Help needed for light detector/ timer circuit design

[Jahooly]

Hi all, I have a problem that I'm sure can be solved with a relatively simple circuit, but I'm not sure exactly how and was wondering if anybody can advise me.

The lab where I work has a motion detector built into the lighting, such that even though the light switch is 'on', the lights will automatically turn off after a certain period of no perceived motion. This becomes a problem when me and my colleagues are sat working at our desks after dark, because the motion detector is not facing us and does not detect our presence, and therefore plunges us into complete darkness at frequent intervals!

My proposed solution is a circuit that could drive a small motor attached in front of the motion sensor. The circuit needs to engage the motor only when the lab lights are already on (since there's no point in the motor spinning round after we've switched the lights out for the night). To save battery power the motor should only turn on for short periods (about 10 seconds) but at regular intervals (probably about 10 minutes, but it would nice to be able to vary this).

So for starters I guess I need an LDR or a photodiode circuit that I can adjust to the intensity of the strip lights (and so that natural light coming in through the window won't set it off). Then what should I use to get the pulse sequence I need (~10s ON, 600s OFF) and will an IC output be able to drive a small motor?

My electronics knowledge is very rusty (and was never that great to begin with!) so consider me a noob. Any help or suggestions would be much appreciated.

 

[Reloadron]

You may have a problem doing this as you suggest. We have the same thing where I work. The motion detectors detect IR (heat) motion such as body heat. If during a Cleveland sub zero winter I walk in from outside covered in cold clothing the units don't respond or they take time till some of my body heat escapes like if I open my jacket.

The same is true of those automatic urinals. Stand there wearing an ice cold jacket and they don't flush when you walk away.

I don't think a simple motor moving something is a viable solution if they respond the way most I have seen do. They are PIR (Passive Infra Red) and respond to heat from the body moving. Possibly a scheme using an IR transmitter would keep one triggered or fooled. Never tried it.

Ron

 

[Jahooly]

Oh, I see - thanks for pointing this out Ron. I'll check whether a stationary blinking IR diode is capable of triggering the sensor - otherwise I might try a line of several IR diodes which turn on and off in sequence to create an illusion of apparent motion. Thankfully it's still warm enough here for the IR sensor to detect me when I first enter the room!

I guess my main question is about how to create the timing that I need. The diode circuit will need to be activated for short periods at regular longer intervals. Can I use a 555 astable for this or would I need to use an astable to trigger a monostable? Also, what are your opinions on the best option for detector of when the strip light is on - an LDR/ photodiode/ thermistor?

 

[Reloadron]

A good choice for the timing would be to program a simple little PIC Chip micro-controller but not everyone is into these little creatures so it is only a thought.

Another possible would be to configure a 556 maybe. The 556 is simply two 555s in a single package. One would be configured as an astable multivibrator but the output inhibited using the reset pin. That would be driven by the other timer. While the output of the first timer was high it would allow the second timer to output a chain of pulses. I just am not sure about the 600 seconds off time (10 min). Ten sec on and 600 sec off and during the On the second timer would output a pulse train. Hopefully someone else will weigh in with some ideas. I am not sure tricking the sensor will work as I suggested as I never tried it.

I can tell you this, In my facility we had similar issues in some areas. They do make remote sensors that can be used with those lighting systems because we got them after the same problems you are experiencing. Not good if someone is on a range perimeter operating machinery and the lights go out. You may want to talk to your facilities people.

Ron

 

[Jahooly]

I can't believe that I've never heard of PIC microcontrollers before - I had to Google them but they sound amazing! I'm not too hot at programming in C (or binary!) but I've found a blockset that will let me write binary files for a PIC from Matlab. I think I'll give it a try just for my own interest... if that fails then I'll stick to a 556.

Having researched the motion sensor in question it appears that it is indeed an PIR detector. Specifically it detects the motion of objects that emit high levels of IR relative to the background (i.e. the lab). This is why even though your body/ coat temperature is not absolute zero (the point at which you would radiate no IR) when you enter your building from the cold, the amount of IR you are emitting relative to your indoor surroundings is not sufficient to trigger the sensor. Hopefully a pair of alternating blinking IR LEDs in front of the sensor will fool it.