Ultrasonic Range Sensor

The Panasonic transducers are no longer available, but the design is still interesting as an example. J1 is the connection to the RCX. D1 - D4 form a bridge rectifier to obtain local power from the RCX which is stored on C7 and regulated to 5V by U1, a low power, low dropout regulator. The microcontroller, U3 is programmed to generate a burst of 8 x 40KHz pulses on pin 6. Q1 switches current into L1 creating
Ultrasonic Range Sensor - schematic

a "flyback" voltage output to Y2, the ultrasonic transmitter, of about 20V peak. The 40KHz is based on a software division of the 10MHz reference (the ceramic resonator, Y1, in this case). Y3, the ultrasonic receiver, is band-limited by L2, to stop normal sound and vehicle vibration from being the dominant input. L2 is chosen to form a resonant circuit with the self capacitance of the receiver piezo. It should be peaked for 40KHz. U2D and U2A are the primary gain stages to get the millivolt level signals up to a couple of volts. Use of a rail to rail amplifier with symmetrical (1/2 rail) biasing makes overload recovery very efficient for large input overloads from very reflective targets. D8 rectifies the returns on C9. D9 generates a exponential waveform on C6 driven from the initial pulse burst. This forms a comparison input to U2B which outputs a logic "1" whenever a return exceeds a range sensitive threshold. This design is only interested in the first return (closest object) since the RCX cannot handle a series of returns from different ranges. Both the analog and logic level returns are input to the microcontroller via analog input pin 7 and digital input pin 4. The current firmware only makes use of the logic input. The microcontroller times the returns and converts them into a varying mark/space squarewave on digital output pin 5. This squarewave is converted to a voltage on C1 and presented to the RCX analog input...

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