This is for my son's 6th grade science fair project:

He wants to have a remotely activated On/Off switch. That is to say, when motion is detected, a lightbulb needs to turn ON -- and STAY ON indefinitely. We don't want the motion detector to send an OFF signal after a predetermined amount of time. Then when the Same motion detector detects motion again, the lightbulb needs to turn OFF.

We need a motion detector (or some sort of sensor that is activated by an interupted beam rather than by physical contact), that can alternately trigger this On/Off sequence, just like closing and opening a circuit by physically turning a lightswitch on and off -- a remote on/off switch.

Doesn't seem like it should be too difficult. Everyone turns their T.V. on and off remotely, we just need the circuit to alternately open and close each time motion is detected rather than by the physical push of a button or flick of a switch.

Anyone know how we can set this up?

Thanks!
Kent

 

have a look at these links .
http://www.discovercircuits.com/S/s-motion.htm
http://www.discovercircuits.com/A/acoustic.htm
both has some motion detection links

also there is an alternate solution using a webcam to detect motion, which needs more programming than electronics.

 

It seems to me that these two requirements contradict each other. i.e.
"We don't want the motion detector to send an OFF signal after a predetermined time" contradicts "Then, when the same motion detector detects motion again, the lightbulb needs to turn OFF"

 

the answer is there "We don't want the motion detector to send an OFF signal after a predetermined amount of time "

ie off signal only after motion is again detected

 

So the OP wants the motion detector to send an OFF signal when motion is detected again, after a predetermined (amount of) time.
You see the difference? Unless you state the problem clearly you will have trouble solving it, "detects motion" is way too vague.
If I walk past a PIR sensor it will "detect motion". One second later it will "detect motion" again, because I don't walk very fast.
I understand the problem, I'm just attempting to help solve it by pointing out the loose ends. What the OP probably wants is more like:
a) If motion detected, turn on lamp and go to state b
b) If no motion detected during a 30(?) second period, go to state c
c) If motion detected, turn off lamp and got to state d
d) If not motion detected during a 30(?) second period, got back to a

Since a) and c) are almost the same, except for the on/off factor, the whole thing could be written as two states, provided that the circuit or program has the capability to toggle the lamp state.

 

I have a similar timing problem with a circuit I designed on another electronics chat site.
It's a Ni-Cad battery discharger circuit that is supposed to shutoff the discharge when the battery's voltage drops to a certain voltage. I used an "all-in-one" LM10 voltage reference/rail-to-rail opamp and a Mosfet for it.
It detects the battery threshold voltage then the opamp begins to turn-off the Mosfet. It has plenty of hysteresis for fast switching and so it doesn't start discharging again when the battery's voltage rises without a load. It gets stuck with the Mosfet half-turned-on and smoking!
I'm adding a delay beween the LM10 and the Mosfet so the LM10 can finish switching its output before the Mosfet turns-off the load.

Attached Images

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