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Upgrading an Old Book Light Torch

from Incandescent Bulb to an Efficient LED

Occasionally, something you've thrown into a dark corner can receive a second chance. Here's an example where my upgrade of an old battery-consuming torch has converted it into a highly efficient LED book light. 

This project was carried out during the COVID-19 lockdown for one of our grandchildren visiting us during that difficult time here in New Zealand.

Introduction

Many years ago, I was given a torch for reading books in bed or in low light conditions. This “torch” was really a fairly specialized and unusual device. It was designed to sit in the pages of the book being read. The bulb-end could be pulled out to “peer over” to directly illuminate the page you were reading. When this was pulled out, the light would turn on.

To show how it was meant to operate, here's a photo from the manufacturer back in 1995 showing the device in use.

After using it for a short time way back in the mid-90's, I found it to have a mix of good and bad features. It did light up the page at night. However,  the NiteOwl used a tiny incandescent bulb. While it was initially bright, the bulb's light fairly quickly changed from bright to faint yellow-orange as it rapidly exhausted the batteries. It sucked the lifeblood out of the pair of AA cells at a collosal rate, drawing almost 500mA while operating!

The pair of AA cells also felt annoyingly heavy in use after even a short time. Also, after some use, the parallel contact strips in the pull-out band became slightly erratic, possibly as a result of the high current demand.








So, all in all, it wasn’t a very successful device. Three or four nights use would easily consume a set of batteries. That got expensive quickly.

It was soon tossed into the back corner of a cupboard where it remained for many, many years.

Demand-driven Design

Recently, our UK-based grandchildren came to visit, and then, with COVID-19 appearing, to stay for somewhat longer than expected time. The eldest likes to read in bed at night when his brother and sister are in bed, and when he wakes up early in the morning. He’d forgotten to bring his torch with him and turning on the bedroom light to read would wake up his sleeping brother.

This being early in the COVID-19 lockdown period, we were not able to just go out and buy a suitable torch. Every shop in New Zealand, apart from those selling groceries and other essential goods, was closed. Even the most basic of torches previously available at supermarkets seemed to have disappeared, too.  As an interim solution, I dragged out my old NiteOwl, dusted it off, and pressed it into service once more. Not surprisingly, the various problems I had encountered quickly surfaced once again.  Within a couple of days, I was faced with changing the batteries.

Many years earlier, I had briefly considered changing out the incandescent blub for an LED but the design of the unit made it quite difficult. One issue - The original bulb is wired into the thin coin-sized module at the top of the light. That left very absolutely no room for additional parts or circuitry, even by using SMD parts.

But the real core of the problem lay in the switching arrangement used in the light. This is integrated into the plastic and metal pull-out strip seen in the photo below.

 
As the plastic strip is pulled out to position the light in use, the bulb is connected to the battery via the two embedded metal strips you can see in the photo. The circuit of the original NiteOwl looks like this:


To upgrade the light, the first option I considered was to simply replace the incandescent bulb with a white LED and a small series resistor, say 4.7 ohms. The resistor could (just!) be squeezed into place inside the device. However, while this approach would not improve the weight, it would extend the battery life slightly. The problem with battery life here is the sharp transition with an LED from full illumination to dim output as the battery voltage falls.

The next option I considered was to use a commonly encountered LED torch circuit:


This option had several advantages. It was relatively simple. Since this circuit runs from a single AA cell, the parts could fit inside the spare space in the battery two-cell battery holder. Also, since this option uses only one 1.5V AA cell, the weight would be reduced considerably. The circuit is also very efficient, able to operate the LED right down to a near-exhaused battery terminal voltage of about 0.7V.

The remaining problems, however, were considerable. First, the arrangement of the NiteOwl’s switch means the circuit is always connected to the battery. The current drain of the circuit, with or without the LED connected, would drain the single AA battery in very short order, probably overnight.

The second and equally fundamental problem was the fact that this same switching arrangement results in the connection and disconnection of the LED from the operating circuit. Without the LED connected, the blocking oscillator output climbs to more than 40 volts. Each time the LED connects or reconnects to the output via the switching strip, this voltage would have resulted in the very prompt destruction of the LED, instantly, every single time.

