# swap cds sensor for pir sendor?



## drwilde (May 14, 2008)

ive got a couple of these cheap sound playback devices. the CDS sensor it has doesnt work in the dark at all. Here is what i want to do. if anyone can help let me know if this is possible and simple. i got a parallax PIR from radio Shack. cant i just remove the CDS and solder on the PIR? if so i dont know what to connect to what. ther are 3 pins on the pir. (-) grd, + and signal. the CDS sensor on has a pair a wire leads to it. i guess + & -. i tried it but didnt get any result. maybe i didnt let it activate long enough? Also there is a jumper for hi lo. not sure hat that is for? any help? I want to make this work without programming and more complicated controllers. thanks!


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## JeffHaas (Sep 7, 2010)

I don't think it's a drop-in replacement. A PIR needs power (the + and - connections) and then it sends a signal on the "out" connection. Radio Shack sells the Parallax PIR, see:
http://www.parallax.com/tabid/768/ProductID/83/Default.aspx

The photocell is a variable resistor that changes when light hits it - see this explanation at Adafruit:
http://www.ladyada.net/learn/sensors/cds.html

The circuit to interpret each of these is different. Usually a PIR talks to a microprocessor, either a Basic Stamp (like the EFX boards) or an Arduino.


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## Otaku (Dec 3, 2004)

I designed an easy-to-make circuit that takes the PIR output and sends it to a MOSFET and thence to a relay. Works good for triggering audio boards and other devices, provided the device will ignore multiple triggers during playback.


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## HomeyDaClown (Oct 3, 2009)

drwilde said:


> ive got a couple of these cheap sound playback devices. the CDS sensor it has doesnt work in the dark at all. Here is what i want to do. if anyone can help let me know if this is possible and simple. i got a parallax PIR from radio Shack. cant i just remove the CDS and solder on the PIR? if so i dont know what to connect to what. ther are 3 pins on the pir. (-) grd, + and signal. the CDS sensor on has a pair a wire leads to it. i guess + & -. i tried it but didnt get any result. maybe i didnt let it activate long enough? Also there is a jumper for hi lo. not sure hat that is for? any help? I want to make this work without programming and more complicated controllers. thanks!


PIRs generally put out a 5 volt signal when they are correctly powered (usually 5 to 12 volts DC) and they see something warm that moves. A CDS simply varies the voltage going through it depending on the light level. The CDS was most likely tied to a comparator circuit to detect light level changes (shadows) and would not be directly interchangable with a PIR module without some interface circuit to mimic the CDS behavior.

I've used cheapo laser pointers aimed at the CDS so that a person would break the beam and activate the prop. Same thing could be done with a small Led clip-on light or flashlight.


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## corey872 (Jan 10, 2010)

Quick and dirty might be to set up a single LED shining either into or parallel to the CDS sensor. The prop could be triggered either when the beam is broken (dark) or when the CDS sees a reflection (light). You could shield the LED so it would hardly be visible. I think those sensors are most sensitive around the green/yellow spectrum.


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## HomeyDaClown (Oct 3, 2009)

On second thought, since you already have the PIR you could do this:










Attach some long wires to the led and mount it so it shines on the CDS. In essence, have the PIR activate the CDS.​


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