On the REM-detecting, white-noise-generating, potato-julienne-ing project front, I have finalized the hardware (and toyed with the firmware) for the REM detector subsystem.
To recap this project: I am developing a sleep mask which will be able to detect the REM (rapid eye movement) phase of sleep, and wake the user up at the 'optimal' point in their sleep cycle. Additionally, it will be able to record the timing and duration of REM sleep phases (potentially useful for improving sleep) and it will be able to generate white, pink or red noise through speakers at the ears to improve sleep.
I mocked up the hardware and some firmware which sends acquired REM detector samples over a serial channel; I also had to put together some software to acquire, interpret and plot this serial information. The results of this effort have allowed me to finalize the hardware design for the REM detector subsystem. As it stands, all I have left to finalize of the hardware is the display and the USB interface hardware; once these two things are nailed down, I can design and order the boards and fabricate the hardware, moving to the firmware-only phase of the project.
Finalized hardware
The hardware has been modified from the schematics I presented originally. These changes are due primarily to two factors: the microcontroller has an internal gain (negating the need for a second external gain stage) and the desire to used a switched-emitter topology (that is, the illumination of the eye for REM detection will only be on for a small percent of the time to save power).
As seen in the schematic below (the left op-amp), the current output from between the phototransistors on the mask is fed into a transimpedance amplifier whose output is fed directly into an ADC pin on the microcontroller. This single-ended signal can eventually be used to set the emitter amplitude. This signal is also fed into the positive side of a differential ADC (with gain); the negative side is fed from a lowpassed PWM output. This negative input can be used to bias the differential ADC channel to maximize dynamic range; the lowpass converts the oscillating PWM signal into a DC signal whose level is the rail voltage times the Duty Cycle of the PWM waveform. The transimpedance amplifier feedback resistor is set to 40kOhm to maximize signal amplitude while preventing saturation under normal (and even some abnormal) usage conditions.
The driver circuitry has been significantly improved in this schematic. Specifically, an op-amp is used to 'linearize' the current control. Above, a sense resistor is used in negative feedback to set the current through the infrared emitter; the set point is determined by a lowpassed PWM fed through a voltage divider. Without the 'linearization' of the sense resistor and negative feedback, the highly nonlinear nature of the emitter's current/voltage characteristic meant that only a few of the possible PWM output levels were 'useful' (that is, corresponding to levels of current we would want to drive our emitter with).
Additionally, the op-amp makes driving the emitter simpler; its high input impedance simplifies the design process for the lowpass-PWM easier. Additionally, it makes it simple to add an enhancement N-FET to allow for switching the emitter on and off (by tying the control input of the op-amp to ground).
Firmware/Software for debugging
To debug the REM detector hardware, I needed to implement a firmware to sample the REM detector input channel and transmit that data to my laptop. I also needed to create software on my computer to acquire and plot the transmitted serial data. The firmware and software are contained in this zip archive.
The big questions I needed to answer were: what do I need to do to keep the differential channel biased appropriately, and how long does the emitter need to be on to ensure that the phototransistor signal is stable before sampling.
The firmware samples the single-ended and differential ADC channels. It then updates the PWM bias setting on the negative input of the differential channel to keep the differential signal centered. It then formats a couple of serial bytes according to the ADC data and sends them to the computer.
To plot this serial stream, I made a function in MATLAB that opens the serial port and continuously samples the incoming bytes, breaking up the stream into sequences, translating them into floating-point numbers and displaying them on the screen, as seen below. The blue trace is the single-ended ADC channel, the green trace is the differential channel, and the red is a moving-average of the green.
I waggled my eyes at the beginning and middle of the plotted waveform; you can see that the signal is well-modulated by eye-waggling, which is necessary for detection of REM.
I used a USB oscilloscope (the Hantek DSO-2090, highly recommended) to check out the settling time for the phototransistor signal in response to switching the emitter. At the end of the day, I established that a switched emitter could be timed to allow for detection of REM using the specified circuit, so I have finalized that circuit and am moving on to validating the USB and display subsystems.
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