by Brian Darnton, UK

(See footnote if unfamiliar with the O.U. microscope)



Some literature on the application of Maplin`s white light LED already exists (ref. 1). Most people seem to use the OU microscope more or less as supplied, but with some form of 3 Volt power pack attached, usually on a short lead. Often this is strapped to the body of the microscope deviating from the original principles of portability and compactness envisaged by the designer. The 3V prefocus bulb is not very economical on power use and the power source has to be replaced or charged quite frequently. The new white LED light source from Maplin seems to have very good characteristics for use with the microscope. It has a brilliant white colour with a hint of blue. It is even strong enough for use with crossed polars.


The existing light fittings can easily be removed from the instrument.
Vertically above the centre of the microscope object position is an aperture with two holes, one on either side. An LED can be inserted downward into this cavity and secured by 2 self tapping brass screws into the pair of holes. The LED leads wrap neatly round the shafts of the two screws Two insulated wires can be taken from these screws through the switch space to the battery area. A compact capping to this assembly would improve the neatness of it.

A small 1K ohm potentiometer and a press button switch are best mounted in one side of the body so that they can be accessed during viewing by ones fingers. As can be seen in the circuit diagram (below) all the components can be connected by soldering in series with one another to complete the circuit. In connecting both the battery and the LED, polarity must be observed or the light will fail to function. Some form of heat sink is required on the LED leads to avoid damage during soldering.

It's a good idea to make a loose mock-up before soldering the components in order to check for functioning. The battery is of a fairly low capacity type but is quite suitable for the casual viewing by the OU Microscope particularly when a press button is used to ensure economic use. It is of course no use at all for long term use, but it has an alternative advantage in that it could be plugged into the 12V source from a motor car provided polarity is observed and the 12 Volt battery disconnected. The components would then remain at the same values. The small battery itself can be taped round and stowed in the old battery cavity and the old slider cover can be replaced to conceal the wires and components.

In order to control brightness, one could of course use a sub-miniature single pole 6 way switch rather than R 1, (the potentiometer), and a bank of 330 Ohm 1/4 Watt resistors connected in series configuration. If the first position is left blank (with no connections) it could also double as the master on/off switch.

As with any microscope, maximum brightness can damage ones eyesight, but manipulation of the the potentiometer should gain complete control of light levels as well as introducing another element of economy. If the microscope is to be boxed and stored it's a good idea to fit an extra slider switch is in series with the press button, to prevent accidental pressure in transit from switching on the light. The measured voltage accross the LED device is about 3.5 Volts and the current should be always limited to less than 25 ma. If the load resistance is reduced to less than the 560 ohm prescribed it could blow up and cause damage!
The system suffers from one disadvantage in that it is only suitable for transmitted light, though it not impossible to mount a second LED that could illuminate the object at about 45 degrees; there is the space for the innovative worker.


This article is intended for guidance only to stimulate innovation. The information is given in good faith for those that have the knowledge to undertake this level of electrical work and the writer can not be responsible for damage occasioned by accidental or faulty procedures. Some warning are given.


VR1 1 K Ohm Variable Resistance
R1 560 Ohm 1/4 Watt Fixed Resistance
B1 12 volt Reserve Battery Soldered on LRV08 at 1.50 +-
LED1 The new Maplin White LED NR73Q
S1 Press on sub min switch.


Size 5mm white LED NR73Q MAPLIN at a cost of 2.54 per one EXCl. VAT & P+P 6/1999. Or at a cost of 1.99 per 25 EXCL. VAT & P+P 6/1999.

Forward Voltage 3.6 Volts
Forward Current 25MA
Reverse voltage 5 Volts
Power Dissipation 100 mW
Colour Temperature 8000 deg K.


1# Tony Saunders-Davies. QMC publication:
An initial summary of the LED's potential use. Article on the Quekett Club website.

A summary of technical information and updated costs on the Maplin web site.

Comments to the author Brian Darnton welcomed.

Editor's Notes:

The OU microscope is the Open University portable microscope based on the McArthur design. This microscope is available secondhand via the Microscopy UK On-line shop.

Maplin is a major UK electronic components supplier. Other countries suppliers may offer a similar LED.


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