Proximity sensor

[linnix]

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.

 

[Jan Panteltje]

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.

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

 

[linnix]

Have you considred ultrasonics?

Ultrasonics would be too expensive to implement.

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

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.

 

[Jan Panteltje]

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.

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.

 

[linnix]

Well, eh, PIR sensors,

Yes, same principle, only smaller.

they come for 10 Euro, set the sensitivity really low,

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

 

[Joerg]

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.

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 ;-)

 

[linnix]

Since you are known as a cost-cutter I guess a coax is out of the
question.

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

The other option would be a guard ring but the 555 isn't a
good candidate for this.

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

Did you try upping the frequency?

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

But mind the EMC, of course. A plate is also an antenna ;-)

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

 

[[email protected]]

Yes, same principle, only smaller.


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

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

 

[linnix]

Just the sensor die, fortunately.

JF

 

[Klaus Kragelund]

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 ;-)

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

 

[Joerg]

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)

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?