Infrared LED / detector design
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From: Matt on 25 Feb 2010 13:11 > If money is an issue Phil H. says you can use solar cells as > photodiodes. I've never tried this and I don't know about the speed > issue. You could just run things at a lower frequency.> George H. What!? No kidding? Interesting, I've got several small 0.3V cells.... I'll see if it works, and at what frequency cool :)
From: Phil Hobbs on 25 Feb 2010 15:08 On 2/25/2010 11:25 AM, George Herold wrote: > On Feb 25, 9:34 am, Matt<hamplif...(a)gmail.com> wrote: >>> What's the time base on that 'scope pic? Or, at what frequency are >>> you chopping the LED? As Tim says a photodiode is faster than a photo- >>> transistor. You've also got a lot more choice in area size with >>> photodiodes. The other thing to be aware of is that IR LED's tend >>> to be slower than their visible cousins. So that delay and turn-on >>> time you are seeing might be from the LED. You can test this by >>> blinking a red LED at your circuit and seeing if the respose time >>> changes. >> >>> George H. >> >> The time base is 10ms/DIV, 2V/DIV (both channels). The top waveform is >> the emitter driven by a 555 at roughly 30kHz. The bottom waveform is >> the detector circuit output. >> >> I'll have to see if I have any photodiodes.... Or if I can get any >> locally. I've got an order put together for DigiKey.... kinda waiting >> on money though. >> >> ..........sorry for the delay y'all. I didn't realize this >> conversation had gone to page 2! > > If money is an issue Phil H. says you can use solar cells as > photodiodes. I've never tried this and I don't know about the speed > issue. You could just run things at a lower frequency. > > Photodiodes use to be cheaper, the price seems to have doubled in the > last few years. > George H. BPV22NF is good medicine for 900 nm-ish, and almost free. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Matt on 1 Mar 2010 10:15 Just an update on building this system from components if anyone is interested: I have the emitter driven by a 555 timer at 30kHz The phototransistor is outputing a respectable square wave, inverse of the emitter.... The Ir is reflecting off a surface about 2" away (for now) and it was apparent I needed an op amp. No problem though. The High and Low pass RC filters are a little broad in their descrimination (SP?) but I'm still going to play with them some more.. Right now it looks like the RC filter will work if I separate my two sensors by a modulation frequency of at least 20kHz.... I was hoping for a little more like a 1kHz or 5kHz at the most difference between left and right.... but we'll see. It would be nice to use the 555 I already have setup to modulate on the detector, but I haven't figured out how to do that yet. I need to look back through all the advice given here again to pick info...... and I found all the components advised on DigiKey.... All of the tuned receivers we out of stock when I checked though. I need to check it again and look at Mouser for the same components.
From: George Herold on 1 Mar 2010 13:58 On Feb 25, 1:11 pm, Matt <hamplif...(a)gmail.com> wrote: > > If money is an issue Phil H. says you can use solar cells as > > photodiodes. I've never tried this and I don't know about the speed > > issue. You could just run things at a lower frequency.> George H. > > What!? No kidding? Interesting, I've got several small 0.3V cells.... > I'll see if it works, and at what frequency > > cool :) According the Phil H's book the solar cells are slow.. He quotes a 3dB corner at 20kHz. But this is using his transistor cascode trick on the front end. The nice thing is you've got a lot of area! Mcuh more than your phototransistor. George H.
From: George Herold on 1 Mar 2010 14:03
On Feb 25, 3:08 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > On 2/25/2010 11:25 AM, George Herold wrote: > > > > > > > On Feb 25, 9:34 am, Matt<hamplif...(a)gmail.com> wrote: > >>> What's the time base on that 'scope pic? Or, at what frequency are > >>> you chopping the LED? As Tim says a photodiode is faster than a photo- > >>> transistor. You've also got a lot more choice in area size with > >>> photodiodes. The other thing to be aware of is that IR LED's tend > >>> to be slower than their visible cousins. So that delay and turn-on > >>> time you are seeing might be from the LED. You can test this by > >>> blinking a red LED at your circuit and seeing if the respose time > >>> changes. > > >>> George H. > > >> The time base is 10ms/DIV, 2V/DIV (both channels). The top waveform is > >> the emitter driven by a 555 at roughly 30kHz. The bottom waveform is > >> the detector circuit output. > > >> I'll have to see if I have any photodiodes.... Or if I can get any > >> locally. I've got an order put together for DigiKey.... kinda waiting > >> on money though. > > >> ..........sorry for the delay y'all. I didn't realize this > >> conversation had gone to page 2! > > > If money is an issue Phil H. says you can use solar cells as > > photodiodes. I've never tried this and I don't know about the speed > > issue. You could just run things at a lower frequency. > > > Photodiodes use to be cheaper, the price seems to have doubled in the > > last few years. > > George H. > > BPV22NF is good medicine for 900 nm-ish, and almost free. > > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text - > > - Show quoted text - Ahh, speaking of the master. Thanks Phil I'll order a few. I was getting PIN diodes from photonic detectors, But they sold out to someone/ slashed the product line an raised the price. So now I'm using diodes from OSI optoelectronics.. (which use to have a differernt name.) I should have tried these years ago because they seem much better than the previous diodes. (better in the there is less C with the same active area.) George H. |