Reading a photo sensor with the Raspberry Pi B+

RE: Reading a photo sensor with the Raspberry Pi B+

tally44xz@hotmail.com — Fri, 01 Jan 2016 23:22:01 GMT

Hey there, I've just copied this nifty project with some unusual results. The numbers returned seem to be coming is sets of three different readings, eg: 2997, 181, 429, 3012, 174, 432 also, holding a flash light over the photocell does not change the values at all. Another strange thing, I have to be physically touching the ribbon cable coming from my Pi for this to work at all. I have no male female connectors so I put the ribbon cable in and use all male. I suspect the capacitor or photo resistor may be bad, I only have one of each. Any ideas would be greatly appreciated!

RE: Reading a photo sensor with the Raspberry Pi B+

D_Hersey — Wed, 24 Dec 2014 20:44:12 GMT

A current source would be a choice, in place of the resistor, if you wanted to tart it up a bit.

RE: Reading a photo sensor with the Raspberry Pi B+

D_Hersey — Wed, 24 Dec 2014 20:38:28 GMT

You can stick it in an oscillator and count it.

What's All This V-F Converter Stuff, Anyhow (Part 1) | Analog content from Electronic Design

RE: Reading a photo sensor with the Raspberry Pi B+

Robert Peter Oakes — Wed, 24 Dec 2014 16:34:06 GMT

another idea based on this great work and a great one for the STEM academy

replacing the photo resistor with an accurate known value resistor will allow you to measure within a reasonable accuracy, the value of a capacitor, the range that this would work may be limited but it should work

Just a thought [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b2f6cc19-2226-4d8d-b9cf-e5f42436589e/8372.contentimage_5F00_1.png:16:16]

RE: Reading a photo sensor with the Raspberry Pi B+

Robert Peter Oakes — Wed, 24 Dec 2014 15:55:17 GMT

Hi Mike

Nice example of basic electronics being applied to a modern system, it is always nice to see simplicity in action and if it is accurate enough for the application then whos to complain, not me for sure. ADCs are nice but add complexity and the additional resolution is often not required.

I believe Agilent (Keysight [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b2f6cc19-2226-4d8d-b9cf-e5f42436589e/1373.contentimage_5F00_1.png:16:16] ) have been using this approach for years in their high end power supplies etc as part of the control circuit as it is so easy to calibrate for (Its all basic counting)

the one thing they do from what I understand is count on both slopes and this may improve the speed a little as there is no need to discharge, only go low enough to trip the logic level the other way, you could then average the reading of up and down or some other fancy math ?

How fast is it allowing you to sample ?

I will be book marking this page for future use [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b2f6cc19-2226-4d8d-b9cf-e5f42436589e/1373.contentimage_5F00_1.png:16:16]

RE: Reading a photo sensor with the Raspberry Pi B+

dragonstyne — Wed, 24 Dec 2014 04:14:29 GMT

Excellent work Sir.

-Steve

RE: Reading a photo sensor with the Raspberry Pi B+

clem57 — Wed, 24 Dec 2014 04:01:27 GMT

So even though we think in 1's and 0's there are points of change giving rise to a square world of digital overlayed in a imprecise/fuzzy world of analog. Cool! [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b2f6cc19-2226-4d8d-b9cf-e5f42436589e/contentimage_5F00_930.png:16:16]

Clem

RE: Reading a photo sensor with the Raspberry Pi B+

Problemchild — Wed, 24 Dec 2014 03:15:08 GMT

This is So 80's ... and that's a complement so many designs had so called analog converters which were merely charge time counters in software.

I'm wondering why you chose a quite large capacitor of 1uf most use say .01-0.1uf. This is quicker of course and I think would probably stress the RPI GPIO pin less when you try to discharge the Cap for the next sample.. What is the discharge time BTW when you set the GPIO low to discharge the capacitor?

RE: Reading a photo sensor with the Raspberry Pi B+

mcb1 — Wed, 24 Dec 2014 02:16:48 GMT

Mike

Despite knowing about RC and charge times, and having been to Adafruit once or twice (mostly shopping), it never occurred to me to try this.

Well done, you deserve a pay rise. ...

Mark

RE: Reading a photo sensor with the Raspberry Pi B+

gadget.iom — Tue, 23 Dec 2014 20:36:01 GMT

Thanks for this [mention:51d5bb89e8a04c63af6a84cefbc03bd9:e9ed411860ed4f2ba0265705b8793d05]. [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b2f6cc19-2226-4d8d-b9cf-e5f42436589e/contentimage_5F00_1.png:16:16]

looking to take light level readings very soon. This is a great approach!

RE: Reading a photo sensor with the Raspberry Pi B+

shabaz — Tue, 23 Dec 2014 20:16:15 GMT

You're on fire with your RPI STEM tutorials : ) Great stuff!! : )

RE: Reading a photo sensor with the Raspberry Pi B+

DAB — Tue, 23 Dec 2014 19:48:24 GMT

Hi Michael,

I like where you are going with these basic tutorials.

Sometimes people just need a few examples to realize the power their boards have hidden.

DAB

RE: Reading a photo sensor with the Raspberry Pi B+

ntewinkel — Tue, 23 Dec 2014 19:06:32 GMT

Another great tutorial, thanks Michael!

I had no idea that reading analog sensors in a digital way like this was even possible.