Jeff Davis firstname.lastname@example.org
Not sure where that quote came from but it sure seems to always come to mind whenever I indulge myself in one of my favorite hobbies, Robotics. In this particular case I had been reading about the Eltec Sensor and was considering adding one to my creation. The first problem I encountered was that I'm CHEAP!... I couldn't see spending $50.00 for one of the sensor kits when I had access to PIR's at a decent price. The following is a description of what I did to interface one of these units and I hope it might help someone who wants to incorporate a Pyro-electric sensor into their robot design with cheap in mind. While these detectors work they way the manufacture intended, they are limiting for the robot hobbyist. A standard motion detector has a pulse stretcher built into its output circuitry as well as a required 12Vdc-supply voltage. The supply voltage isn't too much of a problem if you are using 12 volt as your primary supply, but the timeout on the motion detector can drop sampling times to 10's of seconds. This situation makes the sensor almost useless for our intended application of being able to quickly scan an area for heat. This hack basically allows you to convert a cheap security system motion detector into almost an equivalent to the Eltec Pyro unit. This modification should be capable on most newer units.
Almost all new motion detectors include a 78L05 in their power supply section. A jumper installed between pin's 1 and 3 (On the transistor styled package) allows for a 5-volt input for operation. Please note after installing the jumper don't apply 12 volts to the unit, this would be a guaranteed way to fry the detector. Sensor Output Fix The typical motion detector is supplied with a 3-pin dual element pyro detector. One pin is ground and the other 2 are outputs from the sensor. These outputs are tied to a pull-up resistor and fed to input on an op-amp. On my unit this was a SMT lm358 dual op-amp. For this interface I used only one of the two outputs of the LM358 op-amp. This output was on pin #1 and is connected to one of the A to D inputs on Port E of the 68hc11 controller board. For ease of connection I soldered the small wire to smt resistor that was close to the op-amp.
This output stays at 2.5 v while no "moving heat" is detected. When heat is introduced to the sensor the output of the op-amp makes a drop to 0 volts, then climbs quickly again to the 2.5 volts until there is another heat trigger. With sampling done on the output of the first op-amp you can achieve outputs similar to the standalone Eltec unit.
All security system detectors come with a lens system. The design of the lens has multiple miniature Fresnel lens arrangements so the detector can see movement across a wide field of view. The ridges of the lens can always be seen from the inside side of the unit. We need to fix this... Some detectors come with white adhesive paper to mask off certain areas to help eliminate false alarms. This paper is to be attached to the inside of the lens. Even if your detector didn't come with the masking paper you can make your own with thick adhesive labels like the ones used for printing mailing labels. Our goal here is to mask all but the forward facing zone. This will give our unit a narrow beam of detection that we need to pinpoint the heat source we are looking for. One last note: The detectors are originally designed to be mounted about 7 feet or so up on a wall and look out and down. For our use the unit should be mounted upside down so we detect up and out.