Re: How to design an infra-red circuit to detect small obstacle (e.g. finger) ?
From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 02/13/05
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Date: Sun, 13 Feb 2005 15:11:42 -0800
"anwar" <anwar09@yahoo.com> wrote in message
news:1108333459.965812.199980@o13g2000cwo.googlegroups.com...
> Larry,
> - I do need a narrom detection indeed. So if I have a 2 feet
> separation and there is a narrow beam where if someone place a finger
> or pass his finger thru the narrow beam quickly it should detect that.
>
> - As per level of gain- I need to make a sound of a certain frequency
> when the obstacle is detected.
>
> So, if you can please provide me details on how to make a circuit like
> that it will be wonderful.
I'm not going to do the detail design or any simulation
on this. I have built similar circuits and used the NE567
in this manner, so I can assure you that this technique
will work with appropriate design and adjustments for
your specific application.
The NE567 provides these functional blocks:
1. An RC oscillator with a useful semi-digital output on
pin 5. This can drive a low-power LED directly or, with
an extra transistor, as much as your LED can take.
2. A synchronous demodulator, multiplying the signal on
pin 3 to produce filtered baseband on pin 1. You can
set the demodulator bandwidth according to how fast
you want detection to happen versus how much out-of-
band noise you wish to reject.
3. A reference and comparator to detect when the level
on pin 3 exceeds a threshold, producing an open-collector
output on pin 8.
You most likely need to provide some gain between the
photocurrent signal and pin 3 of the NE567. The net
response of that stage, from photo-current to pin 3,
should have 90 degrees of phase shift to work well
with the NE567 demodulator. (That might be 270
degrees, so the LED might have to be turned around
or the polarity of its BJT driver inverted.) For modest
gain, a phototransistor can be used in a circuit with the
following topology to provide this function:
VCC
+
|
.-.
| |
| |
'.'
.---------o
| | ||
.-. o------||- PhotoRx
| | | ||
| | |
'-' |
| |/
o-------|
.----o |>
| | |
--- .-. o---.
--- | | | |
| | | .-. |
| '-' | | ---
| | | | ---
'----o '-' |
| o---'
| |
=== ===
GND GND
(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
This can be done with a very compact layout near the
receiving phototransistor to minimize noise pickup. The
LED can be driven by some twisted pair.
As for making a tone when pin 8 is driven low by
detection of the beam, (or opened by interruption
of the beam), I will leave that problem to you.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
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