Simple cheap alarm design.

[patrick_g]

A friend of mine is building a house and is concerned with theft of tools. Another friend who recently built a house had the door to his garage kicked in and all his power tools, including table saw etc) and other goods too, taken, never to be seen again.

I told both of them a penny spent on prevention is worth many dollars in tools saved.

One out of two listened and he is interested in an alarm for his shed at the house site.

I built a simple alarm years ago and so I just drew it up from memory and built another one for my friend. It is a pretty basic system, easy for DIY. It is intended for when someone is close enough to hear the siren but could be easily remoted with a radio link. One version I helped a coworker install in his car, paged him if the car's security was compromised. That was sufficient for him because he was within a few hundred feet of his car (but out of sight) when he was concerned with theft.

Anyway I am attaching the schematic for a simple version.

Circuit description:

Batt: I used an old 12 volt car battery but could be a lot smaller, a motorcycle battery or starting battery for a lawn mower or...

Siren: I had an old 12 volt DC motor driven siren but you can buy an electronic siren cheap or use an inexpensive air horn from Harbor Freight or J.C. Whitney. Any 12 volt DC operated noise maker is OK if it doesn't exceed the current rating of the relay contacts.

Relay: The relay I used this time is a DPDT 12 volt relay with 12 amp contacts. A SPDT relay would do fine. I wired the contacts in parallel to get more current handling in case my buddy want a real kick butt noisemaker.

Switch 1: is a simple on/off switch SPST. I used a labeled toggle switch but a $0.69 light switch from Lowe's would be fine.

Switch 2: is a momentary contact SPST switch, essentially a doorbell button.

The other component (not shown) is a battery maintainer which are often on sale for way under $10 at Harbor Freight. By using a battery the alarm system can not be disabled by turning off the AC power
The arrows labeled "to series sensors" in the schematic drawing represent two wires that go to the sensors which are all wired in a "daisy chain" like cheap Christmas tree lights. YOu can put a few nails around a window opening and zig zag some small gauge wire across it. This constitutes an effective window sensor. If the wire is broken the alarm sounds. Restoring the wire does not stop the alarm which runs till the battery goes down (can be several hours) or the alarm is turned off.

You can buy magnetic switches with an accompanying magnet to put on the door and door frame so if the door is opened the alarm goes off. Again, closing the door does not shut the alarm off.

How does it work?

When it is wired up and installed with the series sensors you arm it by pressing and holding down the momentary contact switch (door bell button AKA Sw 2) and turning the Off/On switch (SW 1) to the On position and then release the push button.

The alarm is now armed and the little current draw of the relay coil is flowing in all the series sensors. If a sensor is "violated" this interrupts the current through the relay coil and it de energizes which lets the (C) Common contact of the relay touch the NC Normally Closed contact which puts battery power to the siren until the battery runs down or the Off/On switch is turned off. There is nothing that can be done to the sensors that will shut the alarm off, a desirable trait.

If you buy all the components new (including a battery) and use a new automotive air horn for your noise maker it could cost $50-$100 to make one of these. If you scrounge a bit, get horns from the junk yard, have or can find a used car battery and so forth but buy the relay (less than $10) then you can build one of these easily for $10-$20.

Be creative... the push button can be replaced with a piece of flexible multi strand wire with which you make momentary contact. You may have a battery charger to use on it a few hours a month to keep the battery charged. YOu can scrounge up and substitute for most everything it takes to make the circuit except I don't recommend making your own relay. They can be found under $10 at radio shack or at RadioShack- Audio/Video Electronics, MP3, LCD TVs, GPS, Digital Cameras, DVD Players, Accessories


This same sort of arrangement can be used to alarm a trailer or a tractor or implement or... Wrap a wire around or through the item of interest and include that wire as a series sensor. The wire will be broken if someone takes the item. Sometimes all it takes is a siren going off to chase away a thief, often empty handed.

Of course you could post a sign to the effect that for public safety a warning siren will sound a few seconds before target practice begins. A 12 volt DC light aimed at the protected device and powered by putting it in parallel with the noise maker os a good thing if protecting a tractor or implement. Then in additioin to the loud attention getting noise the would be thief is "in the spot light" so to speak.

Note: the numbering of your relay may be different from the RS unit I bought.
Pat

 

[patrick_g]

For picture oriented folks I am including a snapshot of a rapidly tossed together version that cost about $12 because I used "nice" switches. The rest is crude home brew with parts made from scrap galvanized sheet metal. Galvanized is handy because it solders easily.

This little scrap of MDF holds the switches and relay and has machine screws to hook up the battery, sensors, and siren.

Pat

 

[SPIKER]

Patrick:

i believe that you need a jumper wirer from the normally open contact back to the coil to HOLD the coil in while the juice continues back through all the safety wires on the doors/windows. the jumper wire is used to maintain the coil power . I'm sure it was a simple omission on the drawing only.

mark

 

[patrick_g]

Patrick:

i believe that you need a jumper wirer from the normally open contact back to the coil to HOLD the coil in while the juice continues back through all the safety wires on the doors/windows. the jumper wire is used to maintain the coil power . I'm sure it was a simple omission on the drawing only.

mark

I built this one from the wiring design that I posted and it works. I will take a look and either comment on your "fix" or explain why a change isn't needed. Thanks for looking it over. It wouldn't be my first mistake.

