DIY Workshop Fire Alarm

A brilliant workshop fire alarm you can build yourself

by Julian Edgar

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At a glance...

  • High quality DIY workshop fire alarm
  • Mains powered, battery back-up
  • Very loud pulsing horn
  • Can use salvaged and secondhand parts to reduce cost

If you’ve got a home workshop, it’s probably not under the same roof as the place in which you live. So what happens when there’s a fire in your workshop, late at night? Even if there’s a smoke alarm fitted, you’re unlikely to hear its beeper when the shed and home are both closed-up tight – especially when you’re in bed asleep. Or what if you’re away from home, and your neighbours don’t hear a thing until the flames are consuming your precious tools – and maybe the car you’ve been modifying or restoring?

It doesn’t warrant thinking about, does it...

So here’s a solution. It is powered from the mains and has full battery back-up, uses a high quality photoelectric detector, and has a loud, pulsing horn that can be heard blocks away. Further, you can link two or more smoke detectors together, so covering large areas. You can also use some salvaged or secondhand parts to reduce the total cost.

Components

The components are sourced from a variety of places. The smoke detector is Jaycar photoelectric sensor. This sensor is designed for hard-wired 12V operation and has a relay output. Interfacing with the detector is a fully built eLabtronics Pulser electronic module. Depending on how you mount things, you may also need a box for this module.

The smoke detector is supplied 12V from a sealed lead-acid (SLA) battery that is float-charged by a mains plug-pack dedicated charger. The battery also powers the Pulser module and the horn. What horn is that? Well, we suggest that you use one or two 12V car horns – but you can use any commercially made 12V siren or horn. Car horns are available very cheaply from wreckers or can be found in pretty well any junked car.

Finally, you’ll need some 4-core cable to connect the smoke detector(s) to the battery and control system.

So what’s the all-up cost (Australian dollars)?

  • Photoelectric smoke detector – Jaycar LA-5045 - $19.95

  • Sealed Lead Acid Battery – Jaycar SB-2480 - $24.95

  • 12V charger – Jaycar MB-3517 - $19.95

  • eLabtronics Pulser module – AutoSpeed Shop - $59

  • Car horn – wrecker - $10?

  • Cable – 4-core – 60 cents/metre

If you buy all these components, the all-up cost will be around $150. Remember, that’s not just for a normal smoke alarm, but for a full photoelectric, battery back-up, mains-powered fire alarm that will be heard even if you’re asleep inside a closed-up house.

In my application, I already had a SLA battery salvaged from a uninterupptable PC power supply (they usually have 6 or so in them), a float charger that had been used in a previous project, a couple of car horns that had been picked up from the tip, and plenty of 4-core cable that had been sourced in the same way (ie a discard from the tip). I also sourced a second-hand metal box – a box originally housing control gear for a high intensity light (pictured). With its guts removed, it made a fine enclosure. It was purchased for $2 from the shop at a local tip.

Even with two smoke detectors and two alarm horns, that brought the cost down to about $100 – very cheap security.

So let’s take a look at the layout of the system. Putting it together is largely just a case of connecting wires – no special electronics knowledge is needed.

Ionisation vs Photoelectric

Two different types of smoke alarms are widely available – ionisation (the most common and cheaper type) and photoelectric.

Ionisation alarms are more sensitive at detecting small particles that tend to be produced in greater amounts by hot, flaming fires, those that might have particulate emissions invisible to the naked eye. Photoelectric alarms are better at sensing large particles, of the sort produced by smoking, smouldering fires.

In a workshop, ionisation alarms are best suited for locations that contain a large amount of combustible material – flammable liquids, paint, newspapers, etc. Photoelectric alarms are best suited to locations where the material will burn more slowly, releasing greater quantities of smoke – stored timber, wooden benches, wooden shelving.

One source suggests that most smoke alarms in North America fitted to commercial premises are of the photoelectric type – it is only in domestic settings where the cheaper ionisation type dominates.

Some rarer smoke alarms have dual sensing systems – ie they are both ionising and photoelectric. However, these are more expensive that either ionisation or photoelectric alarms.

The detector used in the described system is of the photoelectric type.

Step by Step

The first step in building the system is to carefully unscrew the back of the smoke detector. As shown here, there is a moveable link positioned on the printed circuit board. Move the link to its other position – this changes the triggered output from being an open relay to a closed relay.

We suggest that you test the system on the bench before installing it, so the next steps are to wire the system up as shown here.

Connect the Pulser module to the battery and horn. Horns don’t usually have a polarity but most sirens and buzzers do – the positive goes to the module. Check your wiring carefully.

Set the Frequency pot half a turn anticlockwise from the fully clockwise position. (Note: These pots are multi-turn so don’t expect to make only one rotation when setting them. Multi-turn pots also don’t have clear end-stops [although they can sometimes be heard clicking when they’ve reached the end of their adjustment] ).

Set the Duty Cycle pot to roughly the middle position in its travel.

Use a jumper wire to temporarily connect the Input terminal to the +12V supply. The Pulser module will then switch on (the on-board LED will light) and the horn should start pulsing. Adjust the frequency pot to change how fast the horn is switching on and off, and the duty cycle pot to adjust how long it stays on each pulse. You can set these to suit your personal preferences.

