Hi

I'm hoping some of you could help me with a design "problem" I'm having.

I'm making a laser harp/theremin for a school project, I'm using a Arduino but are doing most of the signal processing analog.
My laser beam detector design is a voltage divider with a photoresistor -> an active 1st order active low pass filter -> op-amp comparator

As far as I know this is an ok order?

Her in Norway I'm not allowed to own powerful enough lasers (without a permit) to use the reflected laser to measure distance. Since I want to control other aspects of the sound like volume or a filter I'm using a IR diode system to measure the distance of the hand from the sensor. I know I could use this to trigger the note also, but no laser harp without lasers .

In this IR system I'm using a voltage follower -> a differential amplifier with two potmeters to get as close to 0-5V as I can for the Arduino.
I'm adding a low pass filter to this part as well and was wondering on the order or if I need the voltage follower at all (since my first order active low pass filter has a voltage follower).

If I still need it, would it be best to buffer the signal before the filter or after?

 

The first amplifier should be a gain amplifier (if needed).
Then the filter and the comparator.

If you have a comparator, what does the arduino do?

How are you measuring the distance?

 

I'm building it to be midi compatible so the Arduino iterates over the 13 input pins and reads the state of the input. Then sends the MIDI note. We had the choice of using an Arduino or a computer with LAbView so I went for the Arduino.

I probably should do some gain/offset to the photoresistor since I get approx 3V(ambient)-4.5V(with laser) out from the voltage divider. but I have a potmeter on the reference voltage for the amplifier so it's useable.

I haven't gotten the parts yet so I dont know the voltage i'd get from the IR system. Without the filter I had planned to measure the voltage generated from the voltage divider, through the voltage follower and use the differential amplifier to sort out the gain and offset. So what you are saying is that the low pass filter should go after the differential amplifier?

 

So what you are saying is that the low pass filter should go after the differential amplifier?

Yes, the filter should go after the amp.
But that's not a differential amp configuration you show (which would have two differential inputs) it's a differential op amp connected as a single input follower (non-inverting amp with a gain of 1).
Below is an op amp connected as a simple differential amp:
It's output voltage equals the difference between the two input voltages.

 

The first one is a input follower the second one is a different take on the differential amplifier but with adjustable gain and offset. It will allow me to set an optimal range no matter what the signal from the IR receiver is. You can look at it like this: the R12 potmeter is like the voltage divider after V2 in your image and R14 is like the resistor bridging - and out on the op-amp. It's not the easiest to set but really helpful in achieving a correct voltage range out.

The reason I was asking on the order of things was that having the active filter last gets me up to three op-amps for this part and I was hoping to manage with two, swapping the input follower with the active filter.

Schematic with the added filter (ignore some of the values):

edit: the variable resistor at the start is just to model the IR-receiver

 

Okay, sorry I didn't look carefully enough at your schematic.

The LM324 is a quad op amp so why are you concerned about the number of op amps?

But the LM324 will only give about 3.5-4V maximum out with a 5V supply.
To get (a near) 5V output you need a rail-rail output op amp.

 

Only concerned of running out of breadboards in the electronics lab

I'm having 13*2 op-amps for the lasers and 13*3 for the IR system. But if I use one chip per IR diode, is there any reason to maybe have a filter before and after the amplifier? Since it's only a resistor and a capacitor extra I mean.

 

is there any reason to maybe have a filter before and after the amplifier? Since it's only a resistor and a capacitor extra I mean.

Rather depends upon the noise you expect in the signal.
Will it be exposed to ambient light?

 

Yes, the diode will sit facing upwards with only the sides blocked from light. It does have a filter cap, but probably not that good as I ordered it from one of those Chinese online stores.