Introduction: Adding an LED Back Light to an ACCTIM BENTIMA LCD Alarm Clock

The LCD battery alarm clock has been of great benefit to us all over recent years and is a great boon for travellers being small, light and silent. It has also found its way into many bedrooms where it can be superior to mains powered LED clocks which are generally far too bright with no meaningful mechanism for properly reducing the glare. This is especially true with the green ones.

One major disadvantage of the LCD clock is that it has no continuous back-light and cannot be read in the dark unless you press the top plate which deploys a minuscule incandescent bulb for a few seconds. The 1.5 Volt battery has insufficient voltage to operate a LED and continuous use of an incandescent bulb would rapidly discharge it. We get over the problem by introducing a small white LED which is powered externally via a PP3 battery and a ballast resistor.

The development of LED's in recent times has resulted in ever more and more light output and it will be a surprise to some that the LED can function and do useful work when passing current of just a few micro-amps, this being enhanced greatly by the night vision of the user. Many of us are exposed to so much light these days that it can come as a surprise to realise what a powerful attribute our night vision is.

Study of current catalogues gives the hint that LCD alarm clocks are now being designed with more batteries and this does allow a continuous back light, presumably an LED and this may mean that this Instructable is an application a little past its time. However there must be many existing clocks that could still benefit from this Instructable and the new clocks may not be able to match the sheer economy achieved here.

The Acctim Bentima model used here has the great advantage that there is sufficient room inside the case to allow some room for experimentation and the back-plane illumination is accessible. More modern LCD alarm clocks tend to be much slimmer and do not have the room to allow for this treatment--one look inside a SEIKO travelling alarm caused the author to rapidly reassemble it! however, this Bentima model is currently still available in the UK at least and there must be many lurking in the backs of drawers and cupboards.

Step 1: First Get Your Clock and Prepare It--Step 1

Vast numbers of LCD Battery Alarm Clocks have been produced over the years and in the picture we see the Acctima Bentima which is the guinea pig for this Indestructible. The device runs from a 1.5 Volt AA cell, usually alkaline. The time and one alarm can be set via the buttons at the top rear of the casing. This type of clock has the huge advantage that it is completely silent but there is a major disadvantage in that it cannot be seen in the dark. There is a light to enable viewing the readout which is deployed by pressing the centre plate on the top but it illuminates for a just a few brief seconds and there is a very good reason for this. Because the the device is powered by a 1.5 Volt cell you cannot use the obvious choice of a light emitting diode (LED) which needs typically 4.5 Volts plus a ballast resistor, instead these clocks use a miniaturised incandescent bulb similar in principle to those used in torches in the days before white LED's became available. Incandescent bulbs are very power hungry and this gives the answer to those who ask "Why can't you just have the light on all the time?" and that is because you would flatten a battery in a very short order even with the minuscule bulb used here.

We need to get some form of LED in there--it takes a little effort but it is possible.

Step 2: First Get Your Clock and Prepare It--Step 2

Now the process becomes more fraught! That which is not made to be opened must be opened!

The picture above shows the clock duly opened after the removal of two self tapping screws. The important locations have been labelled. The ribbon connecting the circuit board with the LCD display is particularly delicate so take care not to damage it.

There are two small areas where it might be possible to incorporate batteries to power the LED. Two CR2032 cells in series would give 6 Volts which is sufficient for our purpose. These locations are large enough to take a CR2032 lithium cell in each position but not large enough to take button cell holders. You can strap coin cells together in series using self amalgamating tape and interpose wire connections under the tape but I found this insufficiently reliable. Also bear in mind that the clock casing is delicate and will not stand many opening/closing operations. I chickened out here and opted to use an external alkaline PP3 battery. The seasoned hacker may feel sufficiently competent to incorporate an internal battery.

The next task is to butcher the white plastic box at the rear of the LCD display by boring a hole in it through which to insert our LED.

Step 3: First Get Your Clock and Prepare It--Step 3

Now bore the hole in the box behind the LCD display to take the LED as ringed in the picture above. I used a hand held printed circuit board drill for this but if you use a hand drill then you will need extra hands to hold the clock very steadily. It may be easier to melt a hole using a nail heated in a cigarette lighter flame but do not breathe in the fumes! .

Step 4: The LED Circuit

You will not get an Instructable electronic circuit much simpler than this one as it consists of only three components, however, some explanation is due for each individual item. The whole assembly must be as small as possible since the assembly has to go into a clock where space will be at a premium and this is certainly the case with this example. Indeed it may not be possible in the case of a slim clock and some way of mounting the components on the outside may be the only way to do it.

As stated before we are using an external alkaline PP3 battery as the power source.