So, this was clearly not a viable solution. A similar situation existed with a number of similar circuits that I evaluated including the much loved and widely copied “Joule Thief” LED torch circuit. This study of various options had led to the original upgrade idea being pushed aside many years ago.

This time around, however, with a grandchild pleading for a solution, and faced with COVID-19 lockdown preventing access to a simple purchase of a new torch, I was forced into the “three cup of coffee” process. This is where I try to figure out a good solution to a challenging problem in the time taken during the day to consume three cups of relatively strong coffee.

The ultimate solution, fortunately, came after just one cup of coffee, plus, I must admit, a little baby-minding time involving grandchild number three. The solution: A high efficiency boost DC-DC regulator with low quiescent current.

The Boost Regulator Solution

I’ve used boost regulator circuits, those I've made as well as prebuilt modules, a number of times in the past. For example, I used one to power my tiny SWR meter from a single AA cell. That boost regulator generated 5V from 1.5V to allow the SWR meter to deliver good measurement dynamic range, excellent battery life, all within a very compact package. Another favourite is the Roman Black 5V regulator. 

This time, I looked for a 3.3V version in my parts bin. That output would better match the typical requirements of a bright high efficiency white LED. The Pololu U1V10F3 module I found in my bin was perfect.


As far as I can tell, this tiny module uses either an ON Semi NCP1402 (early versions) or a Texas Instruments TPS61201 switching boost regulator chip (later versions), both using the standard circuit shown in the relevant chip datasheet. These are highly integrated chips requiring only an inductor and some bypass capacitors to complete the boost regulator circuit in each case.

Pololu’s version is built on a very compact SMD board. Other similar regulators available from South China are slightly larger, but still very compact. That Pololu module allowed me to easily fit it into the spare space made available by the removal of one of the two original AA cells.

The key features of this module that made it ideal for use in this application include, firstly, the tightly regulated output of the module. With or without the LED connected, the module’s output remained at exactly 3.3V. Connecting and disconnecting the LED via the existing strip switch does not damage the LED or the boost regulator.

Secondly, the quiescent current drain of the regulator, the current drain when the LED is not connected, is about 3½ mA. If I could have used the shutdown features of these regulators, it would have been well under 100uA. However, that value of 3½ mA is fine for this situation. I just have to remember to remove the battery from the NiteOwl if it is to sit unused on the shelf for several months.
 
Looking at some specific numbers, a typical alkaline AA cell has a capacity at this low current level of about 2000 – 3000mAh. That suggests a standby life of over 18 months. Service life is much less in this application, of course. When the book lamp is used, say, one hour each day/night, the current drain rises considerably.

The current drawn by the direct connection of the LED to the regulator output was nearly 200mA and the resulting output current to the LED exceeded its ratings. It was also excessively bright for the application. A 10 ohm 1/4W resistor was placed in series to give a more appropriate light level. Regulator efficiency was around 70% at this point. I think this is an excellent value for this application, especially when compared to the original value with the incandescent bulb. The life is further extended by the change in battery end-of-life voltage from about 1.2V each with the original bulb and the new end-point with the LED with a single cell battery voltage of about 0.6V.

This suggested an ultimate battery life in use of 2 – 4 weeks which, for a low cost alkaline AA cell, is perfectly acceptable.

Construction

As noted, the use of hot glue in the modification made this easy to build and robust in use. It’s not pretty, but in use, all of this is out of sight.


The small 3mm white LED is a high efficiency high output type. The leads were bent at right-angles close to the LED body and the leads fed through the hole left by removal of the original bulb. It was hot glued into the original mounting location of the incandescent bulb. In practice, the blobs of hot glue are nearly invisible.A carefully applied drop of instant glue helped to hold the LED leads firmly in place.

 

Operation

After some weeks of intensive use, the AA battery has lasted, on average, around 4 weeks of one to two hours of use per day. Well, night and early dawn use, really.

So far, the construction has stood up to the demands of our young grandson. The device has met with child and parental approval, and i'm pleased to see something get a new lease on life that would otherwise have ultimately been thrown away..

I’d call that a win.

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