Post by thread:[OT] Need help with phototransistor

Hi guys, I need help on using a phototransistor (PT, for short!) to excite an input pin on a PIC16F84. I am using the typical circuit, that is, the PT with emmiter grounded and collector connected to the i/o pin. I put a pull-up resistor of 10k from the i/o pin to Vdd. It didn't worked! I am using a TIL78 activated by a LED (yellow). Can anyone suggest other cirucuit? Rafael Pinto Vdd | > > 10k > | +-------------> RA0 | | / |/ TIL78 |\ | \| -| | | ----- --- -

part 0 876 bytes -----Original Message----- From: Hardy e/ou Rafael Pinto [SMTP:spam_OUTcppintoTakeThisOuTNETRIO.COM.BR] Sent: Wednesday, April 08, 1998 4:18 PM To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU Subject: [OT] Need help with phototransistor Hi guys, I need help on using a phototransistor (PT, for short!) to excite an input pin on a PIC16F84. I am using the typical circuit, that is, the PT with emmiter grounded and collector connected to the i/o pin. I put a pull-up resistor of 10k from the i/o pin to Vdd. It didn't worked! I am using a TIL78 activated by a LED (yellow). Can anyone suggest other cirucuit? Rafael Pinto Vdd | > > 10k > | +-------------> RA0 | | / |/ TIL78 |\ | \| -| | | ----- --- -

On Thu, 9 Apr 1998 10:01:40 -0400 John Shreffler <johnsKILLspamAVENUETECH.COM> writes: >------ =_NextPart_000_01BD639E.817F5430 >Content-Type: text/plain; charset="us-ascii" >Content-Transfer-Encoding: 7bit > >PTs are sensitive to IR, in the 800 to 950 nm range, and >would ignore the yellow. Try an IR emitter instead. The PT's in clear plastic packages are sensitive to all wavelengths from 950 nm IR through blue, and even some UV. They are *most* sensitive to IR (being about 50% down at blue). Many PT's are packaged in "black" or very dark green plastic that absorbs visible light but still transmits IR. These of course have to be used with IR emitters. IR emitters are also much brighter than regular LED's. So unless there's some particular reason to use yellow, change to red or IR. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]

At 12:23 PM 4/9/98 -0400, you wrote: >The PT's in clear plastic packages are sensitive to all wavelengths from >950 nm IR through blue, and even some UV. They are *most* sensitive to >IR (being about 50% down at blue). Many PT's are packaged in "black" or >very dark green plastic that absorbs visible light but still transmits >IR. These of course have to be used with IR emitters. > >IR emitters are also much brighter than regular LED's. So unless there's >some particular reason to use yellow, change to red or IR. > Mike, I have been looking for blue and green sensitive PTs for quite a while. I was not successful in finding them, even in the clear packages. Most of the ones I have seen have not responded very well to green or blue light. It was a while back and I'm not sure I remember correctly, but I think I took a blue LED and pointed it directly into a clead PT with no distance between them and the PT acted as if the led weren't there. A green LED didn't do much better. Red and IR affected it greatly. Sean +--------------------------------+ | Sean Breheny | | Amateur Radio Callsign: KA3YXM | | Electrical Engineering Student | +--------------------------------+ Fight injustice, please look at http://homepages.enterprise.net/toolan/joanandrews/ Personal page: http://www.people.cornell.edu/pages/shb7 .....shb7KILLspam.....cornell.edu Phone(USA): (607) 253-0315

Subject: [OT] Need help with phototransistor Rafael Pinto wrote: <SNIP> > I am using a TIL78 activated by a LED (yellow). Can anyone suggest > other cirucuit? This is just my guess, but most phototransistors are more sensitive to red and IR light. Perhaps your yellow LED isn't being seen by the PT. Either switch to red, or if you can't for some reason, then a photodiode might be a better choice as they usually are sensitive to shorter wavelengths. I have seen an advertisement for photodiodes down to 214nM. Remember though, that in general a photodiode will produce less photocurrent than a phototransistor (roughly the same detectivity, though). If the light from the LED is weak, then you might have to hook up to additional circuitry to detect the LED. Just my thoughts. Your mileage may vary. Michael REMOVE THE .NS (NO SPAM) FROM MY ADDRESS TO REPLY ***************************************** The strong do what they have to do and the weak accept what they have to accept. --Thucydides

