Ultrasonic Proximity Detector

Perfect, Ultra-Simple Electronics Project

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.

Ultrasonic Proximity Detector

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 feet. 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.

The CircuitPhoto 1

The circuit consists of the following:

40 kHz Ultrasonic Sensor Set (Transmitter)
    1. Crystal Oscillator
    2. Drivers
40 kHz Ultrasonic Sensor Set (Receiver)
    1. 68 dB Amplifier
    2. Cascade Voltage Doubler
    3. Threshold Detector
    4. 70 Hz Oscillator
    5. One-Shot (Monostable Multivibrator)
    6. Relay

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.

Crystal AssemblyFigure 3. 40KHz Crystal Assembly

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" apart on a piece of perfboard (see Photo 2 below).

Device PinoutsFigure 2. Device Pinouts

receiverPhoto 2

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.

Part List:

Component Name Part Description
R1 3K 5% 1/4W Resistor
R2 150K 5% 1/4W Resistor
R3,R10,R14 330K 5% 1/4W Resistor
R4 1.8K 5% 1/4W Resistor
R5 470 5% 1/4W Resistor
R6 10K 5% 1/4W Resistor
R7 2.2K 5% 1/4W Resistor
R8,R15,R17 1K 5% 1/4W Resistor
R9 220 5% 1/4W Resistor
R11,R19,R20,R22 100K 5% 1/4W Resistor
R12 100K Potentiometer
R13 10M 5% 1/4W Resistor
R16,R21 1M 5% 1/4W Resistor
R18 150 5% 1/4W Resistor
C1,C3,C6,C15,C16 100nF Capacitor
C2,C5,C8,C12 1µF Capacitor
C4 33pF Capacitor
C7 47nF Capacitor
C9,C10 22pF Capacitor
C11 22µF Tantalum Capacitor
C13 100µF Capacitor
C14 470µF Capacitor
C17 10nF Capacitor
D1,D2 1N914 Diode
D3 Red LED
Q1,Q2,Q3 2N3904 Transistor
U1 Comparator Dual ±18 Volt 36 Volt
U2 HEX Inverting Buffer/Converter
U3 Standard Timer Single
U4 Standard Regulator 9 Volt 2.2 Amp
K1 12V SPDT Relay
TB1 Terminal Block
J1 2.5mm x 5.5mm Jack
T1 12V 200mA Wall Transformer