A Red LED Desk Lamp

Need a red lamp to provide soft red illumination around the telescope? Described here is an easy way to do this. Take a commonly available desklamp and convert it to LED’s. This is a single evening project, taking just a few hours to accomplish once the parts have been obtained.

Observing Table
A red LED lamp on the observing table
I have found the resulting light extremely useful when observing. No more holding the flashlight in your mouth when reading charts or making observing notes.

Because the light can light up a whole area when turned fully on it can be used as a worklight when breaking down gear and packing it into the vehicle without bothering other observers at a dark observing site. Or you might build the 120V version for a personal observatory.

I have also found that these lights also make great gifts for fellow astronomers.

To accomplish this we will disassemble the lamp, remove the 12V bulb and replace it with an array of LED’s. To control the brightness we will install a variable regulator to allow dimming of the lamp from completely off to very bright. An option is to install another connector to allow DC power to be supplied from a battery for field use.

Before embarking on this project be sure to read this entire article. Having the entire task in mind will help you plan ahead. It is important to use safe electronic assembly techniques to avoid any risk of damage or injury when making the modifications or when using the lamp. We will make no modifications to the high voltage side of the transformer to insure the lamp remains safe to use.

One secret to the lamp is the LED’s. These wonderful devices convert a small trickle of electricity into a very bright red light. Be sure to get ‘Ultrabright’ or ‘Superbright’ types that put out at least 3000mCd (millicandela). I found some that put out 6000mCd that I use in my lights at All Electronics. Perfboard, regulators and many of the other parts can also be purchased there.

12V Reading Light
A typical 12V reading light as purchased

The lamp I choose to start with is shown at the right. It is an inexpensive desk lamp that uses a 12V halogen bulb. Similar lamps are available at many discount and department stores, this one cost me $9.99 at Target.

The lamp offers a good starting point… It has all the needed mechanical parts, a case, a gooseneck and a reflector assembly. Because it uses a 12V bulb (not a 110V bulb) it has a transformer built into the base that can be adapted to power LED’s. This transformer also serves as a weight in the base to keep the lamp upright. This light also has plenty of extra space inside for the added circuitry as well as another connector and a potentiometer to vary the brightness.

Some of these lamps use 120V bulbs, and have no transformer. These can still be used, wired as 12V only versions (see below).

These lamps are also available with a clamp style base that can be useful for some situations.

One last important thing to look for in the light is that it comes apart with screws, it is not ultrasonic welded or glued, and can be easily reassembled once the necessary modifications are made.

Red Lamp Schematic
Schematic of the 120V Red LED Lamp

On the right is the schematic for the light. Shown are the two input connectors, a plug for 120V and a DC jack for 12V. The AC plug and the transformer should have been included with the light. Either or both connectors may be used in the design. We will discuss this option more fully below.

The parts list shows each of the components you will need to build the lamp. These can be ordered from any number of websites such as Digikey, All Electronics or purchased locally if you have a good electronics parts store available. Many electronics hobbyists will have many of these parts on hand.

All Electronics may have everything you need, including the pot and knob, but as a surplus house their stock is sometimes variable.

Part
Qty
All Electronics #
Digikey Part #
Red LED’s 4 LED-94 160-1492-ND
LM317 voltage regulator 1 LM317T LM317TFS-ND
100uF 35V capacitor 1 100/35VR P5165-ND
1N4001 diode 2 1N4001 1N4001DICT-ND
1k potentiometer 1   575SS102-ND
Knob 1   8555K-ND
10 ohm 1/4W resistor 1 10-1/4 10QBK-ND
220 ohm 1/4W resistor 1 220-1/4 220QBK-ND
560 ohm 1/4W resistor 1 560-1/4 560QBK-ND
Perfboard 1 PC-1  
DC power jack 1   SC1049-ND
Disassembled Reflector Assembly
The reflector section of the lamp disassembled

Once all parts are on hand it is time to do the deed and dismantle this perfectly good light and make it into something a bit different…

The reflector assembly is where we will add the LED’s and regulator. This assembly should come apart when you remove the screws. Save all the parts as you will need them for re-assembly.

Cut away the old bulb socket and careful to save as much of the wire as possible without loosing it into the gooseneck. If the wires are too short they can be replaced with longer wires but it can be somewhat difficult to thread the new wires back into the gooseneck.

Potentiometer Wiring
Proper wiring for the potentiometer

Solder two wires at least 6″ long to the potentiometer. Be sure to solder one to the first terminal and the second wire to the second two terminals as shown. This sets up the potentiometer so that the light brightens as the knob is turned clockwise as is customary.

The pot might have a small metal tab. This is normally used to prevent rotation of the body when the knob is turned. Because you do not want to cut an extra hole in the case you can bend this tab down or cut it off. To prevent the pot from turning a small dab of RTV can be used against the body when mounting the potentiometer.

