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| The electrical design was the most complex portion of this project. In order to obtain an emg signal, the emg electrodes were placed on a large muscle (the bicep for this game). A reference ground electrode was placed on a flat, bony area with small amounts of soft tissue, such as the elbow. When the player flexed his muscle, the electrode detected a small action potential voltage. The signal received from the electrode needed to be sent through several stages of signal conditioning in order to become discernable. The first stage was a 1000x amplifying stage, in which the signal from the electrodes was fed into a 1NA114AP instrumentation amplifier. We chose this particular instrumentation amp for its high signal/noise ratio. This stage amplified the millivolt signal to one on the order of volts. |
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The next step was to filter the signal to eliminate high-frequency oscillations. We built two low-pass filters using an LT1124 op-amp. This stage elimintated any high-frequency voltage spikes caused by random muscle action, and ensured that the only part of the signal that remained were the low-frequency signals caused by sustained flexing. After filtering the signal, another LT1124 op-amp was used to amplify the signal 22x and then buffer the signal. After buffering, the signal was sent to a rectifier to eliminate any portions of the signal that were in the negative domain. The next stage of the circuit was an amplifying stage. Since this was the final amplifier stage for the signal, we put a 10k potentiometer in series with a 10k resistor. When the potentiometer was at its maximum resistance value, the amplification was 20x, when it was at its minimum value, the amplification was 10x. This gave us a means of adjusting the gain in order to prevent the signal from becoming the rail-to-rail voltage of the op-amp. Using a potentiometer also gave each individual player a means of adjusting the game to his or her own capabilities. Stronger people who generated a larger signal from the electrodes could reduce the gain and make the system less sensitive to their flexing, while weaker players could increase the gain to make it easier to get a large enough signal. After the final amplification, the signal was buffered once more, and then sent to a comparator. We used an LM311N chip as our comparator, with a +5V reference signal. Whenver our signal went over +5V, the output of the comparator was +5V. If our signal was below +5V, the comparator output was 0V. This step turned our analog signal into a TTL digital signal, which we then sent to the Handyboard. The details of how the Handyboard used this signal are included in the Software page. We also built on/off switches for each player to use by taking a single-pull/single-throw switch and connecting one lead to the Handyboard's +5V digital port, and the other lead to the ground port. When the switch was on, the Handyboard read digital 1 from that port, and would run its program normally. When the switch was off, the Handyboard read digital 0, and did nothing. We constructed two emg conditioning circuits, allowing independent, two-player games. |
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