Photodiode Question
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From: Jim Thompson on 21 Mar 2010 23:10 What current would one expect to see from a photodiode exposed to typical bright room light (used to control a backlight proportional to room light)? As usual, my customer has no clue. My only concern, design wise, is to build a micropower transconductance amplifier with sufficient output drive to handle the maximum available photo current. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Tim Wescott on 21 Mar 2010 23:34 Jim Thompson wrote: > What current would one expect to see from a photodiode exposed to > typical bright room light (used to control a backlight proportional to > room light)? > > As usual, my customer has no clue. My only concern, design wise, is > to build a micropower transconductance amplifier with sufficient > output drive to handle the maximum available photo current. Lots -- I did something like this a few years ago and saw several milliamps. But as you really should know, the real answer is "it depends". It depends on the diode area, whether it's shaded, what the customer thinks "bright" is, ect. Order a few from Digikey, cobble up something with a 9V battery and an ammeter, and go measure it! Then use the sensitivity on that data sheet to extrapolate to what you'd expect from other similar diodes. One thing that you will learn is that your eyeballs have an amazing capability to accommodate changes in lighting, and that "bright" for a room doesn't hold a candle to "bright" for a sunny day. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Bob Pownall on 22 Mar 2010 00:12 Jim Thompson wrote: > What current would one expect to see from a photodiode exposed to > typical bright room light (used to control a backlight proportional to > room light)? > > As usual, my customer has no clue. My only concern, design wise, is > to build a micropower transconductance amplifier with sufficient > output drive to handle the maximum available photo current. > > ...Jim Thompson As Tim said, "It depends". Tim mentioned the "obvious suspects" - how big is the photodiode? (1 mm x 1 mm or 1 cm x 1 cm - it will make a difference...), how bright is "bright"? (IIRC, the eye's photoresponse is logarithmic, but a photodiode is linear over several orders of magnitude in illumination), etc. One thing Tim didn't mention is the frequency response of the photodiode. Do you and your client only care about DC illumination, or does the photodiode have to be able to deal with the illumination switching on and off at several tens or a few hundred kHz. In general, if only DC is of concern, the photodiode can be designed to have better responsivity - the current out per unit optical power in. Also, with the photodiode be reverse-biased or zero-biased? (Reverse bias will improve the responsivity) but not applying an external bias voltage across the PD will make things simpler. Depending on what the client's needs are, you might want to consider using a photoconductor instead of a photodiode. Bob Pownall
From: Vladimir Vassilevsky on 22 Mar 2010 00:23 Jim Thompson wrote: > What current would one expect to see from a photodiode exposed to > typical bright room light (used to control a backlight proportional to > room light)? > > As usual, my customer has no clue. My only concern, design wise, is > to build a micropower transconductance amplifier with sufficient > output drive to handle the maximum available photo current. Photodiode could be not a very good idea, as they depend on everything. A good part for luminance detection is TPS851 from Toshiba. See the datasheet for details. If you want to isolate the light sensor from being illuminated by your own backlight, you can time multiplex the backlight and the sensor. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: pimpom on 22 Mar 2010 02:21
Jim Thompson wrote: > What current would one expect to see from a photodiode exposed > to > typical bright room light (used to control a backlight > proportional to > room light)? > > As usual, my customer has no clue. My only concern, design > wise, is > to build a micropower transconductance amplifier with > sufficient > output drive to handle the maximum available photo current. > > ...Jim Thompson For simple detection of ambient light level with no requirement of high linearity, I find LDRs more sensitive and easier to work with. They correspond fairly well with human spectral response too. |