The white LED should be as small as possible and I used a 3 mm one which was already diffused, see the second picture above..

When you use an LED with a battery you will invariably find a ballast resistor to go in series with the LED however these are normally in the region of a few hundred Ohms so to see a resistor of 1 meg Ohm used may be surprising. In fact white LED's continue to emit light even when passing only a few micro amps and this small amount of light may be usable especially when human night vision comes into play which, of course, it does here. A 1 meg Ohm value for the ballast resistor means that around 7 micro amp passes through the circuit and, since the capacity of an alkaline PP3 cell is 500 milliAmp hours, this calculates to a battery life of 8 years! In practice this would be the shelf life. If you do feel that you need more light then drop the resistor value to 470 k Ohm but the battery life may well be halved.

Note that there is no switch and the LED is on continuously--you just do not see it in the daytime.

Step 5: Complete the LED/Battery Circuit and Insert It Into the Clock

The first picture shows how, with a blob of 'Blu Tack' and a 'third hand' to provide support the PP3 battery lead can be soldered to the LED and the ballast resistor.

In the second picture we see how the whole assembly is fitted in. The LED is fitted in to the hole in the LCD display cover box and held in place with 'Blu Tack' and this also serves to cover the bare wires. Ringed in red we see a small 'U' shaped hole cut into the bottom casing allows the PP3 battery lead to be brought out to the outside world.

The clock may now be reassembled which requires patience as it is rather fiddly.

The last picture shows a rear view of the completed clock which retains all of the original clock functions. The PP3 battery is held in place with a couple of Blu Tack blobs.

Step 6: Some Last Thoughts

The original requirement of this project has been achieved in that a LCD back-plane can be lit with almost negligible battery consumption resulting in a very useful item. One half of our partnership feels that there is nothing aesthetically untoward in having a PP3 battery stuck on the rear of the clock whereas the other thinks it ugly. So there is unfinished business here and the Instructable may receive a second visit. The project will be truly complete when all of the circuitry is inside the clock and can run from the clock's own battery thus avoiding the need to open the clock to replace a battery. This sort of clock is not designed to be opened and simply will not stand repeated access to the inside.

Hopefully a successful revisit will show that this problem is a solution in disguise.

In principle any device that uses an LCD display which van be physically reached could be a candidate for this treatment.


rafununu (author)2016-12-01

As you've already made a Joule thief, you could have simply use something similar to save a battery and avoid this disgracefull dongle. Of course a high resistor value should still be used to minimize current intensity.

Lionel Sear (author)rafununu2016-12-01

One side of our aforementioned partnership agrees with you! I, in the other hand will own up to 'dubious dongle' :-)

My plan B is indeed to go the Joule Thief route with the circuit powered by the clock's own battery. This would allow a completely sealed solution with no further access to the interior required. But. . . there are things to consider. Space is tight! I think it reasonable to allow a budget of 0.2 mA continuous power consumption for the Joule Thief so as not to cause frequent battery changes; this is quite a big ask but possible. The circuit needs to be stable, you do not want to be adjusting the circuit or even having to have a hole in the casing to access a trimming pot. Watch this space.

rafununu (author)Lionel Sear2016-12-02

Joule thieves don't save battery power, that's a fact, but they are usually "crude formwork". Your estimation is of 0.2mA consuming for arround 7µA for the Led itself ? I think a Joule thief can be fine tuned for this particular application in order to reduce loss to its minimal value.

woodvale035 (author)2016-11-30

hi. nice idea for continuous back light. one thing I was wondering with your circuit can you simply add a light or photo sensor to stop the led being on during daylight or would this need some more circuitry.??
BTW. I have a mains powered alarm clock that had a green display which lit up the whole room. I tried a few things, first was covering it with black pvc tape but was too dark. then I tried white tape. it was better but still not great. the solution I use now is clear sellotape that I then covered in black permanent marker. I gave it more coats of marker to get the darkness desired.

Lionel Sear (author)woodvale0352016-12-01

Thanks for your interest.

We have had the same experience with garish green alarm clock displays and had to resort to coloured films and cloths but it was never right and the offending clock went to the charity shop in the end. We got a red one which was much better being in a visible region that is kinder to the eyes but all of them need a facility whereby you can turn the display intensity right down to almost zero.

With the LED battery being external in this case it might, in principle, be possible to turn off the LED during daylight. Semiconductor photocells work in the wrong sense in that they are high resistance in the dark so you would have use one to turn off a series transistor but it might be tricky to control the starvation current of a few micro-amps that we are using. The hope is that our battery will last a very long time.

About This Instructable




Bio: I am a retired analytical chemist living with my wife Cynthia in Cornwall, south west England. I have held the UK radio amateur call sign ... More »
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