Although there are certainly limitations to how much light (or infrared radiation) can be emitted by an LED, there are tricks that can be used to push them to their ultimate maximum limits. This project will demonstrate how a simple infrared illuminator can be “pushed” a little more in order to extend the useful range of a simple night vision system using a camcorder and a low lox monochrome camera. In order to build this project, you will need to have the datasheet handy for the LEDs you plan to use so that you can determine the amount of current the LED will withstand in pulsed mode operation.
Pulsed mode operation means that the LED will be turned on and off at a very fast rate using more current than it could withstand continuously. The purpose of doing this is to force the LED to output short bursts of much brighter light (or infrared radiation) than it normally would, and by keeping the pulse with duty cycle short, the LED will not overheat. Television remotes do this to create sharp intense bursts of modulated light to send out to the receiver, and many low voltage consumer devices do this with visible LEDs to make them appear brighter while at the same time conserving power. A pulsed visible LED may look 10 times brighter, yet consume only half the power in pulsed mode operation. Of course, there are limitations to pulsing LEDs, and you may find that using more LEDs or higher current LEDs to be more effective than using a pulse mode driver.
This project will explore the strengths and weaknesses of the pulsed mode operation of both visible LEDs and infrared LEDs, and compare both using a low lux monochrome spy camera connected to a camcorder and small infrared illuminator.
Figure 1 – This is a pulsed mode illuminator taken from an outdoor security camera
Most newer outdoor security cameras now include an infrared ring light illuminator to enhance their ability to see in the dark. Infrared light falls just below red on the light spectrum, making up the wavelengths from about 750 nanometers to about 1500 nanometers. This light cannot be seen by human eyes, but it can easily be seen by many video cameras, making it useful as a covert lighting method in night vision systems. A common example of infrared light is the medium for communication between your remote control and television set. The LED on the end of your remote sends out pulses of infrared light which is received by the infrared detector on the TV and demodulated back into data. Of course, you cannot see the pulses because they are out of our visual range, but any video camera that is not equipped with an infrared filter can see this light easily.
Figure 1 shows an infrared LED illuminator ring taken from a small outdoor security camera. There are 17 infrared LEDs arranged in a series parallel configuration around a small hole where the camera lens would be installed so that the light is spread evenly around the field of view. This is an older illuminator, and was also a pulse mode system, which is why there are semiconductors on the rear of the circuit board. An illuminator without a pulse mode driver will not have any semiconductors as it is wired directly to the DC power source, giving each LED its maximum voltage and current all of the time.
You may notice that the semiconductors on the back of the illuminator shown in Figure 1 are completely fried , which is one of the downfalls of having more circuitry – more points of failure. This circuit was either zapped by a nearby lightning surge, or simply gave it up after overheating, causing a massive failure in almost all of the transistors in the circuit. Luckily, the LEDs survived and found their way into my junk collection. I have a very extensive collection of infrared enabled security cameras, and the interesting thing is that almost all of the newer ones have better night vision capabilities and do not use pulsed mode LED drivers. Maybe the manufacturers decided that better LEDs made more sense than pushing lower quality LEDs to their ultimate maximum ratings?