Now make sure the MOSFET (the output switching transistor) isn’t getting too hot! How hot it gets depends on your ratio of horn ‘on’ to ‘off’ times, and how many horns, sirens or buzzers you are running. In my testing, using a fairly long ‘on’ to ‘off’ time (ie high duty cycle) and running two conventional car horns, after a few minutes of (noisy!) performance the MOSFET started getting warm to touch.

I then fitted the small heatsink shown here and the temp was then fine. Note: the MOSFET can get quite hot without there being a problem – as a guide, you should be able to keep your finger on it without pain!

Once you have the horns pulsing happily and the MOSFET not getting too hot, you can disconnect the battery, remove your jumper and then wire-in the smoke detector. Again, I suggest that you first do it on the bench to confirm that everything is working properly. Follow this diagram and don’t forget that you need to change the position of the jumper link on the smoke detector, as described above.

With the system set up on the bench, press the ‘test’ button on the smoke detector and the horn should sound. Blow smoke into the detector (anyone still a smoker these days?) and the horn should again trigger. Note that if the sensitivity of the detector is too low, remove the back and adjust the small pot (arrowed).

At this stage you probably won’t need to charge the battery, but for completeness the charger is included on this circuit diagram.

Horns

Car horns are grossly underestimated as effective alarms. They are super loud, very cheap, work off low voltages and – with the use of the Pulser module – can be cycled on and off without effort.

When using two horns, it makes sense to use two different designs – even two from completely different cars. This is because when they are sounded together, the different pitches make for a discordant sound, which works well at gaining attention. Note that if you have two identical horns, they can be made to sound different by rotating the ‘pitch adjust’ screw found on most horns.

Horns should always be mounted using the original strap brackets. Most times, these comprise leaves of thin metal, allowing the horn to vibrate when operating. This increases the sound output over ‘hard’ mounting a horn. Therefore, when buying or salvaging old horns, ensure you also always also get the mounting strap.

Installation

With everything working and tested on the bench, pull the system apart and mount it in the workshop. For convenience and best smoke detecting ability, the smoke detector will normally be mounted remotely to the battery/horns/Pulser.

The location of the smoke detector is important – a poor choice of location can severely reduce the effectiveness of the fire alarm. The following points are based on advice provided with Chubb smoke detectors.

Do NOT place smoke alarms:

  • In turbulent air from fans, doors, windows, etc. The rapid air movement may prevent combustion particles from entering the alarm. This is especially important if you have permanent workshop ventilators operating eg roof-top whirlybirds.

  • In dead air spaces such as the peak of an "A" frame ceiling. "Dead Air" at the top may prevent smoke from reaching the alarm in time to provide early warning. In rooms with simple sloped, peaked or gabled ceilings, install smoke alarms on the ceiling 90 cm from the highest point of the ceiling.

  • Less than 30cm from the wall when mounted to the ceiling.

An alternative to mounting on the flat surface of walls or the roof is to make a support on which the smoke detector mounts and then suspend that support from the roof on a chain. In my application three smoke detectors were used, being hung on chains to drop them out of the “dead air” near the top of the A-frame roof.

I chose to mount the horns on the outside of the salvaged metal box. However, there’s nothing to stop you mounting the horn (or horns) remotely – eg outside under eaves or in some other semi-sheltered spot.

The battery and the Pulser module were mounted inside the box....

...with the battery held down by a folded piece of aluminium. A piece of rubber is sandwiched between the top of the battery and the strap.

The Pulser module is mounted on a piece of insulating plastic.

The wiring that connects the smoke detector power supply and output to the battery and Pulser can be of quite thin gauge – these wires take very little current. However, use normal gauge (eg 5 amp) hook-up wire between the battery and the Pulser, and between the Pulser and the horn(s).

On/Off Switch?

Depending on the activities that occur in your workshop, you may wish to place a manual on/off switch in the system. This switch can also be used to turn off the system should it ever false alarm.

If you place the switch near the battery, the alarm horns will make a short sound whenever you turn the system back on.

If you place the switch at the horn, the system will work when it detects smoke but no sound will come from the horn. The system won’t beep when switched back on.

Important note: if you leave the system off, it won’t detect when your workshop starts to burn down....

Conclusion

Whether you have a very large workshop or one that’s more modest, this DIY fire alarm system with give you much greater real-world protection than a conventional cheapy smoke alarm. By using salvaged or secondhand parts, the system can also be made quite cheaply.

Finally, after you’ve installed the alarm, don’t forget to familiarise neighbours with its sound so that if you’re away, they can react appropriately.

Dual Detectors or Horns?

Using two (or more) detectors requires that they’re wired in parallel, as here. Don’t forget that each detector needs to have the internal jumper link moved to the second position.

Horns or sirens can also be wired in parallel. If they are polarity conspicuous, don’t forget to wire them correctly.

Testing

The system should be tested on a monthly basis. Always switch off the battery charger when doing the test, to ensure that the battery is still holding charge properly.

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