Have a question about Sensors? Ask our expert Jane Awittor

Good day Ma'am,

I need your expert advice. I am having a problem right now about my PIR sensor lhi 968. You see, i only have the sensor itself that's why i am creating circuits from scratch with reference from circuits i search at the internet. I am currently using a LM324 with the PIR sensor. Well, the circuit works... as a heat detector and not that very efficient. It will only activate when the tip of the soldering iron is near the sensor, about 1cm. My problem is i don't have the lens for it so i tried using it bare. Is that possible? And also, can i make a motion detection project from this without the use of lens (fresnel lens) ?

Thanks
Robert

I was hanging fire to see if any one else would help you - apparently not.

Can you post the ciruit you are using and it should be possible to help.

MK

I was hanging fire to see if any one else would help you - apparently not.

Can you post the ciruit you are using and it should be possible to help.

MK

Thanks for the reply mike!

here is the circuit i am using right now: https://09fa4394-a-62cb3a1a-s-sites.googlegroups.com/site/osamaslab/Home/projects-list/pir-sensor-based-alarm/PIR.pdf?attachauth=ANoY7cqVwgjo8GD24M7gtClV6sAl2cPgP0NfLvO1AP0Zi4yFmPhbgJvvoTZO7mJf2pBeTekgYgfKvWU7dQ50jj9HmgmhRzINktReAirBsOeuw3dek-nmx_7uyC0xq4jD7MgWPKofB7F5bQbAv3jTRHDopDBoekpy7n4TIKufaa2DDlMjf980tJfJ9y-3xlP0L4xCvDoImHdpV_F6z3ieGcQLSILAwMJc36UNXmNgnCLqRQ1JVwEbqH-Xn_nzKE9hW58u7zxRVUX6&attredirects=1

i think i really should use a fresnel lens or any substitue for that.. and that's my problem.. i think.. :/

Robert, from your first post there is more wrong than just a lens - you should be able to detect hand waving easily.

If you can make some DC measurements we can tell if your ciruit is working OK.

Can you measure (with respect to earth), no handwaving but sensor must be connected.

DC volts on

ZD1 cathode (junction with R10)

U1, pin 1

U1, pin 7

U1, pin 14

U1, pin 8

MK

here's my measurement:
ZD1 cathode (junction with R10)     4.4V

U1, pin 1     355mV

U1, pin 7     1.728V

U1, pin 14     285mV

U1, pin 8     358mV

handwaving or not, the voltage doesn't change.. do i have a defective PIR semsor? :/

Well let's work it out,

What happens if you briefly connect the sensor S to ground with a 47k resistor.

(It should make  a big signal and activate the relay).

I expect the DC voltage on S is  about 335mV - could you please check.

MK

Hello,

     I'm very sorry for the late reply. I think my first sensor was defective. I am now using a new sensor i bought with a fresnel lens using the same circuit. But the problem is, it's still not that sensitive for hand waving or any movement surrounding it. Well, atleast the distance improved when i am using the soldering iron tip for about 4inches. When i probe the voltage reading between the sensor's S while i am waving my hand, 2-5 millivolts of changes occur. But when a soldering tip is near, a significant change occurs in the voltage reading. How can i adjust the sensitivity so that it can react to a simple hand waving etc.?
thanks

Hello Robert,

You would only expect quite  a small signal - tha amplifiers in your ciruit have  a gain of 9000 and the detection threshold is about 1V so you need a signal of about 0.1 mV to set off the alarm. A DMM is not the best tool to look at the signal with because it is in a difficult frequency range - too slow for AC settings on a DMM and changing too rapidly for DC settings. Use a scope if you have access to one and look on the output of the first or second amplifiers.

MK

Hello Sir Mike,

Yes, i think so too. And that is my problem, i have no oscilloscope. But, my problem is already solve! I used another circuit . Here it is: http://320volt.com/wp-content/uploads/2012/10/pir-kontrol-devresi-hareket-sensoru.png.Since, i don't have the cd4538 IC, i just put a LED to determine if i have an output from diodes D3 and D4 and yes, success!! The voltage reading ranges from a hundred millivolts to 3.+ volts when i wave my hand, make any sudden movement from a meter distance. I think that's enough for me, i can incorporate that output with an ic or a transistor to activate a relay. Thanks by the way sir for assisting me with my problem. You're very helpful .

but i will still look into that, i also want to learn on how to determine the gain of the amplifier in a circuit etc. so i can better understand it not just following the schematic. Maybe i'll ask you again about that if you're still willing to educate me. Again, many thanks to you sir!