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Figure 2 - This is a pulsed mode illuminator taken from an outdoor security camera
Figure 2 - This is a pulsed mode illuminator taken from an outdoor security camera

There are several varieties of infrared LEDs, ranging in size, field of view, output power, and effective light color. The most commonly used infrared LEDs output 940 nanometer infrared light which is far beyond the human visual range, and fairly detectable by any non filtered video camera. There are also infrared LEDs available for the 800 to 900 nanometer range, and these are even better for use in night vision applications, but there will be an ever so slightly detectable red glow as the human eye can faintly detect this band of light. If you have seen an outdoor night vision security camera after dark, then you are probably familiar with this dull red glow. The infrared LEDs shown Figure 2 are common 940 nanometer infrared LEDs with a forward voltage of 1.2 volts.



Figure 3 - Maximum current pulse will be shown on the datasheet for your LED
Figure 3 - Maximum current pulse will be shown on the datasheet for your LED

In order to attempt any kind of pulsed mode operation for visible or infrared LEDs, you need to look at the datasheet in order to find the maximum pulse current rating as well as the recommended duty cycle. Some LEDs can only handle a small increase of current during pulsed mode operation, making them unsuitable for the task. Figure 3 shows the section of a typical infrared LED that details the important values needed for pulse mode operation.

According to the datasheet segment, this LED needs 5 volts and has a maximum rating of 100 milliamps for normal operation. But, in pulsed mode operation the LED can withstand 1.5 amps, which is 1500 milliamps, or 15 times the current! This is amazing really, but don't expect the LED to output 15 times the amount of light or infrared radiation. One of the things you will realize when experimenting with pulse driven LEDs is that visible LEDs may appear to be twice as bright to your eyes, but the difference is not that great when using them to illuminate security cameras. There is still a small amount of gain to an infrared illuminator, but you may find the added complexity of the circuit to be not worth the effort.

If your LEDs have a pulse mode rating that is any less than 10 times the continuous current rating, then it is probably not worth your time to use them in pulsed mode operation. Also, check your data sheet for a recommended on/off time (duty cycle), to ensure that you don't over drive the current in pulsed mode operation. The circuit shown here is probably safe for most LEDs as it will deliver a very short pulse of between 8 microseconds and 10 microseconds. In order to drive your LEDs up to their ultimate maximum brightness, some experimentation will certainly be necessary. Expect to send a few LEDs to the graveyard along the way!

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