Just a question... Aren't LED's supposed to be able to work as photodetectors? If so, has anyone tried using a blue LED as a detector of the light from another blue LED? Of course, this might be a complete waste of time, but I just wondered. CIAO - Martin. On Thu, 9 Apr 1998 17:13:17 -0400, Sean Breheny <EraseMEshb7spam_OUTTakeThisOuTCORNELL.EDU> wrote: {Quote hidden}>At 12:23 PM 4/9/98 -0400, you wrote: >>The PT's in clear plastic packages are sensitive to all wavelengths from >>950 nm IR through blue, and even some UV. They are *most* sensitive to >>IR (being about 50% down at blue). Many PT's are packaged in "black" or >>very dark green plastic that absorbs visible light but still transmits >>IR. These of course have to be used with IR emitters. >> >>IR emitters are also much brighter than regular LED's. So unless there's >>some particular reason to use yellow, change to red or IR. >> > >Mike, > >I have been looking for blue and green sensitive PTs for quite a while. I >was not successful in finding them, even in the clear packages. Most of the >ones I have seen have not responded very well to green or blue light. It >was a while back and I'm not sure I remember correctly, but I think I took >a blue LED and pointed it directly into a clead PT with no distance between >them and the PT acted as if the led weren't there. A green LED didn't do >much better. Red and IR affected it greatly. > >Sean Martin R. Green elimarspam_OUTNOSPAMbigfoot.com To reply, remove the NOSPAM from the return address Stamp out SPAM everywhere!!!

I used a blue enhanced photodiode. I then added a gain stage. This worked well with a blue LED as well as a "Pure Green" LED. At 05:13 PM 4/9/98 -0400, you wrote: {Quote hidden}>At 12:23 PM 4/9/98 -0400, you wrote: >>The PT's in clear plastic packages are sensitive to all wavelengths from >>950 nm IR through blue, and even some UV. They are *most* sensitive to >>IR (being about 50% down at blue). Many PT's are packaged in "black" or >>very dark green plastic that absorbs visible light but still transmits >>IR. These of course have to be used with IR emitters. >> >>IR emitters are also much brighter than regular LED's. So unless there's >>some particular reason to use yellow, change to red or IR. >> > >Mike, > >I have been looking for blue and green sensitive PTs for quite a while. I >was not successful in finding them, even in the clear packages. Most of the >ones I have seen have not responded very well to green or blue light. It >was a while back and I'm not sure I remember correctly, but I think I took >a blue LED and pointed it directly into a clead PT with no distance between >them and the PT acted as if the led weren't there. A green LED didn't do >much better. Red and IR affected it greatly. > >Sean > >+--------------------------------+ >| Sean Breheny | >| Amateur Radio Callsign: KA3YXM | >| Electrical Engineering Student | >+--------------------------------+ >Fight injustice, please look at >http://homepages.enterprise.net/toolan/joanandrews/ > >Personal page: http://www.people.cornell.edu/pages/shb7 >@spam@shb7KILLspamcornell.edu >Phone(USA): (607) 253-0315 > > Larry G. Nelson Sr. KILLspamL.NelsonKILLspamieee.org http://www.ultranet.com/~nr

At 05:17 PM 4/8/98 -0300, you wrote: {Quote hidden}> Hi guys, > > I need help on using a phototransistor (PT, for short!) to excite an >input pin on a PIC16F84. I am using the typical circuit, that is, the PT >with emmiter grounded and collector connected to the i/o pin. I put a >pull-up resistor of 10k from the i/o pin to Vdd. It didn't worked! I am >using a TIL78 activated by a LED (yellow). Can anyone suggest other >cirucuit? > > Rafael Pinto > > > > Vdd > | > > > > 10k > > > | > +-------------> RA0 > | > | / > |/ TIL78 > |\ > | \| > -| > | > | > ----- > --- > - I've been doing this for about 3 years without problems. I use a 33k to 100k pull DOWN resistor, instead of a pull up resistor. I think the threshold voltage, with 5Vcc is about 1.8 volts.