Maximum Number of discrete photodiodes?

Hi all,

I'm looking to build a large metal ring with a number of photodiodes mounted and spaced evenly along the circumference facing into the centre of the ring. Each one will have it's own laser pointed straight at it, and I'll measure the intensity/brightness of each diode individually.

How many diodes is a feasible maximum if, say, I wanted continuous and roughly simultaneous read out from all of them?

I've built simple photo-diode circuits before and plotted them in real-time through python so I'm pretty confident I could build the thing, but I just want to maximise the size of the data set.

Thanks!

Really, as big as you want.
Use multichannel ADCs with SPI interface, can cycle thru a ton of them really quickly
For example, 8-channel 12-bit ADC:
http://www.digikey.com/product-search/en/integrated-circuits-ics/data-acquisition-analog-to-digital-converters-adc/2556291?k=mcp3208
Need SCK, MOSI, MISO, then all other pins can be chip selects.
So with an Uno for example, with serial pins left free, that leaves 15 pins available for chip selects.
15 x 8 channels = 120 inputs.
With a '1284P, 27 x 8 channels = 216 inputs.
With a Mega, 48 x 8 channels = 384 inputs.

Add some shift registers for chip select control, and the number of chip selects/channels becomes limitless.
At some point signals will need buffering to maintain integrity due to capacitance of the signal runs; that is easily overcome.

So how many did you really want, and at what resolution?
With a laser, are you likely to get results other than on/off?

richardhall:
Hi all,

I’m looking to build a large metal ring with a number of photodiodes mounted and spaced evenly along the circumference facing into the centre of the ring. Each one will have it’s own laser pointed straight at it, and I’ll measure the intensity/brightness of each diode individually.

How many diodes is a feasible maximum if, say, I wanted continuous and roughly simultaneous read out from all of them?

I’ve built simple photo-diode circuits before and plotted them in real-time through python so I’m pretty confident I could build the thing, but I just want to maximise the size of the data set.

Thanks!

Are you trying to make a display that shows if the beam is centered in the ring or not?

If so, all you need is 4 photo detectors, spaced 90 degrees apart.

Imagine the 4 diodes labelled A, B, C and D where A is at the 12 oclock position, B is at 3, C is at 6 and D is at 9.

The X axis will be (B - D) and the Y axis will be (A - C).

You can use the X and Y values directly to light a pixel on a graphic display which will show you exactly where the beam is (like this, pseudo-code):

[b]while (1) {
    a = readDiode (A);
    b = readDiode (B);
    c = readDiode (C);
    d = readDiode (D);
    X = (b - d);
    Y = (a - c);
    setCursor (X, Y);
    setDot (ON);
}[/b]

I did a thing similar to this a long time ago. It was a sun tracker and it moved the sensor to make X and Y both zero. Any value of the X or Y axis was an error signal used to drive the appropriate servo.

What is the intended use of this device?