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Step 4Some Technical Details
An LED is a current device, meaning it responds to current change change not voltage change. The Voltage across an LED is always fixed (red LED have a voltage of 1.2V, white LEDs have higher voltages). So technically this device covered in this instructable is a CURRENT CONTROLLED RESISTOR.
However you can add a voltage to current converter to drive the LED. The simplest voltage to current converter device is a resistor.
A resistor converts voltage to current by means of OHMs law, so that the output current I=V(across R)/R.
The voltage across a resistor in series with an LED is Vinput-VLED (in a red LED it's Vinput-1.2V).
It is always a good idea to put a resistor in series with an LED because it protects the LED from over current which burns out the LED.
Another detail, the output light from an LED is not linearly proportional to the input current but changes exponentially.
I plotted the input current vs. the output resistance of the voltage controlled resistor in this instructable below. It clearly fits an exponential trend line.
However you can add a voltage to current converter to drive the LED. The simplest voltage to current converter device is a resistor.
A resistor converts voltage to current by means of OHMs law, so that the output current I=V(across R)/R.
The voltage across a resistor in series with an LED is Vinput-VLED (in a red LED it's Vinput-1.2V).
It is always a good idea to put a resistor in series with an LED because it protects the LED from over current which burns out the LED.
Another detail, the output light from an LED is not linearly proportional to the input current but changes exponentially.
I plotted the input current vs. the output resistance of the voltage controlled resistor in this instructable below. It clearly fits an exponential trend line.
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