The Pointer Trainer

Designers:Samuel Kuo and Peter Wang
Client Coordinators:Mary Caldwell, Duke Hospital Pediatric Rehabilitation Unit
Supervising Professor: Dr. Larry N. Bohs

Approximately 2 million acquired brain injuries (ABI) occur each year. Acquired brain injury refers to severe brain damage caused by a traumatic event. Brain injured children typically suffer from head balance and communication difficulties. The goals of the Pointer Trainer are to improve head coordination in brain injured children, and to help them learn to communicate using a laser pointer. The device will be used by therapists in the Duke Hospital Pediatric Rehabilitation Unit.

The Pointer Trainer consists of a target box and a laser diode. To improve head coordination, the laser diode is mounted to a headband so that children with ABI can use their head to point the laser diode at the target box. Upon hitting the target for a specific period of time (1 or 3 seconds, selectable by the therapist), an audio response is activated. This response is recorded prior to the therapy session, and can be associated with a picture or word near the target box. The design allows children to communicate by pointing to a target box with the desired audio response.

How this project helped
The physical aspect of training and strengthening neck muscles through pointing the laser and holding it for a duration of time in order to activate the target box helps children improve their muscular control. During their recovery, children without the ability to speak will have a simple form of communication through the audio response of the target box. The ability to interact with other people despite their limited physical and communication abilities gives children with ABI greater independence.

A positive focal length Fresnel lens (2″ diameter, 1.3″ focal length) is positioned as a collector in the target box. Light hitting the Fresnel lens is directed towards a photoresistor placed at the focal length of the lens. A trigger circuit determines the amount of time that light strikes the photoresistor.

The main components of the trigger include the photoresistor and a LM311voltage comparator (see Figure 2). The resistance of the photoresistor decreases with increasing light intensity. The trigger circuit automatically adjusts to the ambient light level as follows. The voltage at the negative input of the LM311 drops instantly as light hits the photoresistor. However, the voltage drop at the positive input is delayed by a capacitor/resistor pair. The voltage at the positive input eventually drops below the voltage at the negative input as the circuit approaches steady state. Voltages at both inputs vary with ambient light intensity, thereby eliminating the need for a fixed reference voltage. The output of the voltage comparator is low during steady state and goes high when the photoresistor is illuminated by the laser.

The comparator circuit uses hysteresis to eliminate output oscillations. These oscillations result from noise on the comparator inputs. By setting the hysteresis voltage greater than the noise, oscillations are eliminated.

When the laser hits the photoresistor, the CD4017counter is enabled. The counter is a decade counter with a separate output pin for each count. The clock input is from a 555 timer with a 415 ms clock cycle. At approximately 1 and 3 seconds, the counter counts to “3” and “7”, respectively. The two pins corresponding to these outputs are connected to a switch that selects the desired activation time. The position of the switch determines which output triggers the record/playback circuit.

The record/playback circuit uses an ISD1520 chip, which allows the therapist to record a message up to 20 seconds long. An LED illuminates to indicate recording. A condenser microphone is used to record the audio message. The ISD1520 drives a LM386 audio amplifer to provide output to the speaker. The entire circuit draws 21mA at resting state and 50mA during an audio feedback response.

On the rear of the target box are momentary switches to operate the record and playback functions, a 10k( potentiometer for volume control, and sliding switches for power and to select the desired activation time.

The cost of the project was about $200.

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