to snow, rain, fog etc this in return activates the Fog horn

i am curious if anybody can tell me if this fog horn circuit or one similar could be modified using some kind of laser diode action etc to activate the circuit in the same way a real one does.

any info would be much appreciated as I cant seem to find anything along the lines of this idea anywhere.

Cheers

 

Turning your horn on or off would be easy. I think the hard part will be rigging the proper detector. The laser signal will need to be modulated - like a TV remote signal, which could be another option - so that it can be easily picked out of background light sources. Do you know what wavelength is used?

Is this what you are thinking of?

 

hi there

Im unsure of the wavelength or unsure if it matters, i have found it hard searching info about how real foghorns work, from what ive read it says a laser OR "photo beam" is used if that makes sense.

This is where im confused, I am aware of circuits involving laser beams and photo cells, where the beam is received by the photocell prompting actions, however im completely unaware of how a laser beam could be used to trigger a device if it is not received by a photocell and instead directed into the open sky

I take it, it must be possible for the laser to change its current or something in some way if it is interrupted or visible due to fog, rain etc

im kind of wondering along the lines of a laser measuring device , I take t these send a beam out and when it hits something somehow changes the data of the device

would this possibly be a similar theory

cheers

 

It looks like there are lasers used for locating the distance to a fog bank, but I think that's not what you're after. I think you just want to detect scatter (reflection) of the beam in a region very near the source.

If it was me, I'd forget the laser and use a TV remote with an IR detector module looking in the same direction. In clear air, the signal will not be reflected back to the detector. At some level of droplets in the air, the detector will see the signal. You can do the same thing with a laser but the TV remote and detector are nearly free and are easy to experiment with. Maybe save the laser upgrade for version 2.0

 

yeah this sounds good, I already have a few IR LEDs and IR detector modules,

I understand the process more from what you've said, I get it now, the IR light points into the air along side the detector and if it reflects rain droplets etc the glare is detected by the IR detector , that makes a lot more sense to me now in the theory of real fog horns as the fact they are placed near lighthouses on rocks etc in the dark sky the slightest change of light is much easier to detect.. n some ways helpful to me to as im on a 15th floor.

using the schematic above do you think it will be a relatively easy process to add this small circuit involving the IR light and detector to trigger the horn in place of the switch, and apart from the IR lights and Detector what extra components am im likely to need.

cheers

 

I'd switch the horn on using an n-channel MOSFET as a switch on the ground line (not the V+ power line). Just about any will do. I have a bunch of IRF540N that I use, and I think Radio Shack stocks the IRF505 if you are in the U.S. (guessing you are not?) Either would be fine. Just be sure the one you get is rated for a continuous current at least 2X whatever your horn circuit draws.

The MOSFET needs a voltage (usually >10V, less for a "logic level" FET) on its gate to turn on like a switch. So you need something between your detector and the MOSFET that will do that. A 555 monostable might work.

You need to define how you want it to work, for how long after the detector triggers. Manual reset? Auto-reset? Continuous horn, or just 30 minutes?

We'll also need to know what detector you have - some hold a steady output state when receiving a signal but most do not.

 

The wavelength used is very important since fog, smoke, rain, and other phenomena have reflectivities that are highly dependent on the wavelength. That doesn't mean that you can't start your experiments using whatever emitter you can find, just keep in mind that if you aren't getting any results it might be because the fog is too transparent that that wavelength.

 

Some reading on transmissometers might be useful for the fog detector part of your project.

 

The wavelength used is very important since fog, smoke, rain, and other phenomena have reflectivities that are highly dependent on the wavelength. That doesn't mean that you can't start your experiments using whatever emitter you can find, just keep in mind that if you aren't getting any results it might be because the fog is too transparent that that wavelength.

The general rule is that longer wavelengths (RED) transmit further/better through a cloudy substance than the shorter wavelengths (BLUE).

It's basic physics, same as the fact that longer sound wavelengths (BASS) transmit further through a lossy medium than shorter wavelengths.

 

You need to define how you want it to work, for how long after the detector triggers. Manual reset? Auto-reset? Continuous horn, or just 30 minutes?

We'll also need to know what detector you have - some hold a steady output state when receiving a signal but most do not.

hi wayneh, on the subject oh how im wanting t to work, ideally it would sound on for a couple of seconds then off for a couple of seconds, until manually switched off or the fog etc cleared.

as for the detectors I have I am unsure of the component numbers, they are in my salvage bin taken from an old hifi system, dvd players etc, i would be glad if you could suggest the best type so I can purchase them from the net,

The wavelength used is very important since fog, smoke, rain, and other phenomena have reflectivities that are highly dependent on the wavelength. That doesn't mean that you can't start your experiments using whatever emitter you can find, just keep in mind that if you aren't getting any results it might be because the fog is too transparent that that wavelength

I am unaware much about wavelengths obviously looking for one which is likely to work in conditions of rain fog snow etc I checked out the page apco 1 posted on the subject of transmssometers it mentions a wavelength of 550 nm but i notice it mentions these devices although working in the same way im talking about are targeted at a receive as apose to a reflection

 

hi wayneh, on the subject oh how im wanting t to work, ideally it would sound on for a couple of seconds then off for a couple of seconds, until manually switched off or the fog etc cleared.

OK, well the on/off function can be done easily with a 555 IC timer circuit at a low frequency.

The function of coming on and staying on can be accomplished with a comparator circuit with hysteresis. Once triggered on, the fog would have to reduce substantially before it would turn off. The hysteresis can be adjusted to avoid "fluttering" at the trigger point. Like how your furnace runs until the temperature changes a couple of degrees, not just 0.1°.

What's still fuzzy to me is how to deal with the detector. As I mentioned, most will briefly change their output when a signal is detected, but they don't hold the value. I think the transmitting LED can be pulsed in bursts, like for 100ms every 2 seconds or such, to keep the detector output held steady. Keeping the duty cycle low helps conserve energy and LED life, too.

 

thanks for the info it has made me far more aware of the needs for this circuit to work, ill keep you posted in my progress with it.

 

The function of coming on and staying on can be accomplished with a comparator circuit with hysteresis.

Another 555 timer, or the second timer on a 556 could be used to accomplish the latching. So a single IC might do all you need.