Post by thread:[PIC]: How to connect photocell to pic a-d

Hello, I am somewhat new to pics and would appreciate some help with using a photocell on a pic A-D port. The range of light on the photocell will not cover the full resistance range of the photocell (whichever one I use), so I don't know how to interface this with a pic to provide maximum range. Assuming that the resistance varies from R1 (lowest ohms) to R2 (highest ohms), I can set this up in a voltage divider with the fixed resistance being R0 with +5V to R0 to photocell to Gnd, and Vout coming from the point where R0 connects with the photocell. Now, V1 = 5V x (R1/(R1+R0)) and V2 = 5V x (R2/(R2+R0)). For some test values I've calculated, this leaves me with V2-V1 being around 1V-2V. Is there something other than a voltage divider I should use for this? I would prefer to keep the vrefs at +5V and ground since I will probably use other things on other A-D channels. TIA Brian _________________________________________________________________ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

> 5V x (R2/(R2+R0)). For some test values I've calculated, this > leaves me with V2-V1 being around 1V-2V. Is there something > other than a voltage divider I should use for this? I would prefer > to keep the vrefs at +5V and ground since I will probably use > other things on other A-D channels. An A2D pin has a maximum recommended signal source impedance of 10k. You don't mention which photocell you're using but if it is more than 10k at any time then you'd need to buffer it Perhaps this way, using a dual op-amp. One 1/2 to produce a DC offset voltage and the other 1/2 as a summer amp to add and amplify the DC offset and the photocell output. By making the DC offset and gain adjustable you can make the photocell's output span between 0 and 5V at the A2D input -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu>

> I am somewhat new to pics and would appreciate some help with using a > photocell on a pic A-D port. The range of light on the photocell will > not cover the full resistance range of the photocell (whichever one I > use), Photocells convert light to electrical energy, and don't operate on varying resistance. Your best bet for measuring light intensity is to measure the photocell's short circuit current. This can be done with an opamp and feedback resistor. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu>

On Wed, 16 Apr 2003, Dennis J. Murray wrote: > You're right about photocells not operating on resistance unless he's using > Cadmium Sulfide cells, Olin (Heaven forbid)! ;-) What's wrong with CdS cells? I've been using one for a while as the daylight detector for my yard light timer, works fine. I've seen them run those dumb dusk-to-dawn outdoor lights, the ones that look liks gas lights, for many years. Dale -- It's a thankless job, but I've got a lot of Karma to burn off. -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu>

Personally, I've had a higher failure rate in extreme conditions with CdS cells than I've had with semiconducter types. Logic says that shouldn't be, since it doesn't get any simpler than a CdS cell. May have been coincidence, but it tends to color my judgement. You want to use Cds? Have at it - no offense meant. Dennis {Original Message removed}

On Wed, 16 Apr 2003, Dennis J. Murray wrote: > You want to use Cds? Have at it - no offense meant. None taken, I was just wondering. It's the first time I've used one, but they have been around for a long time and I've seen 'em forever. So far it's working fine under normal outdoor conditions, but of course it's out of direct rain & sunlight. Dale -- It's a thankless job, but I've got a lot of Karma to burn off. -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu>

I've used a CDS photocell from radio Shaft as a "dawn and dusk" detector for the anchor light on my sailboat for 12 years now. It is inside a window, not exposed to the elements. Works like a charm. Larry At 11:16 AM 4/16/2003 -0500, you wrote: {Quote hidden}>On Wed, 16 Apr 2003, Dennis J. Murray wrote: > > > You want to use Cds? Have at it - no offense meant. > >None taken, I was just wondering. It's the first time I've used one, but >they have been around for a long time and I've seen 'em forever. So far >it's working fine under normal outdoor conditions, but of course it's out >of direct rain & sunlight. > >Dale >-- >It's a thankless job, but I've got a lot of Karma to burn off. > >-- >http://www.piclist.com hint: To leave the PICList >piclist-unsubscribe-requestspam_OUTmitvma.mit.edu Larry Bradley Orleans (Ottawa), Ontario, CANADA -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-requestKILLspammitvma.mit.edu>

> On Wed, 16 Apr 2003, Dennis J. Murray wrote: > > > You want to use Cds? Have at it - no offense meant. > > None taken, I was just wondering. It's the first time I've used one, but > they have been around for a long time and I've seen 'em forever. So far > it's working fine under normal outdoor conditions, but of course it's out > of direct rain & sunlight. I think the biggest problem with Cds cells is part to part variation. The stability of a single part is quite good, but the variability between parts can be quite bad. Especially if you get parts from different production runs. TTYL -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu>

At 07:48 AM 4/16/03 -0400, Olin Lathrop wrote: >Photocells convert light to electrical energy, and don't operate on >varying resistance. The problem is that the term "photocell" refers to at least 3 different sensors, each with radically different characteristics. These are: Light Dependent Resistors (LDRs), solar cells, vacuum tube photocell. (side note: The last time I saw one of those vacuum tube photocells when I was a teenager - I do remember that it wasn't a photo-multiplier - the tube had 2 elements within: curved plate and a vertical rod up the center. It was so long ago that I can't remember if the curved plate was anode or cathode.) Anyways, what Olin is referring to could be either photo-diodes or solar cells. However, other types of photocell (CdS) *do* change resistance with changes in light level. Also note that a photo-transistor can give the illusion of a resistance changing value in response to changes in illumination. dwayne -- Dwayne Reid <RemoveMEdwaynerTakeThisOuTplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 19 years of Engineering Innovation (1984 - 2003) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestspamBeGonemitvma.mit.edu>