Proximity sensor

Apr 27, 2008 11 Replies
L

We are working on short range (5cm to 10cm) proximity detection. First of all, I still think MEMS IR sensor is the way to go. But it would take time to build, so perhaps next year.



Meanwhile, we are trying capacitive sensor using a 555 timer. Based on input capacitance, the 555 output a periodic signal monitored by a microcontroller.



The circuit works somewhat, but only when close to the timer chip. The question is how can we isolate the trigger pin and attach it to external detection plate. Apparently, most of the electrostatic fields leaked between the pins (trigger and ground) of the chip, so external plate does not help much.



Thanks for any suggestions.


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J

On a sunny day (Sun, 27 Apr 2008 09:46:21 -0700 (PDT)) it happened linnix wrote in :

Have you considred ultrasonics? Also, if it was possible to have some RF coil on the object, you could just pick up the signal?

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L

Ultrasonics would be too expensive to implement.

The object is an approaching hand or finger, which is a natural IR transmitter anyway. The idea is to have micro-mirrors to focus on the IR signal.

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J

On a sunny day (Sun, 27 Apr 2008 10:34:38 -0700 (PDT)) it happened linnix wrote in :

Well, eh, PIR sensors, they come for 10 Euro, set the sensitivity really low, perhaps it would work? I mean the passive IR for security applications that you can buy in many places.

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L

Yes, same principle, only smaller.

But we need to make it for 10 to 20 cents @ 100,000.

places.

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J

Since you are known as a cost-cutter I guess a coax is out of the question. The other option would be a guard ring but the 555 isn't a good candidate for this. Did you try upping the frequency? But mind the EMC, of course. A plate is also an antenna ;-)

Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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L

What would I connect the outer shield to? Certainly not ground.

My next try would be an op-amp bridge detector.

No matter what frequency, the signal shorted between pins are just too high, as compared to the free air dielectric between external plates.

I did not believe capacitive sensing would work well enough. I built the test circuit to prove myself wrong, but I failed.

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J

The entire thing, or just the sensor? JF
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A

just

places.

A IR LED and a photo thingy? I mean what do all those automatic faucets and urinals use?

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L

Just the sensor die, fortunately.

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K

Regarding the EMC, the proximity application does not need to be active all the time. Using a duty cycle on-time for the entire sensor will reduce EMC (cheat the quasi-peak test receiver filter)

Cheers

Klaus

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J

Yes, but be careful. A week ago I did another EMC session in the valley and the guys hinted that just like the dithering trick it can be risky to rely on quasi-peak regulations to remain unchanged. All it takes is a few guys in Brussels ... well, let's not go there :-)

Were you able to shake the pneumonia with "donating" some of it to family members?

Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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