hand wave motion detector

[hansknec]

Does anyone here know what the latest technology is for hand motion detectors? Need ultra-low current consumption for battery powered unit. Similar to the automatic paper towel dispensers showing up everywhere.

Seems there are three possible routes: Pyroelectric, Active Infrared "radar", and passive light change.

Detector must work in complete darkness, so passive is out.

There must be something very cheap that they use in these halloween displays that sense a hand entering the candy bowl.

Thanks.

John

[Ttelmah]

The commonest one for short range, is missing from your list. Capacitance. Look at Maxim APP1903. This example, is more complex than most, but in simpler forms, this is the basis of most of the detectors on lights etc..

Best Wishes

[hansknec]

Way overkill. I'm sure there is a $0.50 part out there somewhere.

I remember seeing an article about pulsing an IR led at 40kHz and then measuring time to reflection using a common IR detector that provides a ttl when a modulated light is received. This is an all digital approach appropriate for micros, and will likely be my solution.

I was just wondering if anyone knows of something new in an all-in-one chip.

[Ttelmah]

Seriously, you can do capacitance detection for that sort of budget.
The Maxim note, is complex, because it performs temperature compensation, but the simplest for motion, rather than proximity, is just to filter the output straight from a detector plate, and use a simple bandpass filter, to remove the carrier, and the very low frequency component, to get rid of stationary signals. Capacitance detection, is used in half a dozen cheap toys that I know of.

Best Wishes

[Guest]

I understand. Thanks.

Unfortunately, my application can't use it. I need to detect a hand approaching a rear doorknob. If a hand touches the front doorknob it must not trigger the signal.

John

[newguy]

Can you stick a switch in/under a doormat?

[Guest]

No, It must be an all-in-one solution right at the doorknob. My kid was raised by wolves and he would just chew up the mat.


just kidding. this is for a commercial application.

[ckielstra]

I don't understand your application very well. How far apart are the two doorknobs?

Do you have to detect movement before the knob is touched, or is allowed to detect someone (just) touching the doorknob? I'm asking this because I'm thinking about the capacitive sensors you see in some of those touch buttons.

[Fabri]

Hi to all,

Very interesting, I'm working around a movement sensor based on buzzer piezo and it's mecanical properties. Also my application is reserved.

Is this you are looking for ?

Regards,
Fabri

[Guest]

I don't want to deal with capacitances and doorknobs. 20,000 volt static sparks are common on the door knob in dry winter air.

I'm almost there with the Infrared now. I happened to have a PNA4612M00XD sitting around. They cost $1.20 ea from digikey. They are designed to put out a signal when they receive a 38 kHz modulated infrared signal. They are immune to all other light, so room light doesn't affect it.

I have a short timer loop that generates 38 kHz to drive an IR LED.
high 11us, low 12us does the trick with my 18F1220 on 8MHZ internal oscillator.

I have temporarilly put a visible led at the output of the detector so I can see when it turns on. It clearly goes from dim to bright as my hand approaches. (LED and detector facing straight up from the proto board)

Now I need to start thinking about current and saving battery power. I will gate power to the detector on and off from the PIC. I will put the PIC to sleep and then have it wake up 3 times a second for 100usec or so. When it wakes, it will pulse the LED and look for the reflection signal from the PNA4612. If it sees a reflection, (ADC read above a certain threshold or CCP module) it will send out my "hand present" signal. If no reflection then sleep again and conserve power.

Shoud run for about a year on a 3V coin cell at the currents I'm using.

John

[hansknec]

Additional research for those interested in low power consumption.

There have been many posts from people asking how to make a PIC wake up from sleep. Most answers usually involve the use of a watch crystal on timer1.

I will use a PIC12F635 because of its Ultra low-power wake-up module and 1 nA sleep current. The ULPWU module eliminates the need for an external watch crystal for wake up because it creates an interrupt on change of pin GP0 as a capacitor slowly discharges. See microchip.com application note 879 for detailed information about this module.

The chip will drive all peripherals and they will be turned off during sleep.

When running, I will choose the minimum oscillator speed necessary to create the 38 kHz signal for the LED drive. Current consumption when running is tied to oscillator speed. I could probably do something with an external chip, but the run time to sleep time ratio is so low that I can afford a few more microamps from the PIC. The highest power consuming unit by far will be the LED when pulsing.

Choose your LED carefully. Choose a high efficiency LED that can run at a lower current to provide the desired output power. The cone angle of the output beam is also very important for concentrating the LED power to the desired beam profile. This will be application specific, but I have seen LED's with a divergence of 1 degree (OSRAM) while others have a 40 degree cone. Avoid infrared lasers unless you know what you are doing. They are not toys. They can cause retinal damage because you have no blink reflex for the invisible light. They are also easily damaged by ESD or power surges that an LED will shrug off.

While I have your attention, let me caution you about high power LED's as well. Looking at the OSRAM and other datasheets you may find infrared LED's that produce incredibly high peak powers when pulsed at high currents. (9 watts at 1 amp for <100us) These LED's WILL CAUSE OPTICAL DAMAGE to the eye if mishandled, yet they looks like any other common LED. If these LED's had existed when I was a young sprout, I would probably be blind now because the first thing a kid does when experimenting with an infrared LED is to hold it up to his eye and try to see if there is anything visible coming out. Use a simple $10 web camera to view them instead. Most CCD cameras are sensitive to IR light.

John

[John P]

Don't forget the theremin (and Theremin):

http://en.wikipedia.org/wiki/Theremin