A Simple Constant Brightness LED Light
Quick Link to Computing Resistor Values
|I was recently asked to present a hands-on workshop for the
Pittsburgh Garden Railway Society that had participants construct a
constant brightness LED headlight. You can give the same
circuit a try by following these step-by-step directions.
LEDs are ideal for use in model train lighting as they can be wired so that they attain their full brightness at a low voltage, a must if working from track power. In addition they also draw very little current, important for those of us who use battery power, and can be extremely bright.
There are some precautions that must be taken, however, as LEDs can easily be destroyed if they are fed an improper electron diet!
In order to protect the LED from excessive voltage and current a voltage regulator is used to keep the voltage to the LED constant. Once the voltage is constant a current limiting resistor can be selected that will deliver an appropriate amount of current to the LED.
Detailed notes on selecting appropriate current limiting resistors are at the end of this article.
Most of these parts are available from Radio Shack and other vendors. To simplify things for those of you who would rather build than track down parts the author has complete parts kits available. See the notes at the end of the article.
The schematic is below. The positive voltage (marked +12) can be anything from around 7 to 20 or more volts. The circuit draws very little current and the regulator is rated for a maximum input voltage not to exceed 30 volts. If you use more than 12 volts it is a good idea to touch the regulator and make sure it is not heating up as excess power must be dissipated as heat. Note that the symbol for the diode, D1, has a vertical bar on the right side in this schematic. Diodes must be installed with the proper orientation or they will not work properly. There is a painted bar on one end of a diode, called the cathode, that corresponds to the bar on the right side of D1 in the diagram. The other end of the diode is referred to as the anode.
Here are the components needed. From left to right they are 78L05, 1N4004 diode, LED and 470 ohm resistor.
The three leads on the regulator are, from left to right with the flat face showing as below, output, ground and input.
Cut the leads on the resistor, LED and resistor as shown below. Note that the lead that is to be cut on the LED is the longer one. It is the positive lead. If you are not sure which is which there is a notch or flat spot on the body of the LED next to the shorter, negative lead.
Carefully bend the leads on the voltage regulator as shown. Make sure the flat, labeled face of the regulator is towards you.
Remove a bit of insulation from each end of the hookup wire.
This article is not meant to be a soldering tutorial but a few tips are in order:
Push the leads of the resistor and LED into a piece of foam to that they are held in proper orientation during soldering. Note that the resistor and LED lead are not quite touching. This was done so that the individual parts would show up in the photo. Before soldering make sure that they touch evenly.
|Clean the tip of your soldering iron on a damp sponge and tin it
with solder until it is bright and shiny. Put the tip of the
iron on the joint between the LED and resistor until heat transfers
and you can melt a bit of solder onto the joint. Remove the
iron and solder and allow to cool for a few seconds. Try to do
this quickly as heating the LED to an excessive temperature can
|Here is the soldered joint.
|Push the unused LED's negative lead into the edge of the foam.
Push the input lead of the regulator into the foam so that the left
hand, output lead of the regulator and the remaining resistor lead
are lined up as shown. Solder them together being
careful not to touch the iron to the foam!
|Align the negative lead of the LED and the center, negative lead
of the regulator and solder as below.
|Bend the input lead of the regulator up a bit and push the
positive lead (the one without the silver band) of the diode into
the foam so that the banded lead of the diode and the input lead of
the regulator line up. Solder them together.
|The circuit is just about complete and can be tested.
Carefully touch the center lead of the regulator to the negative
terminal on the battery. Touch the positive end of the diode
to the positive terminal. The LED should light!
|Shorten the red wire about 1/2 inch and tin the ends of
the hookup wire with a bit of solder.
|Tin the positive end of the diode then solder the positive (red)
wire to it. In the photos below the battery is being used as a
weight to hold the components in place for soldering.
|Solder the negative wire to the center lead on the regulator.
|Here is the completed circuit.
|Test by touching the red wire to the positive terminal on the
battery and the black wire to the negative terminal. Note that
hooking them up backwards will not hurt the LED or regulator as the
diode only permits current to flow in one direction.
The wiring for the larger 7805 voltage regulator is identical with one exception, as you can see from the photo below. The 78L05 is wired with its labeled, flat face towards you but the 7805 is wired with the labeled face away from you.
Leads on the voltage regulator, resistor and diode can be shortened to give a more compact package. The following configuration is suggested.
The negative, shorter, LED lead goes to the center lead of the regulator and the positive lead of the LED goes to the resistor. Power leads are soldered to the center and right hand leads of the regulator.
Surface Mount LEDs
If space is at a premium and you have a good magnifying glass, surface mount LEDs and resistors can be used. The device on the right, marked 510, is a resistor and the other device is the LED.
The negative terminal of this surface mount LED is marked with a line on the back of the case. Other surface mount LEDs may be marked in a different manner. If in doubt apply voltage, through a 1000 ohm resistor, to each side of the LED. Reverse the leads if it does not work.
Use a piece of electrical tape to hold the LED in place. Push the resistor against the positive terminal and hold it in place with a penny.
Using fine solder and a fine tip iron place a bit of solder in the joint between the LED and the resistor.
Note that the resistor being used below is marked "510" - this is a 51 ohm resistor. (Take the first two digits, 51 in this case, and multiply them times 10 to the power of the third digit, 10 to the zero power in this case, which is 1 - a resistor marked 513 would be 51,000 ohms as 10 to the third power is 1000. 51 x 1000 = 51,000)
Add wires to the negative side of the LED and the resistor, connect to 5 volts and you are done!
Here you see a 3mm white LED on the left and the surface mount unit on the right. The brightness is similar but the 3mm unit is much more focused. The surface mount is more of a flood lamp.
Selecting Current Limiting Resistors
LEDs must be used with a current limiting resistor. If a resistor is not used the LED has the potential to draw enough current from the circuit to destroy itself.
Some folks prefer a mathematical computation to determine the value of the current limiting resistor. If that is your preference see item 5 in the list below. If you are one who prefers a more "hands-on" approach one of the first four items in the list below may be for you.
Inexact method - no tools or special components- can destroy the LED
Here you see two 1000 ohm resistors in parallel.
Inexact method - no tools or special components- can destroy LED
Inexact method - requires a basic volt-ohm meter - can destroy LED
Note the wiring of the pot in the schematic and photo below. It has 3 leads, one on either side and one in the center. The center lead and the one on the right are wired together.
Here the resistance of the potentiometer measures 495 ohms.
Exact method - requires a basic meter- unlikely to destroy LED - you need to have the LED's specifications
Note that the meter is set to the 200 ma range and that the display shows a bit over 20 ma. The LED being tested is rated for a maximum of 30 ma forward current but is quite bright at 21.4 ma.
One can, of course, determine the proper resistor mathematically. Light Emitting Diodes don't adhere strictly to Ohm's Law but it is a simple matter to work around this.
A complete parts kit is available from the author. Contact firstname.lastname@example.org to order.
The kit contains:
The cost is $4.00 per kit + $3.00 shipping for up to 5 kits.
Other LEDs are available including 5mm white, 3mm green, 3mm red