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Instant On Circuit

:: EPE Chat Zone ≠:: ≠Radio Bygones Message Board :: » EPE Forum Archives 2005-2006 » Archive through 14 May, 2006 » Instant On Circuit « Previous Next »

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Scott2734
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Username: Scott2734

Post Number: 69
Registered: 10-2005

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Posted on Tuesday, 02 May, 2006 - 03:21 am:   Edit Post Delete Post Print Post

In another thread i asked about breaking a beam between a ir emitter and detector, and running the output throu a transitor to a pic pin.I got that part working, but i get a slow increase or decrease in voltage , or it will hang around one voltage on the lower half without cutting all the way off.I was wondering if anyone knows of a way to have the output from the transitor either on or off all the way without any floating voltages? This has been a challenge to me because as you know, if you move something in between the beam slowly, the voltage goes up slowly, and vise versa. i also built a op amp circuit that acts the same way. of course thats the way it is suposed to act.Any help would be thankfull.
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Obiwan
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Username: Obiwan

Post Number: 455
Registered: 12-2005

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Posted on Tuesday, 02 May, 2006 - 03:48 am:   Edit Post Delete Post Print Post

Post a schematic.
Do Not Hit The Fly That Lands On The Tigers Head.
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Scott2734
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Username: Scott2734

Post Number: 70
Registered: 10-2005

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Posted on Tuesday, 02 May, 2006 - 05:59 am:   Edit Post Delete Post Print Post

I chose to go with the op amp circuit because it runs off of 5 volts and it seems more stable. But still have the same problem with the output voltage .whoops, i forgot to value the resistors. will do tomorrow.

beam

(Message edited by scott2734 on 02 May, 2006)
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Mark
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Username: Mark

Post Number: 33
Registered: 05-2005

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Posted on Tuesday, 02 May, 2006 - 10:07 am:   Edit Post Delete Post Print Post

Not sure about the op amp feedback arrangement, but with the right values it might be OK.
You need to follow this circuit with a Schmi(d)tt trigger circuit - use one gate of a CMOS 4093. The output will then switch cleanly.

Mark
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Paul_goodson
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Username: Paul_goodson

Post Number: 383
Registered: 05-2005


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Posted on Wednesday, 03 May, 2006 - 08:26 am:   Edit Post Delete Post Print Post

Hi Scott
I have a circuit that shows a pre-amp operating as a Schimitt Trigger for use in a broken beam circuit.
and includes a VR to adjust the threshold output level.
If you wont it drop me an email, as I donít seem to be able up load it to the forum.

Paul

(Message edited by paul_goodson on 03 May, 2006)
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Mike_b
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Username: Mike_b

Post Number: 171
Registered: 05-2005

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Posted on Wednesday, 03 May, 2006 - 10:28 am:   Edit Post Delete Post Print Post

Why dont you configure the opamp as a schmitt trigger - sorry I dont have the cct to hand but there are loads of examples on the net.

rgds

mb
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Mark
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Username: Mark

Post Number: 38
Registered: 05-2005

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Posted on Wednesday, 03 May, 2006 - 12:20 pm:   Edit Post Delete Post Print Post

Try a 2M2 between pins 1 and 3 to give some positive feedback. That should give schmitt trigger action. Lower the value to give more feedback if necessary.

Mark
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Scott2734
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Username: Scott2734

Post Number: 71
Registered: 10-2005

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Posted on Thursday, 04 May, 2006 - 03:26 am:   Edit Post Delete Post Print Post

Thanks everybody. Mark i will try your sugestion.Wouldnt i need to take the feedback network out of pin 1 and 2 if im going to run the feedback from 1 and 3? I'm starting to learn everything in electronics is not black and white.I either want a logical 0 volts or a logical 5 volts, im learning it dont work like that.Me being picky is geting the best of me as usual lol.
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Steerpike
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Username: Steerpike

Post Number: 108
Registered: 05-2005


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Posted on Tuesday, 09 May, 2006 - 05:19 pm:   Edit Post Delete Post Print Post

An op amp intended to work as a comparator (eg LM311) might work better than a linear amplifier type.
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Grab
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Username: Grab

Post Number: 189
Registered: 05-2005


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Posted on Wednesday, 10 May, 2006 - 09:52 am:   Edit Post Delete Post Print Post

If you do use a comparator, check how the output behaves. Many/most comparators have an open-collector transistor output, with the transistor turned on when the comparator switches, which is radically different from how a linear op-amp will behave. The comparator version is usually *more* useful (for starters, you get a better 0V out of it), but you need to design your circuit with this output in mind.

Graham.
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Mark
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Username: Mark

Post Number: 49
Registered: 05-2005

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Posted on Thursday, 11 May, 2006 - 10:00 am:   Edit Post Delete Post Print Post

You don't need to take out the negative feedback for the schmitt trigger action to work. It's a matter of getting the proportion right between the two (negative and positive) feedback paths. Trial and error is easy in this simple circuit - you can figure out the 'maths' later

Mark
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Thomas
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Username: Thomas

Post Number: 94
Registered: 04-2005

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Posted on Saturday, 13 May, 2006 - 08:59 pm:   Edit Post Delete Post Print Post

Hmmm, not sure I fully understand, but what about a 555 monstable, the trigger pin biased almost to triggering, and wire up your detector via a d.c.-blocking cap to trigger on detection? i.e. D2 and R2 remain as in your diagram, with the d.c. blocking cap from their junction to the 555 trigger pin. Thomas.

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