This is a ultrasonic motion detector (or movement detector) circuit. The circuit claimed has a high movement sensitivity. Even air moving (hot air rising, wind blowing) will trigger it when the trimpot is set near the most sensitive position.

The transmitter sends out a steady ultrasonic tone at 40kHz. At this frequency the wavelength is about 6 mm. Any reflected sound is detected by ultrasonic receiver. The signal is then amplified by IC1:A and IC1:B. IC1:A is selfbiasing via C2 & R5. The time constant of the first amplifier is set to let the 40kHz signal through. Between the first & second amplifier there is a negative peak detector (diode D1 & R8) which follows the envelope of the 40kHz signal. If there is no movement the envelope is just a straight line. The time constant of IC1:B is much slower so that it will follow this envelope. All the amplifiers are AC coupled to prevent DC bias problems.

Then the signal is fed through a window detector IC1:C which detects both positive and negative pulses. When there is no movement the potential at pin 7 sits at half the supply potential and neither D2 or D3 can conduct. The potential at pin 8 is low. If the signal rises D3 conducts causing the output to go high. If the signal falls then D2 conducts which also causes the output to go high. Thus the name window detector circuit because it detects potentials which move both below and above a given range.

A low pass filter screens out unwanted spurious signals, then an amp IC1:D set up as a monostable flip flop converts any signal that gets through the filter into a substantial pulse to turn on the BC639. This turns on the LED and provides a Signal Out to drive a separate relay or any other device you may wish to signal to. The time constant of the monostable flip-flop is about half a second and is set by C8 & R10. D4 is used to separate the charge & discharge time constants. It lets the circuit switch on immediately movement is detected but allows about a 1/2 second delay for the reset.

The circuit available in kit, you can order the kit at kitsrus.com or electronickits.com. If you want to built the ultrasonic motion detector by yourself, just download the PDF manual document for detail explanation and parts list for this circuit:

Ultrasonic Movement Detector Circuit Project

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Ultrasonic Transmitter Parts List:

IC1 = NE555 timer IC
VR1 = 10k variable resistor
R1 = 4.7k resistor
R2 = 18k potentiometer
R3 = 1k resistor
R4, R5 = 220 ohm resistor
C1 = 680 picofarad capacitor
C2 = 0.01uf capacitor
D1, D2 = 1N4148 Diode
T1 = SL100 NPN transistor
T2 = SK100 PNP transistor
S1 = SPST momentary contact switch
XMTR = ultrasonic transmitter 40-50khz

Ultrasonic Receiver Parts List:

RCVR = Ultrasonic Receiver 40-50khz
RL1 = 6volt 200ohm resistor
IC2 = CA3140
VR2 = 250k Variable Resistor
R6 = 390k Resistor
R7 = 470k Resistor
R8, R12 = 15k Resistor
R9 = 12k
R10, R13 = 10k
R11 = 4.7k
R14 = 100k Resistor
R15 = 33 ohm Resistor
C3 = 0.22uf ceramic capacitor
C4 = 0.1uf ceramic capacitor
C5 = 560n ceramic capacitor
T3,T4 = BC548 NPN Transistor
T5 = BC558 PNP Transistor
T6 = SL100 NPN Transistor
D3,D4,D5 = 1N4148 Diode

The ultrasonic transmitter uses a 555 based astable multivibrator. It oscillates at a frequency of 40-50 kHz. An ultrasonic transmitter transducer is used here to transmit ultrasonic sound very effectively. The transmitter is powered from a 9-volt PP3 single cell. The ultrasonic receiver circuit uses an ultrasonic receiver transducer to sense ultrasonic signals. It also uses a two-stage amplifier, a rectifier stage, and an operational amplifier in inverting mode. Output of op-amp is connected to a relay through a complimentary relay driver stage. A 9-volt battery eliminator can be used for receiver circuit, if required. When switch S1 of transmitter is pressed, it generates ultrasonic sound. The sound is received by ultrasonic receiver transducer. It converts it to electrical variations of the same frequency. These signals are amplified by transistors T3 and T4. The amplified signals are then rectified and filtered. The filtered DC voltage is given to inverting pin of op-amp IC2. The non- inverting pin of IC2 is connected to a variable DC voltage via preset VR2 which determines the threshold value of ultrasonic signal received by receiver for operation of relay RL1. The inverted output of IC2 is used to bias transistor T5. When transistor T5 conducts, it supplies base bias to transistor T6. When transistor T6 conducts, it actuates the relay. The relay can be used to control any electrical or electronic equipment.

Circuit notes:

1. Frequency of ultrasonic sound generated can be varied from 40 to 50 kHz range by adjusting VR1. Adjust it for maximum performance.
2. Ultrasonic sounds are highly directional. So when you are operating the switch the ultrasonic transmitter transducer of transmitter should be placed towards ultrasonic receiver transducer of receiver circuit for proper functioning.
3. Use a 9-volt PP3 battery for transmitter. The receiver can be powered from a battery eliminator and is always kept in switched on position.
4. For latch facility use a DPDT relay if you want to switch on and switch off the load. A flip-flop can be inserted between IC2 and relay. If you want only an ?ON-time delay? use a 555 only at output of IC2. The relay will be energised for the required period determined by the timing components of 555 monostable multivibrator.
5. Ultrasonic waves are emitted by many natural sources. Therefore, sometimes, the circuit might get falsely triggered, espically when a flip-flop is used with the circuit, and there is no remedy for that

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