February 07, 2014

Making a Human Interface Device Using SpikerBox

Q said to Bond: It's activated by nerve impulses from the wrist muscles. This dialog from the 1979 James Bond movie, Moonraker, seems to fit into place when we talk about interfacing biosignals (surface EMG, in this case) with an actuator. [Watch a clip from the movie below]

In order to achieve this human-machine interface using our good not-so-old spikerbox, we need to estimate the output signal amplitude coming from the extensors of the wrist (around 5 mV), amplify the signal by some 300 times by cascading two LM 386 IC (or any opamp taking care that the device doesn't go into saturation) to about 1500 mV. We can then connect the output from pin 5 of the IC via the 250 MFD to a *1K resistor (* = value to be experimented with) to ground. Next we connect a wire from negative terminal of the electrolytic capacitor to pin 13 of CD 4066, a quad bilateral switch IC. The Boolean output (on/off logic) signal may then be driven into terminals of a 'laser pointer', whose push button terminals will have to be replaced by the output from pins 1 and 2 of CD4016/4066. A sketch of it is shown below. Pardon my bad drawing 'skill'! Interfacing should not be a problem as the power supply is 5V and CMOS ICs are happy with that.

Output level of LM386 is at half the supply voltage level (i.e. at 2.5 V). At +5mV signal level, voltage at pin 5 becomes 4.0 V [2.5 + (5*300*10^-3)], making the control pin of 4066 go logic high. A low resistance (250 ohm) path is established between pin 1 and 2 of this IC, hopefully switching the laser on. We better not play with darts at this moment!

Another idea is to hook up an optical mouse in such a way that its internal potentiometer is swapped with a twin light-dependent resistor (LDR) as shown below. The picture at the left/top panel shows the component side of the mouse and the soldered side at the right. The three leads in the rectangle 'marked' in the right/bottom picture panel on the soldered side may be replaced by the new pot using a twin LDR, as shown in the middle panel (its middle two leads connected to form the central/common lead of the new three terminal pot as shown in red arrows). 

The input maybe obtained preferably in stereo from the sternocleidomastoids of both sides. And the output amplified and fed into two LEDs separated by a window which communicate in a 'line of sight' with the 'twin LDR' setup. This would couple the two circuits (the SpikerBox setup and the laptop via the 'doctored' mouse) optically. The scrolling function (up/down) may then be undertaken by moving the neck sideways. The experimental setup may be as per this link. Details to be updated at a later date. You too may contribute.

Disclaimer: This circuit is a mere prototype, liable to have errors. I will check it at a later date, got to hone my electronic skill a little and get out of any further procrastination.

Creative Commons License
This work by Amiya Sarkar is licensed under a Creative Commons Attribution 4.0 International License.

Last modified: Apr20, 2014
Reference: hyper-links, unless specifically mentioned
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