Sensor Design and Construction

Homemade Door Frame Trip Sensor Construction:

As described in the detection method page, we used three different sensors to detect a person's presence.  Some of these, like the PIR and microphone, were purchased for a relatively small price.  It was clear that we needed to find a new detection method once our video detection efforts were deemed unfeasible.  We determined that for the two-room test room setup we planned to build we would need 4 sensors that could detect a person walking through a door frame.  We experimented with an active IR transmitter-receiver circuit, but it's limited range made it an undesirable choice.  Instead we opted for a laser transmitter/CDS receiver setup.  The circuits shown below are similar to those used as safety beams for garage door systems.  Those components typically cost $30-50, but we were able to make the sensor shown below for about $5.  The sensor uses a TTL "OR" gate to ensure output is low when the beam contacts the CDS cell and 1 when the beam is broken.  Note that the resistors used in the laser transmitter are 1W resistors.  These were necessary to handle the relatively high power requirements of the laser.  The resistors were in short supply, so only two laser/CDS sensors could be constructed.  Two IR sensors were used for the remaining two door frame trip sensors.

Laser Transmitter Circuit Diagram:

CDS Receiver Circuit Diagram:

Laser/CDS Sensor Assembly:


IR Emitter Diagram:

IR Receiver Diagram:

IR Sensors:



Relay Construction:

 The DE2 board provides a 3.3V signal to any output GPIO pins.  In order to switch our speakers on and off, a relay and step-up circuit were used to control the speakers' 120V AC source.  The DE2's logic-controlled 3.3V signal was used to turn on a Darlington transistor pair that allowed the constant 5V from the DE2 to close a relay for the 120V AC source.

Relay Circuit Diagram:

Relay Package:

Since the relay handles an 120V AC source, the relay and wall outlet were installed in a plastic hobby box to prevent tampering and promote safety. 


PIR Sensors:

The PIR sensors were packaged to ensure a stable and durable platform.



Project Links:

Detection Method 

Project Proposal

MATLAB Image Processing

Test Room

VGA Sensor Display 

Control Logic


Conclusions and Future Work