Design of a Wheelchair Pressure Relief Timer

Final Design The overall schematic of the design is shown in Figure 1. The final design for the timer circuit includes a 555 timer chip whose output oscillates between 0 and 9V (off and on states). This output is shown in Figure 2. The amount of time the chip spends in each state depends on the values of the two resistors and one capacitor, as shown in Figure 3. The off-state lasts for approximately 25 minutes and the on-state lasts for about 30 seconds, during which the speaker and LED circuits (Figures 4 and 5, respectively) are activated. If the user performs pressure relief properly (lifting off of the seat), the pressure switch will be opened and thus the alarm circuits will not be powered. After the 30 second on-state, the 555 timer chip will automatically reset to the off-state. The LED circuit of Figure 5 is similar to the timer circuit because it utilizes the oscillating output in order to create a flashing LED effect. In order to allow the user to completely turn off the device, an on/off switch is located at the 9V battery source, as shown in Figure 1. Another switch allows for the user to select between a silent, visual alarm only and both the visual and audible alarms together. This feature may be useful in quiet environments where a loud alarm is not appropriate.
Figure 1: Overall Schematic of Design
Figure 2: Timer Circuit Output
Figure 3: Timer circuit Schematic	Figure 4: Speaker Circuit Schematic	Figure 5: LED Circuit Schematic
In Figure 6, the pressure switch design incorporates the use of a momentary pushbutton switch to monitor the pressure relief exercises. If the button is pushed down, then the user must be seated and the alarms will sound as usual. If the button is not pushed down, then the user has lifted off of the center of the seat and the alarms will not sound. In order to avoid preventing the weight of the cushion itself to push down the button, a ring of polyurethane cylinders are placed around the switch to provide enough resistance against the cushion. They will compress under the weight of a user weighing more than 15lbs. The polyurethane cylinders also serve a second purpose of preventing damage to the button from excessive force. The button and the cylinders are attached to a clear, hard plastic disc to keep everything in place. Another plastic disc is used to make contact with the pushbutton switch. This entire assembly is enclosed within foam and then wrapped in a vinyl fabric to simulate a typical wheelchair seat cushion.
Figure 6: Pressure Switch Design

Figure 7: Main Unit	Figure 8: Open Main Unit
The main unit, shown above in Figure 7, houses the circuitry, the dual-alarm system, the on/off switch (on left), and the alert selection switch (on right). The 9V battery can be changed by removing four screws on the top lid.	Inside the main unit, all connections are soldered and secured. The mini-speaker is located at the far right. The two lead wires on the left connect to the pressure switch inside of the seat cushion.
Figure 9: LED Operation	Figure 10: Pressure Switch
The two LEDs, pictured above in Figure 9, flash at a rate of 2Hz when the speaker is off, and 3Hz when the speaker is on.	The pressure switch, pictured above in Figure 10, show the polyurethane cylinders that provide resistance and resilience to the two plastic discs. Another polyurethane ring surrounds the bottom of the pushbutton switch to protect the exposed connections. The three posts provide guidance as the plates move up and down.
Figure 11: Overall Product
Figure 11 shows how the complete device is situated on a common wheelchair. The pressure switch assembly is embedded between layers of foam to prevent the user from sensing its presence in the cushion.	Click on the logo above to watch a short video clip of the two alarms.