A Simple Constant Brightness LED Light

revised 11-24-08

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.

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.

1.  Brightness only:

Inexact method - no tools or special components- can destroy the LED

1. Insert a 1000 ohm resistor in series with the LED and the 5 volt supply
2. Observe the brightness of the LED
3. Temporarily place a 2nd 1000 ohm resistor in parallel with the first resistor - this gives a resistance of 500 ohms
4. Observe the brightness.  If it increases repeat with a 3rd 1000 ohm in parallel giving 333 ohms.  If it does not get brighter use this resistance value
5. Repeat as needed making sure you don't get more than five or six 1000 ohm resistors in parallel, dropping the resistance below 100 ohms

Here you see two 1000 ohm resistors in parallel.

2.  Temperature:

Inexact method - no tools or special components- can destroy LED

1. Use the above procedure checking the temperature of the LED at each step
2. The best way to check the temperature is to touch the LED with a finger.  Once you are sure it is not too hot touch it to your lips or to your cheek.
3. Once the LED starts to get the least bit warm use that resistance or one a bit higher in value.
4. The objective is to keep the LED from getting anything but a bit warm.  Never let it get hot.
5. Remember, the higher the resistance, the lower the current and the dimmer and cooler the LED will be
3. Variable resistor:

Inexact method - requires a basic volt-ohm meter - can destroy LED

 The meter used here is a very inexpensive unit that is available from HarborFreight.com for under \$10.00.  I have seen them on sale for less than \$5.00 at the local Harbor Freight store.
1. Use either of the above procedures with a variable resistor or potentiometer in place of the 1000 ohm resistors
2. Insert a 1000 ohm variable resistor in series with the LED and 5 volt power supply
3. Make sure you start with the potentiometer set to its HIGHEST value
4. Adjust the pot, turning it so the resistance decreases, until the brightness of the LED ceases to increase or until it begins to get a bit warm
5. Remove the pot from the circuit and measure its resistance.  Use that value in a fixed resistor

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.

4. Current used:

Exact method - requires a basic meter- unlikely to destroy LED - you need to have the LED's specifications

1. Find the LED's specification sheet.
2. On the spec sheet find the recommended current for the LED
3. Set the meter to the milliamp (ma) setting
4. Put the probes from the meter in series with the 5 volt power supply and the resistor
5. Using either a 1000 ohm potentiometer or a number of 1000 ohm fixed resistors as above light the LED and take note of the current reading on the meter
6. Change the resistance.  When the current  shown on the meter reaches the recommend value you have identified the proper resistor to use

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.

5.

For those of you who are more mathematically inclined!

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.

1. Gather the recommended current and forward voltage (sometimes called forward voltage drop) from the LED's data sheet.  The specifications for the LEDs I am working with here show 3.4 volts forward voltage and 30 ma maximum current.  To be safe keep it under the maximum and use 25 ma

2. Before applying Ohm's Law subtract the forward voltage from the supply voltage.  In our case we are supplying 5 volts so the difference is 5 volts - 3.4 volts = 1.6 volts

3. Divide this voltage by the current (in amps).  This would be 1.6 / .025 (25 ma = 0.025 amps) = 64 ohms

4. Another example:   I have bright red LEDs that have a forward voltage of 2.1 volts and a recommended current of 20 ma.  The resistor, assuming a 5 volt supply, would be (5-2.1) / 0.020 = 2.9/0.020 = 145 ohms

5. The formula is         Resistance = (Supply Voltage - Forward Voltage) / Current (in amps)

 Parts Kits: A complete parts kit is available from the author.  Contact dave@davebodnar.com to order. The kit contains: 2 @ 3mm white LEDs (very bright!) 2 @ 470 ohm current limiting resistors 2 @ 7805 or 78L05 voltage regulator (your choice) 2 @ 1N4004 (or similar) diode short length of hookup wire 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