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Ultrasonic Motion Detector

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New Member

I need some help in the motion detector circuit which i have built recently using this manual available online


I haven't purchased the kit for it i have paid money to a professional PCB maker who designed the PCB for me using the layout given in the manual I have verified the PCB using layout, it seems okay, then i have purchased the parts from the market and sold them very carefully. Also i have verified them after soldering using the layout and schematic given in the manual.

Now the problem is that whenever i power up the circuit the LED which should trigger on motion detection is turned on continuously, also the output signal is on always no matter i cover the ultrasonic sensors or change the sensitivity level using variable resistor the circuit remains on always.

Now the only changes i have made in this circuit are

1. I have used non-polarized capacitors rather than capacitors which have polarity to avoid the mistakes in polarity.

2. I am using a 39.270 KHZ crystal rather than a 40 KHZ crystal (40 KHZ is mentioned in manual) because exact 40 KHZ crystal is not available in the market here so i have chosen the closest value of 39.270 KHZ. Both ultrasonic sensors operate on 40 KHZ.

so please tell me does anyone built this circuit and verified it ? Also please tell me that does the changes i have mentioned above can cause this circuit to operate in an abnormal way (Always ON).

Please help me out i need this device for my project. Thanks in advance.


Well-Known Member
The xtal freq your using should be ok. The only polar cap is a 100uf filter cap. Are you in fact using a 100uf cap? If you are using too small a value, you may be getting an oscillation which will hose up your circuit. You should try to isolate the problem in stages. Maybe start by putting a 1uf cap from ground to R6, this should kill any signal and the LED should go off. If it does then you can work your way back. Also make sure you installed the diodes in the correct direction.


New Member
Thanks for the reply.

I am not very good in electronics, i will check 1uf capacitor and than tell you the results for further assistance.

Thanks again.


New Member
i also have the same problem plus if i do not connect the sensor,the LED still on when the supply is given.Please help me also with these matter
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