Build an Ultrasonic Proximity Detector
Whether you want to build an automatic flush toilet for your home or design a machine to taunt your cat every time she walks by, our friends from Nuts & Volts Magazine have this great ultra-simple, ultrasonic proximity detector.
The circuit in Figure 1a and Figure 1b are the schematics of an
ultrasonic proximity detector. It has a maximum detection range of
approximately 6 ft. When someone passes in front of the ultrasonic
transceiver board, the contacts of relay K1 will close. The relay will
remain energized while the device continues to detect a reflection. When
there is no reflection, the relay contacts will open after 20 seconds.
Photo 1 shows the assembled circuit on an experimenter's breadboard.
a. Crystal Oscillator
b. Drivers
40 kHz
Ultrasonic Sensor Set (Receiver)
(P/N 139492)
a. 68 dB Amplifier
b. Cascade Voltage Doubler
c. Threshold Detector
d. 70 Hz Oscillator
e. One-Shot (Monostable Multivibrator)
f. Relay
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Step1: Transmitter
U2C forms a 40 KHz oscillator (figure 3 shows the 40 KHz crystal
assembly). This oscillator is connected to U2D and U2E while the
inverted oscillator signal (U2B) goes to U2A and U2F. These parallel
gates provide more current and drive the ultrasonic transmitter. Note
that it may take a couple of seconds after the power is applied for the
oscillator to stabilize.
Step 2: Receiver
Q1 and Q2 amplify the reflected 40 KHz signal picked up by the
ultrasonic receiver by 2500. Q2 is capacitively coupled to the voltage
doubler formed by D1 and D2. The rectified signal is connected to the
negative input of voltage comparator U1A. R12 (the Sensitivity
potentiometer) sets the threshold voltage for U1A. When the threshold
voltage is exceeded, the open collector output of U1A goes
high-impedance. This enables the 70 Hz oscillator formed by U1B. When
this oscillator is on, the LED glows and the one-shot formed by U3 is
repeatedly triggered. The output duration of the one-shot is set by R16
and C11 and is equal to 1.1*R16*C11 seconds. U3's output turns on Q3.
As a result, K1 closes its normally open contacts. C13 dampens the
inductive kickback when K1 is turned off, preventing the circuit from
triggering due to this noise source. The unit is powered by a 12 VDC
200mA unregulated wall transformer. U4 provides a regulated 9VDC to
power the circuit. Device pinouts are shown in Figure 2. The 40 KHz
transmitter and receiver are mounted 4 inches apart on a piece of
perfboard.
To calibrate the device, have someone stand at the desired detection
distance in front of the transceiver board. Next, turn the Sensitivity
Control (R12) until LED D3 glows. Then rotate R12 slightly in the
opposite direction until the LED goes out. Noise may cause the device
to trigger randomly, causing the LED to blink. If this happens, turn
R12 slightly to reduce the circuit's sensitivity. One application for
this circuit is turning on a door chime. Another use is detecting a
person on a porch and turning on an exterior light. This can be
accomplished by wiring the normally open contacts of the relay in
parallel with the lamp SWITCH. Have fun with your Ultrasonic Proximity
Detector.
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