Access Card Sensor
Each game required an access card to be inserted into a slot before activating and responding to user interactions. The access card was a black and white pattern (the letters "SPDL" in this case) printed on a 3x5" card (Fig. 1). We designed a detector that would recognize the card if fully inserted with the text facing up. Our detector was not sensitive to the orientation of the card, and mechanically prevented false readings.
Our approach was to measure two points in a constrained slot. We used packaged IR LED/phototransistor "tape sensors" to identify black and white print. By measuring a point in the back, we could guarantee that a card had to be fully inserted to trigger the sensor. A second sensor looked for a black spot that corresponded with the pattern. The tape sensor circuit was designed to produce a digital high when the sensor was facing a white card and a digital low when facing black ink (Fig. 2). By inverting the black spot sensor and AND'ing the two signals, we were able to produce a single digital high/low where high meant a card was detected. From a software standpoint, we were able to treat this input as a simple button.