Tag: MAX44000 sensor
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This article discusses several ways to increase the MAX44000 proximity detection sensor drive current through infrared LEDs, ranging from very simple to complex methods that allow the user to detect targets that are further away from the proximity sensor.
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The MAX44000 integrates powerful features in a tiny, 2mm x 2mm x 0.6mm package to provide a compact, efficient infrared proximity detection solution. The sensor monitors by pulsing the infrared emitting diode and detecting the amplitude of the reflected signal. The larger the amplitude of the signal, the closer the target is to the sensor.
In a typical configuration (Figure 1), when there is no glass above the sensor, the effective range of the device is approximately 13 cm (18% gray board). In this case, one of the limiting factors is the output power of the chip transmitter. The limited transmit power limits the range that the sensor can detect and affects the implementation of certain features, such as (human) presence detection. Fortunately, there are a few simple ways to improve the performance of the MAX44000.
Figure 1. Standard configuration of the MAX44000
Increase output power
The easiest way to increase the output power of the transmitter is to drive the LED through a FET or other type of transistor. Figure 2 shows an implementation in which the DR4 pin of the MAX44000 drives the pMOSFET on and off, which in turn drives the current through the emitter. The current value is set by R5.
Figure 2. The easiest way to increase the LED current
The circuit can be further improved by setting the current through the transmitter more precisely, as shown in Figure 3. In this case, a simple op amp current source sets the current through the LED. In the figure, a voltage is applied to the non-inverting input of the op amp, and the voltage is converted into a current (in this example, the sense resistor R5 is 1 Ω, and the voltage generated by the 1 V voltage is 1 A). This voltage can come from a fixed source, such as a voltage reference, or from a digital to analog converter (DAC).
However, the IR emitter must operate in a pulsed manner, adding a primary circuit for optimal combination with the MAX44000. The analog switch sends the set voltage to the op amp current driver. The normally closed input is connected to ground and the normally open input is connected to the corresponding voltage. By connecting a pull-up resistor to the control pin, it is possible to switch the analog switch from the NC input to the NO input.
Figure 3. Using a current-regulating circuit to increase LED power
This method is only one way to increase the output power of the transmitter, and other similar Maxim devices can be used. Other methods include using a pull up on the DRV to send a voltage signal to the controller that switches the DAC output between 0V and the voltage corresponding to the desired current.
The effect of increasing the transmit power is obvious: the current flowing through the LED is larger, meaning that it can support a farther detection range. Figure 4 is a schematic diagram showing the increased range of the circuit of Figure 2. At a distance of 30 cm (or about 1 ft), the signal produced by 400 mA and below has a count of approximately 10: very close to the noise floor. Increasing the current to 750 mA gives a signal count of 32 at the same distance, eliminating the need for a lens or other optical focusing device.
Figure 4. Support for further detection distances when increasing current
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