Well, I looked over the drawing and it is what I wanted. It works as drawn and has been delivered and demonstrated successfully at the customer site.

To reiterate the sequence of events. First, of course, you ensure that all the daisy chained series connected normally closed sensors are closed. Then you press and hold the momentary contact switch and while holding the momentary switch down yo turn the off/on switch to the on position and then release the button (normally open momentary contact switch)

As a practical matter the current through the momentary switch is in parallel with current through the siren so the siren runs just a very little bit when the off/on switch is first turned on but when the relay energizes because the momentary switch provides a current path to energize the coil the path for siren current is interrupted when the relay energizes.

If I left you in the dark (sometimes my explanations are not the clearest), please ask a specific question and I will try to answer it. The circuit does work AS DRAWN and AS BUILT and are the same.

The push button provides a temporary shunt around the NO contacts of the relay to power the coil. Once energized, the NO contacts provide current to keep the relay energized until a sensor goes open. Of course the de-energized relay lets the NC contacts turn on the siren until the off/on switch is turned off, the battery runs down, or the cows come home.

The little shot of siren when first "set" is mostly a function of the "soft" battery substitute I used (small battery charger) which is pulled down by the starting surge and causes a brief delay in energizing the relay which shuts the siren off. It is just a small fraction of a second. With a real battery for power (pretty stiff source) it is likely the motor driven siren will hardly spin any if at all. If an electronic siren is substituted for the motor driven one it will probably just make a click sound when the system is set.

Pat

 

[Soundguy]

Spiker.. it's a simple 'latching' relay design.. the circuit appears to function as drawn, IF the lines going to the series sensors have continuity and the main spst on/off switch is on, and you hit the momentary button.. that causes the relay to engage, because the common relay contact, fed by the on/off switch is carried over to the coil thru the NO contact, the relay is latched.. IE.. it's own actuated contacts are feeding power to the coil.. this circuit trips ( unlatches ), when the path to negative ( sensors ) are cut. ( for a glass window I'd use foil tape.. for doors.. I'd use magnetic switches.. etc.

Once the circuit unlatches, the siren is powered by the battery thru the NC contact and the common contact, and can only be reset by reconnecting the sensors, and then flashing the coil field with the momentary SPST switch... causing the relay to once again latch.

And spiker.. you may need to have them glasses checked.. the NO contact is tied to the coil line #8, while #7 finds (-) thru the sensors.... in this diagram.. no jumper wire needed...

soundguy

 

[Soundguy]

Upon further review, you coul add a capacitor to this circuit to add an entry delay.. I see 2 places it could be placed.... delay value based on entry delay needed. You could also add grounding SPST momentary ( NC ) pin switches to certain areas that could be made to make the relay unlatch without breaking the series sensor line by the addition of another relay with NC contacts in line with the pictured relay's field line with one side of the new relay's field powered, and the other side of it's field hooked to the pin switches which are NC, but 'held open' by the door / window being closed.. when that window was opened, it would energize the 2nd relay, break and unlatch the field line to the first relay... etc. All of these pin switches could be wired parallel to the 2nd relay.. IE.. only 1 other relay needed.. etc.

From there.. lots of other circuits could be added that all trigger when the first circuit unlatches...dialers, flashing lamps also based on unlatched relays.. etc.. Could even drive a timer circuit that does all that and then resets.. even add a circuit that 'tries' to reset the primary circuit axter 'x' time.. of course it would only be able to reset if one or both of the 2 fault conditions were not present... etc..

Could jazz it up and use an x-10 lamp controller or any off the shelf rf controlled recep block, and make this thing with a wireless entry / keychain fob setup that drives a relay setup and arms or disables it...

Soundguy

 

[patrick_g]

SoundGuy, As you say, there are a jillion ways to make it more sophisticated. You can buy more sophisticated at a number of stores. I shared this design as it is the simplest for a DIY guy to whip up that works well.

As you noted the holding contacts on the relay keep the siren from being silenced by closing the door or whatever and restoring the series sensor chain.

I built this for a friend who has a well house with electricity about 50 feet from a shed that doesn't. He is using the shed to secure a lot of power tools. I instructed him to nail the unit in the snapshot to the wall inside the well house and hook up a batt to it (using a batt maintainer to keep it charged.) I loaned him a 12 VDC motor driven siren loud enough to wake the dead. A $20 air horn or a 12VDC solid state siren would have been OK. The siren goes at the gable end vent of the well house where it can be heard at the trailer he is living in while building.

The sensors are magnetic switches on the window and door and some thin wire stretched between nails across the window to prevent entry without breaking the circuit. (No glass tape readily available out here on the fringes of civilization and Radio Shack stores are essentially telephone boutiques and toy stores nowadays.

