Saturday, March 28, 2009

Ultrasonic Sensor Circuit4

40kHz Ultrasonic Driver Circuit for a few pounds

Here is a circuit diagram for an ultrasonic driver circuit suitable
for the the Speed of ultrasound and Lloyd's Mirror using
Ultrasonic Waves experiments. Test the circuit on a plug-in
prototype board to make sure I've transcribed the component
values correctly. I got the circuit to work on a plug-in prototype
board before it was transferred to a printed circuit board (PCB).
The circuit should work with strip board so you shouldn't need
to make a PCB. If somebody does do PCB artwork (or a strip
board layout) they should publish it on the www and I'll link to it.
Many basic electronics books explain the operation of the 555
chip (the 556 chip is a package of two 555 circuits in a single chip).

Ultrasonic Reflectance Detector
This circuit detects motion within approximately 5 inches of
a piezo-ceramic element ultrasonic transducer. The detection
distance is much smaller than obtainable with other ultrasonic
techniques, however, it only requires a single transducer, as
opposed to the two-transducer arrangement typically found in
other designs. The short-range detection is adequate for many
applications, such as proximity-operated commodity dispensers
or tamper alarms for merchandise cases.

Ultrasonic Range Sensor
1 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
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.

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