Comments welcome re. amplification/scaling and offset/bias CCT

[Nick Mulder]

Hi,

a few months back I made this cct so I could control the output of an LDR (or phototransistor) to give me an output that would cover a 0~X Volt range (X could be anywhere from 12V down to zero).

for instance if the LDR output was 2~4V volts I would minus 2 Volts with the bias single gang pot and then amplify by x6 to get 0~12V or by x2.5 to get 0~5V using the scaling dual-gang pot (shown twice).

I didn't have much time to get this cct working and not knowing much about op-amps left me with a couple of not so perfect factors, it was still fine for the application - (16 of them were put together on vero board)

I want to make a PCB version next and would like any advice to stop the following issues:

a. The offset will easily let the output fall into the negative (ie. setting the zero point takes more time than it should)

b. The amplification will range from being unresponsive to sudden changes - its like I turn it half way to get the first half of what I need then 2mm around to get the rest

I'm also putting a LPF on the LDR output to get rid of light flicker from spurious sources - we were getting a 48Hz square wave from it once when a film projector as pointed at it....

I hope the schematic is easy to read, its my first and I'm not sure of conventions.
Please feel free to comment on any other odd bits you may see could be done better.

Any help greatly appreciated,
thanks,
Nick

View attachment 40006

 

[AN920]

Dual gang pots are normally made for audio and will likely be log not linear. This could be why you see the rapid increase at the end. I think if you look at the pot markings there is a B for linear or A for audio or log.

more here http://sound.westhost.com/pots.htm#using

 

[Nick Mulder]

these are linear - I checked...

I was thinking maybe log would be the answer ...

thanks for the reply though,
always appreciated...

Nick

 

[AN920]

Your diagram does not indicate how the sliders are connected. As it is drawn it does nothing when you vary the pot. Indicate the direction that slider B1 will move in respect to B2 when adjusted. This is all important. Maybe fix the drawing to avoid confusion.

 

[Nick Mulder]

It is a dual-gang pot - two pots mechanically connected to provide the same (close enough) resistance as the other, they are not connected electrically.  Theese particulars ones are 'B1=B2' ...
I haven't been around the electronics world long enough to encounter a different set-up...

I dont know the convention for how they are represented in schematic form (tried searching, but its one of those things that gets you a thousand irrelevant results) -

they are not available in my CAD package libraries as a drag and drop component either...

hope they aren't too confusing

(the cct works btw, it just needs some adjustments or maybe a rewire or three to get it going really well)

 

[AN920]

This what I mean. Also I would add a buffer to prevent any loading by the B2 on the LDR when B2 goes low resistance which will place R6 in parallel with R10.

View attachment 40008

 

[Nick Mulder]

righto - ok - thanks for the drawing.. really helpful

I'll try the buffer, it'll be easy as the op-amp is a dual type, I also need to put a Low pass filter on the cct as I only need light change transistions no faster than say 200mS ...

I was thinking it would be best placed on the input side rather than output, this would also be behind the buffer  ?

thanks again, - drawing is just what I 'm after

Nick

 

[AN920]

You can try this. Play with the RxC constant to reduce flicker response. Last variation will allow you to use smaller C value

View attachment 40009

 

[AN920]

You could also try this clamp idea. It will require 1 extra opamp. This will prevent your offset value from going higher than your LDR value and will then prevent the opamp going negative. As long as the offset is lower than the LDR value it will have no effect on the circuit. Just a idea. If you want a visual indication when the offset is adjusted below the LDR value add the extra LED and resistor.

View attachment 40010

 

[Nick Mulder]

the main cost in the units will be the PCB - so adding a quad op-amp IC per 1.3333 cct's wont be too much of a hassle ...  Any room for another op-amp in there  ;D ?

I will breadboard this all up later tonight and see how it goes -


thanks again for your help - I've got a little understanding of op-amps but working with them on an application really gets me eager to learn

Nick

 

[AN920]

If you don't know what to do with the spare amp put it at the top as another buffer by breaking at the blue line.
Circuits can quicly get out of hand and you end up with 10 opamps! ;D

View attachment 40011

 

[Nick Mulder]

I suppose when you are adding op-amps to sort problems that op-amps youve already added to solve problems have themselves caused ...

..then its time to stop 

I like the way your drawing has now color - very exciting !

Nick