I instructed him to take a cheap household indoor extension cord and cut it in the middle. Use one end to connect to the sensor connections on the "brains" of the alarm and wire the other cut ends to the sensors in the shed. You now insert one of the big extension cords at the job site between the two cut ends to complete the sensor circuit. If "THE BAD GUY" tries to steal the extension cord or unplug it to kill the alarm, it sets off the alarm. There is nothing that the bad guy can do at the shed to stop the alarm. He would have to break into the locked windowless well house and find the alarm system or siren. If he isn't hearing impaired before then he will be afterward. Did I mentioin the alarm is LOUD!!!

I don't know if my friend will take my advice to add a light in parallel with the siren. I suggested he mount a 12 VDC spotlight under the eaves of the well house and aim it at the window and door of the shed. The would be thief would be well illuminated and the alarm should make him self conscious as well as an easy target for my friend.

The latching relay approach is handy for a simple anti theft device for vehicles with a key switch too. A secret push button switch is mounted in the car and the "signal" (current supplied by the switch) from the key switch is routed through it. Turning the key to the on position will do nothing unless the secret switch is pressed, after which everything works as per normal until the key switch is shut off and then the secret switch must be used again.

I had this system but with a slight improvement in my 1943 Ford built military Jeep. I incorporated a 6NO30 (6 volt Normally Open 30 second thermal delay relay) into the system. If the bad guy didn't give up easily and opened the hood and hot wired the ignition to start the Jeep it would start and run for 30 seconds and then shut off and flash the lights and honk the horn. The theory was that this should give him time to get into the road in traffic and the stopped vehicle with flashing lights and honking horn would make him self conscious so he would slink away or alert me so I coilid have a discussion with him.

Pat

 

[Soundguy]

No criticism was intened. I like simple robust designs like this one. makes for a good unit.

I made a circuit very similar to this as a back door alarm at a building i used to work at, alone... simple, cheap, and effective!

This can be added to a vehicle as you mention.. especially if you can hid a small power source seperate from the battery!

soundguy

SoundGuy, As you say, there are a jillion ways to make it more sophisticated. You can buy more sophisticated at a number of stores. I shared this design as it is the simplest for a DIY guy to whip up that works well.


Pat

 

[patrick_g]

No criticism was intened. I like simple robust designs like this one. makes for a good unit.

I made a circuit very similar to this as a back door alarm at a building i used to work at, alone... simple, cheap, and effective!

This can be added to a vehicle as you mention.. especially if you can hid a small power source seperate from the battery!

soundguy

Soundguy, I did not think of your comments as negative, just in the spirit of incremental improvement. My response was to clarify my intent (and may or may not have succeeded.) I think this latching relay approach is the simplest DIY alarm that is truly useful which is why I posted it. There have been various discussions regarding loss prevention and this design is a decent first step.

I offered to put a radio remote module on it to ring in his house if the alarm sounded but he declined saying the siren was plenty. The alarm we put in my friends VW bug was silent and directly paged him (he wore a Motorola Pageboy model pager which we alerted via a small transmitter sharing his car's radio antenna.)

Automotive alarm systems that do not have their own battery are subject to being disabled by anyone who can manage to get cutters on either of the battery leads. The alarm system (store bought not DIY) that I have in my Dodge truck has an electronic siren with its own battery. Once this module turns on it can not easily be turned off. If you disconnect the vehicle battery that will set off the siren module.

I had the truck in for maint and asked if they were going to do anything that would require disconnecting the truck battery. They said ABSOLUTELY NOT, NO WAY! Later I get a call at home and in the background I can hear a very loud siren. I had to drive 20miles to turn off the module with a high security round key. It was specked to run at full volume for several hours on its internal battery which was charged off the car battery (diode isolation.) An impound driver told me the tilt switch on a car he hauled turned on the alarm like that and it ran all the way to the impound lot and ran for hours.

There is no alarm that is both useful and impregnable to being circumvented but with a decent installation virtually all BAD GUYS will exit the scene empty handed if a truly loud siren audible both inside and outside of the building goes off and can't be quickly silenced.

In these days of remotely monitored security systems, most bad guys know to cut your phone lines before breaking in so the alarm company is not called by the system. I pay $5/month for a cellphone backup system. If my phone line is inactive, the "rules of engagement" automatically change and if while my phone line is inactive an alarm zone is violated the monitoring company is notified via the cell phone module and the alarm company does not call the house to see what is up they just dispatch the cops or fire department depending on what sensor(s) tripped. The assumption is that the odds are if your phone is down AND a zone is violated it is not coincidence.

Pat

 

[Soundguy]

Yep.. simple can be good.

I've used latching systems on tractor and stationary engines as low oil pressure cutouts and ignition isolation. like if an old gasser stalls, the points don't set there and burn.. or if you leave it unatended to pump.. say.. water, and the oil pressure drops ( closing a switch t'ed into the oil line ).. then it would either cut ignition power.. or activate the key 'off' diesel fuel solenoid.. etc... they work well for those situations. Kinda like a murphy safety switch. Just have a 2nd push button to bypass during cranking, and / or, depending on design.. just use the key switch, and then let the relay latch and go.. couple different ways to wire depending on what you want to do.

soundguy