Cheap, Quick & Dirty Automatic Emergency Light

It's rather normal for us electronics tinkerers that, from time to time, we sit in the dark - because we often perform our tinkering under favour of night, and then blow a fuse or trip the circuit breaker, by accidentally shorting or overloading our workshop's mains supply. In the UK this may be somewhat different, since there the fuses are located inside the mains plugs, or so I was told - and if you blow one of these, with some luck the circuit breaker in the fuse box remains on.

If your house is equipped with an earth leakage circuit breaker, even only touching the protective earth conductor is sometimes sufficient to produce a blackout.

If this happens, in addition to having been given a fright, your lights are off. And in the worst case, there is something on your desk that is smelling bad, smoking or sizzling - which you can smell and hear, but not see anymore (and if you can see it anyway, it is high time to fire up your fire extinguisher!). It is important for you to first switch off whatever might still be switched on in front of you and then quickly find the breaker box to get some light again. All these tasks are performed much more easily if there is some emergency light available. Apart from that, if your house is as tidy and well organised as mine, you can avoid tripping in the dark over your junk box or something else that's blocking your way.

You might, of course, purchase a commercial escape light equipped with a rechargeable battery.

But for this simple application - just a little help finding your way in the dark - it doesn't have to be anything sophisticated; a small light can help a lot in this situation. I tried getting it on the cheap by using materials lying around and waiting for a new purpose.

So check your junk box!

You need to find:

• A retired wall wart (or perhaps a charger for the now obsolete NiCd batteries)
• In case the wall wart only provides AC or unsmoothed DC, you add some diodes and/or an electrolytic capacitor.
• Any battery-operated LED light (such as a cheap torch light or an unused bicycle head lamp)
• A relay that is small enough to fit into the wall wart's case. Its coil voltage has to approximately match the one of your wall wart and needs to have a normally closed contact
• Perhaps a resistor (depending on the wall wart and the relay)

And, last but not least, the most important ingredient:

• The lucky idea of how to connect that all

(In order to not complicate this instructable too much, more information on reusing wall warts will soon be given in a separate instructable with the apt title 'Reusing Wall Warts'.)

This somewhat depends on the torch light and the wall wart you have available. But the principle is simple:

Your emergency light unit is plugged into a wall outlet. This outlet must be fed from the same circuit breaker that trips when you produce a short circuit. It should be located next to the exit of the room where your tinkering takes place, and its output is only used for one single purpose: it activates a relay as long the socket provides power. As soon as this power goes missing, the relay drops off. Its normally closed contact will close and switches the torch light on, providing some light in the room in order to allow you finding your way. Since your emergency light unit is only plugged into the wall outlet (instead of being permanently installed, e.g., above the exit door), you can pull it out and take it with you on your way to the breaker box - in case the whole house should be dark.

(Above, you see a block diagram showing first the two units while still being single, then after they were married. The next picture is the circuit diagram of the finished assembly.)

In my case this rather was a battery charger instead, made by Sony (BC-7DY). It was plugged directly into a wall socket and provided a charging bay for one of the small, flat, box-shaped (and now obsolete) NiCd batteries. These were used in the olden days for e.g. walkmen or wireless headphones. The charger's type label says 'DC 1.2 V --- 250 mA'. This must not be taken for granted, since it is true only as long as a NiCd battery is inserted and charging. Under no-load conditions its output voltage is considerably higher. In fact, with a smoothing capacitor it is about 7.4 VDC - which is a bit too much for the particular small relay I selected. There was no smoothing capacitor contained, which is ok for charging a NiCd battery, but not ideal for operating a relay.

So some minor modifications were required:

• The case had be opened (for, at least, having a look at what's inside). Since the two case halves were glued together, I had to separate them by using a bit of patience, a hacksaw, and brute force. In the end, after re-glueing the case, it will look a little less pretty. But fortunately there is no beauty contest to be entered.
• The case contains a small PCB on which all the electrical components are installed (the mains transformer, two silicon rectifier diodes and a 1 Ohm resistor), including two wire contacts that protrude into the charging bay; these contacts are no more used and were removed. The two strange wire loops standing up are, btw, contacts for connecting the transformer to the inner contacts of the mains plug when the case is closed.
• I installed a 100 uF/25 V electrolytic capacitor for smoothing the output voltage. Observe the correct polarity when installing it; the positive wire is usually the longer one, sometimes marked with a '+' sign, and the negative wire is marked often with a wide band and a '–' sign.
• I replaced the 1 Ohm series resistor by 120 Ohm in order to bring the voltage down to about 4.5 Volts which is sufficient for a 5 V/500 Ohm relay to energise.
• And the relay: I found a tiny Elec-Trol RA30511051 reed relay in a DIL14 package in my junk box. This manufacturer exists no more, but similar products are available, such as the HE721B0500 by Littelfuse. The relay features a 5 V/500 Ohm coil, and its contact rating is sufficient for the low current of my LED torch light. It is important that the relay is equipped with a normally-closed contact, since you want the light to be on when the relay is not energised. The pins of the DIL package are somewhat delicate - in order not to damage them, I soldered the relay on a leftover piece of prototyping board. I fixed this assembly within the case the cheapo way with a few drops of hot glue. There exist reed relay versions having a suppressor diode in parallel with the coil; if you have one of these, make sure that you connect the + supply to the pin going to the diode's cathode (usually no. 2) in order to avoid shorting the supply voltage with the diode.

Once everything is installed, you drill a small hole in the case, feed two wires from the relay contacts to the outside, and close the wall wart by its screws, glue, adhesive tape, or whatever is convenient. Remember, it doesn't have to be pretty but rather useful instead...

This is a very cheap unit with several bright white LEDs, powered by three AAA dry cells (also called LR03, AM4 or Micro) connected in series. The dry cells are contained in an extra battery box with a center contact each front and back. The cylindrical body has a screw-on cap at the end that also contains an on-off switch.

This torch light needs a little modification in order to be married with the wall wart, but this isn't complicated at all. You need to separate the rear contact of the battery box from its counterpart in the screw-on cap and feed both of them into the wall wart containing the relay, which then acts as the new on/off switch for the torch light.

An easy way to separate the contacts is using a small cutoff piece of double-sided, copper-clad PCB material (from the junk box as well). This is some stiff insulating sheet having both surfaces covered by a thin copper film. If you solder each of the wires coming from the wall wart to one of the copper surfaces and then put this piece between the two contacts, your torch light can be switched on by connecting these two wires. A hole must be provided in the body for feeding these wires through. You must not forget, however, to switch the torch light's own switch on. (Because two switches connected in series can also be thought of as a logical AND function - the lamp is on if, and only if, one AND the other switch are on - this is clearly visible in the circuit diagram in step 2 above.)

(The 1st picture above shows the two contacts in the torch light - one at the end of the battery box, the second is the spring in the screw-on cap. The 2nd picture shows the PCB contact piece before inserting it into the body)

Once everything is connected and working, you attach the torch light to the wall wart by whatever seems reasonable - glue, zip tie, gaffer tape, you name it. Then plug it into a wall outlet at your desired location, as shown above.

Don't be disappointed by the fact that, from this moment on - thanks to one Mr. Murphy and his law - there will be no more short circuits in your workshop. But better be safe than sorry, anyway.

Nevertheless, you can test your emergency light whenever you like - just pull it out of the wall outlet (as shown in the first picture of this instructable). If it doesn't light up, either its switch is off, or you need to replace the torch light's batteries - but this won't happen frequently since you will never need your emergency light in the future :-)