Rutgers Physics Student Response System

Student Station Hardware

The student station is centered on an ordinary telephone keypad, which is mounted together with 3 LED lights on a small printed circuit board. A standard keypad is simply a set of twelve normally open SPST switches, each of which, when pressed, shorts one of three column wires with one of four row wires. The three column wires are directly and separately connected from each of the (up to) sixteen keypads to the one subsubstation controlling them. During the polling period which gathers keypress information, one of these 48 wires is grounded, while the others float. If any of the four buttons in that column is depressed, the corresponding row will be driven low. Each subsubstation has four lines for the rows, which are pulled up with a resistor, and sensed. The voltage on this line will be low only if the one button connecting this row to the one grounded column is pressed. Therefore by measuring the voltage on the four row lines, during 48 periods corresponding to the grounding of each of the 48 columns, we can determine the state of the 192 buttons involved.

There is one difficulty that needs to be avoided. If a troublemaker holds down all the buttons in one column, whenever the button-press of another student is about to be sensed, not only will the corresponding row be grounded, but also the troublesome column, and therefore all the rows, so that any single button press would register incorrectly as all the buttons in one column being pressed. To avoid this problem the common row lines pass through diodes on each keypad. Thus the grounded row is unable to pull down the troublesome column, and the other rows are therefore unaffected.

Each subsubstation also controls one light-enable line, which is driven high when the light in the currently grounded column is to be lit, and not otherwise. Thus each of the LED's is lit at most 1/48 of the time, but this is sufficient to make a clear signal that the light is lit. (The polling process cycles 18 times a second, so the blinking is fast.)

The keypad, four diodes, three LEDs and an RJ45 socket are connected to a printed circuit board I designed, and this combination is mounted in a solid black plastic armrest designed to hold it. We discovered that the exposed LEDs were subject to being wiggled back and forth until they broke off, so we added a small transparent plastic cover over the LEDs. Both the armrests and the covers were produced on a computer-controlled milling machine in the Department's mechanical shop.


Revised: October 27, 1995