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More  Light Detector Circuits

40KHz Light Detector with Sunlight Immunity
designed by David A. Johnson, P.E.

This hobby circuit below was designed to turn on an external 12v relay, whenever it detects light from a nearby LED light source, modulated at 40KHz to 50KHz.  This circuit was originally designed to operate from a fast moving vehicle. The light transmitter was positioned at a stationary position, while the matching receiver was mounted on the vehicle.  This hobby circuit has high ambient light immunity and in most cases, can operate in direct sunlight.
An array of about 30+ infrared LEDs, powered from a +12v source, is used as the source of 40KHz modulated light. An alternate light source, made from an array of 7 visible red LEDs housed in a 10mm “jumbo” package, is also shown in the schematic below. A simple 555 timer, wired as a 40KHz oscillator and connected to an n-channel FET, drives the LEDs, powered from a +12v DC source.
A small inexpensive photo diode, housed in a clear plastic package, similar to a 5mm LED, is used as the light detector. It is reversed biased with a +5v supply and connected directly to a 100mH coil. The photo diode leaks current into the inductor at a level which is directly proportional to the light intensity. The inductive load provides an efficient way to separate the weak AC current signal generated by the modulated light source from the strong ambient light current, which includes direct sunlight. The voltage that appears across the inductor is a combination of AC and DC components. The resistance of the coil means a DC voltage will appear across the coil equal to the leakage current times the coil resistance.

But, the reactance of the inductor forms a much higher load impedance to any AC component from the photo diode. At 50KHz, the reactance is equal to about 30K. Using a capacitor, the DC signal across the inductor is blocked, allowing the AC signal to pass. The AC signal is fed to a transistor Amplifier, which uses a NPN Darlington device. A smaller 10mH inductor forms the load impedance of the Amplifier and insures that only signals lefted at about 40KHz will be Amplified. The transistor Amplifier provides a voltage gain of about ten. 

The output of the Amplifier is fed to the input of a voltage comparator circuit. With the component values show, the comparator will produce a logic level output when the 40KHz from the Amplifier reaches about 50mv peak to peak or greater. The output of the first comparator is fed through a filter network, which will generate a negative voltage swing, whenever a 40KHz signal is detected. The second filter network requires multiple comparator transitions before the DC voltage swings low enough to toggle the second comparator. The second voltage comparator, within the dual comparator package, is used to produce a positive logic swing, when several cycles of 40KHz signal is detected. The output of the second comparator is connected to a power FET, which drives the external relay.

I have been asked many times if we can predict the signal level at the light detector end.  The answer is yes, we can. The typical half angle light emission pattern from these LEDs is about 15 degrees. At a distance of 30 feet, the light will form a circular pattern about 15 feet in diameter. This comes from the equation D = (2)(S)(tanL), where D is the diameter of the light pattern, S is the distance from the LED light source and detector and L is the divergence half angle of the LEDs. Assuming about 10 milliwatts of light from each LED, the total light power launched from the 30 LED array would be about 0.3 watts. The area of illumination at 30 feet would then be about 200 square feet or about 30,000 square inches. The light detector area is about 0.030 square inches. Therefore, of the 0.3 watts of infrared light launched, only about 0.3 microwatts will be collected by the photo diode. With a conversion factor of about 0.5 milliamps of photo diode current per milliwatt of 880nm light, the photo diode current will be about 150 nanoamps. The reactance of a 100mH coil at 40KHz is about 25K. So, the expected peak to peak signal induced across the coil from the 150 nanoamps of current from the photo diode will be bout 7mv peak to peak. This is a bit on the small side for a direct conversion method, so an Amplifier was needed. With a gain of about X10, the 7mv peak to peak signal across the coil will be turned into a 70mv peak to the input of the comparator circuit, which is enough to toggle the circuit.   
When using the 7 10mm jumbo LEDs, the light pattern is a bit tighter.  Each device has a half angle of about 5 degrees.  This yields an illumination area of about 3,000 square inches instead of 30,000 with the 5mm LEDs.  Although the 7 jumbo LED source launches only one fourth as much light power, the smaller illumination area yields an improvement of 2:1 over the 30 LED light source. 
Can this light detector respond fast enough if it is installed on a fast moving vehicle? At a speed of 100 mph, a vehicle would travel 147 feet per second. In 10 milliseconds, the vehicle would travel only 1.4 feet. Since the width of the light beam is about 15 feet for the 30 LED array and 5 feet for the 7 jumbo LED array, there is plenty of time to toggle a relay as the vehicle passes through the spot of light.

Click on Drawing Below to view PDF version of Schematic

Circuit Circuit 40Hkz Light Detector & Transmister, designed by David Johnson

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