For a motion detecting application I found a sensor with code LHI 778.

If you look at the datasheet in the link, the characteristics of its internal FET transistor is not specified and there is no information about its internal sensor element. There for this sensor is too vague for me for designing a practical circuit. My aim is to design a circuit for security purpose which detects people passing by a window or house gate, both at night and daytime.

Can you tell me more about this sensor and offer a practical circuit which lights a LED when there is detection of any movement.



  • The circuit diagram below shows a basic means of operating a relay or LED

  • Many other circuits are provided via the reference link

  • A Fresnel lens or some other lens will be required to allow this detector to function as intended. Links to various explanations of how lensing systems work with PIRs and links to suppliers of typical lenses are provided.

The line in the LHI778 data sheet labelled "output impedance" tells you what you need to know for interfacing purposes.

The sensor is either on or off.

When on and when you use a 47 kohm load resistor the FET looks like it is a resistor with resistance between 5 and 10 kohm.

Looks like load should be from source to ground with drain at V+ - not with load in drain as below. Arithmetic still applies as below.

You can design from there.
eg if you ground FET source and put 47 k to drain from V+ then.

  • When FET is off drain is at V+

  • When FET is on drain is at from 5/(5+47) to 10/(10+47) of V+.

eg if V+ = 9V drain will be from 5/52 x 9 =~ 0.9V to 10/57 x 9 =~ 1.6 V.

This output can be used to drive a transistor or op amp circuit of your choice. Or an LED plus series resistor could be used in place of the relay coil (LED will have the opposite polarity to the diode shown - remove relay and diode and add LED plus series resistor.). If that advice is not enough and you need detailed circuitry then you probably need to do some more reading on the subject. See references below.

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The above circuit is from here

* Many more PIR circuits here *

Fresnel and other lenses for PIR use:

Note that the device by itself is not suitable for your task. You need to add a Fresnel lens or similar optical system that causes changes in input as te detected body moves between "zones". Searching the web for

PIR Fresnel

should turn up many explanations of what is required.

Many many many images plus links to web pages

Various PIR components including lenses and sensor ICs

PIR Fresnel lenses and cone optics

Various Fresnel lenses for PIR use

Still more Fresnel lenses

Alibaba - vast range of lenses andelated materials

Wikipedia on PIR detectors

Fresnel lens tutorial

And more ...


This is an addition to Russell's answer, so read that first.

First something about driving the transistor. You have to make sure you supply enough current to drive your load, especially if that's a relay. The second schematic in Russell's answer is poorly designed in that aspect. The 100k\$\Omega\$ base resistor will limit the base current to a low 0.1mA. The BC547B's \$H_{FE}\$ is minimum 200, so the collector current may not be higher than 20mA, which may be too little to drive a relay. So, keeping the PIR's 10k\$\Omega\$ output impedance in mind, a 1k\$\Omega\$ resistor is a much better choice.

Then the lens. At first I also wanted to remark that you need a lens with the PIR sensor, but that may not be so. The sensor has a wide viewing angle and won't detect motion once the person is in its view. Russell talks about a Fresnel lens to remedy this.

Fresnel lens

A Fresnel lens is just a common lens which is flattened in a way that retains the lens' curvature. This is needed because IR light won't pass if the lens is too thick. For the rest the lens works like a normal lens, focusing the IR radiation onto sensor.
This way it's possible to detect motion if the person comes within the sensor's view. So why wouldn't you need a lens? If you want to detect a person passing a window the window has the same effect as the lens, limiting the sensor's view, so it will react to a person who's behind the wall and then walks by the window.
What you can do to better detect motion is to use a fragmented lens, which divides the detection area in smaller parts:

napion detection zones

This image shows the detection zones of a Panasonic NaPiOn PIR sensor. So whenever a person moves from one detection zone to another this will be detected by the sensor, making it very sensitive to even small movements. A common Fresnel lens doesn't offer this sensitivity.


There is this page describing usage of PIR sensor.

Here is a sample circuit from that page:
PIR Sensor Circuit

If you connect a LED and a 1K resistor to the collector of 2N2222 in series instead of the relay in the picture, the LED will light as you asked in your question.

  • \$\begingroup\$ Is there any difference in operation between day and night? \$\endgroup\$ – Erdem Sep 6 '11 at 16:04

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