This night light provides a warm soft glow after dark and is battery powered, so it can be placed anywhere - bedroom, dark stairway, hallway - or even carried with you if you awaken in the night and have to find your way in the dark ! Its most dazzling feature is that it will operate for over four months on a single D-cell.
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The "Electronic" part of this night light derives from the so-called "Joule Thief" which is the real secret of its performance. The Joule Thief circuit drains practically all the usable energy out of the battery. The circuit also has a feature that turns it off in daylight to further conserve its battery.
The Joule Thief is by now a well-known type of circuit for driving an LED from a single cell battery. The circuit boosts the battery voltage to the 3.5 volts or so that a white LED needs, and continues working until the battery has been drained down to around 0.5V. This is the voltage at which the single transistor in the circuit can no longer function, so everything stops. At this point you can feel pretty good knowing that you've made use of virtually all the energy that battery had to give. The light will get a little less bright as the battery wears down, but it is still an effective night light near the very end. You can even make use of batteries that have become too weak to brightly light your flashlight or play your radio. One of the author's radios refuses to play when it has run the batteries down to around 1.2 Volts, but there is still enough energy in them to keep one of these electronic night lights going for a month or two!
Variants of the Joule Thief circuit have been described in a number of articles, Instructables, and websites. This author learned the basics of how to make one from the following interesting website: www.bigclive.com/joule.htm .
This is the schematic diagram of the Joule Thief circuit as made for this night light project. It is more or less what has become the "standard" Joule Thief circuit with two minor modifications. The first is that the resistor R value has been increased to 10K (from the usual 1K). This modification prolongs battery life by reducing the current draw; the illumination output is decreased, but still makes a very effective night light. You could use a lower resistor value for more light, but battery life will be cut shorter. You may in fact wish to put in a potentiometer instead of the fixed resistor so you can easily adjust the light level to your liking. But be aware that current consumption can rise significantly. For example, the author measured the current draw of the circuit from a battery at 1.1V and found that with the specified 10K resistor the current drawn from the battery was 12mA, with a 5K resistor it was 20mA, and with a 1K resistor it rose to 40mA.
The second modification is the addition of the Photo Transistor Q2. This turns off the night light in daylight to conserve battery energy. When light falls on Q2, it conducts and brings the base of the transistor Q1 to ground, thus shutting it off. The circuit still takes a tiny bit of power through resistor R and the primary of the transformer T, but the current draw is drastically less than when the circuit operates, and thus conserves the battery when the night light is not needed. Of course, if you are going to put the night light in a place that is always rather dark, like in a corner or inside a bookshelf, then Q2 can just be omitted; the light will still work night and day for two or three months from a D-cell.
The phototransistor that the author typically uses for these night lights is a Ledtech LT9593-91-0125 purchased from www.allelectronics.com . Another that worked equally well was a tiny Stanley PS5022. A CdS photoresistor also worked fine. Several photodiodes were tried also, but only one worked as well as the above.
The transistor Q1 can be a 2N2222 or 2N3904. Either one works fine.
The toroidal transformer for this night light was made on a ferrite core 1/2"OD by 1/4"ID by about 3/16" long. This is available from www.sciplus.com as well as other electronics parts suppliers. The magnet wire usually used by this author is #32, but the gauge is not that critical. An 8 foot length doubled over (to 4 feet) is wound through the ferrite torus as neatly as patience will allow. Again, the length is not too critical. The website www.bigclive.com/joule.htm , does a thorough job in describing the transformer construction and how to sort out the wire ends after winding. There are other good sites also, as a search for Joule Thief will quickly reveal.
With all the parts ready, it is most prudent to first hook up the circuit on a solderless breadboard and make sure it works just the way it should. When all the bugs have been driven out, then you can go ahead with confidence to make the soldered up version.
The permanent circuit can be built on a tiny piece of stripboard as shown in this drawing. This is a view from the top with the copper strips shown in gray on the underside, where all the soldering is done. Note the break in the leftmost strip.
A hole is shown drilled through the board for attaching the circuit to the wooden upright, but it could be just be glued in place; glue dots or double stick foam tape is good for this kind of application. A nylon machine screw was used to secure the circuit in the author's example shown here in the photographs. In some of the photos it may look like the screw also holds the transformer in place as well, but in fact the toroid is fastened permanently to the stripboard with epoxy. Actually a generous amount of epoxy is used over the transformer to protect and secure the windings and wire leads.
If you do drill the mounting hole through the stripboard as shown, it will also form a break in the copper strip, so the positive battery lead and the two transformer leads could alternately be soldered into the center strip through the lower two holes; then no break would have to be cut in the left strip.
The Joule Thief is positioned at a height that illuminates the image on the glass to best advantage. The LED that is shown here has a wide viewing angle to spread out the light.
Leads are taken up to the phototransistor that peeks out of the lid to sense daylight or dark.
Here is another electronic night light made by the author (featuring St. John Bosco). This one does duty on the bottom of the stairway going up to the loft which is handy when awakened at night by a thunderstorm and one of us has to go up and unplug the computer to protect it from possible lightning damage!
Still another one of our night lights (this one is kept at the top of the loft stairs) has a jumbo white LED mounted near the top cover and angled downward. This spotlights the main image on the night light bottle and also brightens up its lower reaches.
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