The reason for the 2 cells is this. The circuit is connected so the two cells basically short each other out. Here is a basic diagram I will use for the following examples. http://www.geocities.com/travis5765/seismo/images/photocell_sensor.png There are 3 examples. Figure 1 with the light spot centered between cells 1 and 2, figure 2 with the spot on cell 1, and figure 3 with the spot on cell 2. Because the cells are shorted, if they both have the same light intensity, such as the case in figure 1, the signal at A and B is NULL. no signal. In figure 2 and 3, the signal is the difference of cell 1 and 2. Lets say for this example that signal B is connected to the + input of the recorder and that signal A is connected to the - input. with this connection, cell 1 outputs a positive phase signal and cell 2 a negative phase signal. We can also say for this example that each cell outputs 5 volts with the light source. So in figure 2, the light spot is on cell 1 and not at all on cell 2. so cell 1 outputs the full 5 volts. so we figure +5v + -0v = +5v. In figure 2, the spot is fully on cell 2 and not at all on cell 1. +0v + -5v = -5v. anywhere in between the two cells can be calculated the same way. Say for instance, the spot is positioned in such a way that cell 1 outputs 3 volts and cell 2 outputs -2 volts. then we have +3v + -2v = +1v. The signal output can be calculated to tell almost exactly where the light spot falls on the 2 cells. Using one cell as you mentioned can be done. you would only have a single phase output though. The system would have to be setup so that the spot fell on the place needed to output a centered signal (5 volts at full spot would need it to be positioned so that 2.5 volts were outputted. So like I said, it can be done but it is a little more work to calibrate. I hope I didn't cause a lot of confusion (my explanations seem to do that sometimes). if you need me to clarify something, let me know. ~Travis >From: "meredith lamb"
>Reply-To: psn-l@.............. >To: psn-l@.............. >Subject: Re: Simple photoelectrics >Date: Sun, 27 May 2001 16:21:04 -0700 >MIME-Version: 1.0 >Received: from [22.214.171.124] by hotmail.com (3.2) with ESMTP id >MHotMailBCDACB0A00544004311AD864E08204F70; Sun May 27 15:30:37 2001 >Received: from dnvrpop5.dnvr.uswest.net (unverified [126.96.36.199]) by >sys5.webtronics.com (Rockliffe SMTPRA 4.5.4) with SMTP id > for ; Sun, 27 May >2001 15:21:36 -0700 >Received: (qmail 82182 invoked by uid 0); 27 May 2001 22:20:06 -0000 >Received: from dnvrapanas87poolb199.dnvr.uswest.net (HELO qwest.net) >(188.8.131.52) by dnvrpop5.dnvr.uswest.net with SMTP; 27 May 2001 >22:20:06 -0000 >From larry_cochrane@.............. Sun May 27 15:32:16 2001 >Message-ID: <3B118BE0.663F30E8@.........> >X-Mailer: Mozilla 4.51 [en] (Win98; U) >X-Accept-Language: en >References: <66.f635a71.284245ec@.......> <3B112589.9FF9D37D@.........> ><001201c0e6f5$5e4aa4a0$7efdb3d1@george> >Sender: psn-l-request@.............. > >Is there really....any technical advantage to using two side by >side photocells (misc variety in a differential bridge), as opposed >to using just one alone? I'd think one cell would be simplier >mechanically but I'am not sure if it would react the same >circuit wise or as well (?) as two cells. > >Its kind of relivant, as I'am on a large bright LED and single >CDS (photoresistor) sensor project at the moment...which could >be changed over time. > >Meredith Lamb > >George Harris wrote: > > > > > The very simple and quiet photosensor system can be made from a bright, > > large LED and > > two small silicon solor cells. The cells are placed side by side and > > connected in parallel > > opposing across the + and - inputs of a quiet opamp. (I use OP 77's). > >__________________________________________________________ > >Public Seismic Network Mailing List (PSN-L) > >To leave this list email PSN-L-REQUEST@.............. with >the body of the message (first line only): unsubscribe >See http://www.seismicnet.com/maillist.html for more information. _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>