It does not matter if the wires from the potentiometer are reversed with this arrangement.

Mounted Potentiometer
A hole drilled into the plastic and the potentiometer mounted

At this point it is probably a good idea to plan where you will fit everything in the case and to drill the hole for the potentiometer.

Find a position for the potentiometer where it can be easily adjusted, usually on the top-front of the reflector assembly. Also be sure to choose a spot where the heat reflector (the metal piece) can be re-installed without hitting the potentiometer. Be careful when drilling the hole as the plastic can be hard and brittle. Often the easiest way to create the hole is to carve it out slowly with a penknife.

Install the potentiometer into the housing with the supplied nut and attach the knob. Note the four wires hanging loose that will be soldered to the circuit constructed below.

LED Circuit Board Assembly
A completed LED circuit board assembly

Find and identify each of the electronic components you need for the circuit.

Another important trick you can see here is to file the tops of the LED’s off before soldering them into the circuit. The dome at the top of the LED is effectively a lens that focuses the light. For this application we want a smooth and even wash of unfocused light from the lamp. Filing the top away removes the lens and creates just the right light source.

The circuit is assembled on a small piece of perfboard as shown. The arrangement of components shown here allows the entire circuit to be assembled without the use of any jumper wires, simply using the component leads to make all connections.
Bottom view of the completed circuit board

LED Circuit Board Assembly
A completed LED circuit board assembly

Be certain to observe correct polarity of the LED’s as well as the capacitor. The resistors do not have any polarity issues and can be used either way.

The LED’s are spread out as much as possible to aid in providing as even a light source as possible. This will soften shadows around a thin object like a pen while writing.

The wires from the base of the lamp as well as the potentiometer will all attach along the lower side of the circuit board.

If a single power source lamp is to be used (see below) then the necessary 1N4001 diode can also be placed on the circuit board. Otherwise for a dual supply (120V or 12V) lamp the diodes must be located in the base.

Mounting the LED Assembly
The LED assembly mounted into the reflector

When the circuit is completed it can be installed into the reflector assembly

Be sure to note which wire from the base is to be the positive supply.

The LED assembly is secured into the reflector with a little silicone RTV. Use a little more RTV around the wires where they pass through the metal heat reflector to keep the edges from cutting into the insulation.

Replace the glass cover and the upper assembly is complete.

Red LED Lamp 12V Schematic
The schematic of a 12V only version of the red LED lamp

At this point the plan becomes different depending on what type of lamp is to be built. A 12V only version for battery operation in the field, a 120V version for use in a personal observatory, or a dual version that can use either.

The schematic at the right shows the 12V version with a DC power jack.

If no transformer was supplied with the lamp you must also build the 12V version. A 12V version can be used on AC power by using an external 12V wall transformer in place of a 12V battery

Clamp Light Base
An example of wiring a clamp style light with a connector in the base

This is also how a clamp version of the light must be wired. For this version as well is is a good idea to include one diode on the circuit board to protect against polarity reversal.

For the 12V only version any transformer that came with the light is to be completely disconnected. The two wires from the LED assembly must be wired to the DC Jack. In this case you must be careful to observe the correct polarity. Connect the center pin of the DC Jack to the positive connection of the circuit as shown in the schematic.

The photo shows the base of a clamp version showing that the wires lead to a DC jack with no other complication.

Red LED Lamp 120Vac Schematic
The schematic of a 120Vac only version of the red LED lamp

A 120V version can be used in a personal observatory where AC power is available. The original transformer in the lamp provides 12VAC voltage to the bulb. As long as you included the 1N4001 diode on the perfboard as instructed above the light should power up when voltage is applied.

Caution: You must use a transformer to reduce the voltage to 12V, if 120V is applied directly to the circuit it will die a spectacular death, components will explode and burn turning your work into irreparable junk. It could also injure you, keep in mind that you could also be irreparable if you touch 120V.

The schematic for the 120V version is shown to the right.

Base and Transformer
Preparing the lamp base and transformer for reassembly

The photo on the right shows the base with the transformer in place ready to connect.

Caution: any 120V connections must be completely insulated for safety. Use heat shrink tubing or electrical tape to fully cover any exposed 120V connection.

To create a dual power version takes a little more wiring in the base. Both the transformer and the DC jack must be wired in using a pair of 1N4001 diodes to allow power to come from either source as shown in the first schematic.

Once finished the lamp is ready for use a day later when all the silicone RTV has cured. The photo here was taken in the field with the lamp powered by a 12V gel-cell battery, it ran all night and didn’t even start to discharge the 4Ah battery I had with me. The lamp uses about 50mA and should run for 80+ hours on that battery.

With the lamp setup beside the scope you will find that observing becomes much more convenient without fumbling for the red flashlight all of the time.

Good luck!

By the Telescope
A red LED lamp by the telescope

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on Mauna Kea, Hawai'i.

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