Need help modifying a motion detector circuit
This modification consists of two parts: an easy part and a slightly harder part :)  You'll need a:


74HC32 OR gate IC in a DIP package
Another PIR sensor, identical to that in the kit
A scrap of perfboard/veroboard/...
More of the phone wire called for by the kit
And a 200k&Omega;, 1/4W resistor (Radded in the schematic below)


The first step is to enable the circuit to work with an ORed set of PIR sensors, while disabling the light sensor function.  For this, you'll need parts listed above.

Make up two PIR sensors according to the kit instructions, but do not include the CdS photoresistor or R1, and do not connect them to the kit.  You can then make up the OR gate perfboard according to the following schematic, with R1 being the 10k&Omega; resistor the kit has you install in the sensor enclosure:







simulate this circuit &ndash; Schematic created using CircuitLab

Once this is made up, the +5V, OPTO, MOTION, and GND leads from the perfboard can be wired to the mainboard in place of the original sensor, while the two modified sensors can be connected to +5V1/MOTION1/GND1 and +5V2/MOTION2/GND2 respectively.  This hardwires the system in "dark" mode, and means that either or both motion sensors active will activate the relay.

Now, to the easy part of the modification, and that's changing it to turn OFF the device it's controlling when it's triggered.  That's simply a matter of taking the wire from the relay to the HOT side of the output and wiring it to the normally closed contact of the relay (the currently-unused pin on the 2-pin end of the relay) instead of the normally open contact of the relay (the center pin on the 3 pin end of the relay).