Author Topic: Smallest Photo Cells out there?  (Read 814 times)

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Ian MacMillan

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Smallest Photo Cells out there?
« on: February 15, 2014, 09:33:00 PM »
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I'm looking at adding some crossing circuits to the new layout and while I had originally planned on using a BDL168 I've decided after seeing the reliability of a friends set up in HO (hes a signal maintainer for PanAm Railways) using the Logic Rail Tech's Grade Crossing Pro2, I was looking at doing it that way. I don't want to use the IR version, and instead am looking at using the photocell version.

I know that the cells with the kit are more for fitting between HO ties, but I'm wondering if anyone out there has used or found any smaller cells that may be more N scale worthy.
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C855B

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Re: Smallest Photo Cells out there?
« Reply #1 on: February 15, 2014, 10:28:05 PM »
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This is the smallest "regular" photocell I could find quickly, at Digikey: http://advancedphotonix.com/wp-content/uploads/PDV-P9203.pdf. The same size comes in a couple of ratings.

Now if you want to tinker with your friend's help, here is an "ambient light sensor" in a 1206 SMD (.12" x .06"), available at Mouser: http://www.avagotech.com/docs/AV02-0053EN. It may or may not be a drop-in replacement for a photocell.
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peteski

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Re: Smallest Photo Cells out there?
« Reply #2 on: February 16, 2014, 10:18:46 AM »
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The photocells Mike found are probably the smallest ones available. Photocells are photo-sensitive resistors. There are smaller photo-sensitive electronic devices, but those are either photo-transistors or photo-diodes.   If the crossing gates circuit is designed to with with photocells, it will not work with photo-transistors or photo-diodes.

I'm not too keen on using any detectors which depend on ambient light for sensing. What if you decide to have a night-time operating session?
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C855B

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Re: Smallest Photo Cells out there?
« Reply #3 on: February 16, 2014, 11:02:58 AM »
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The detector system Ian is using accommodates photocells and IR pairs. I am curious if that SMD sensor would work with the photocell logic since it's looking for a resistive load, and is an analog device. It might require a hack.

Yeah, I have that concern about ambient detection, too. For that matter, one of the features of GC&W I'm designing into the room is night ops with special lighting effects. HOWEVER... detection to me is not an issue of one system versus another. Occupancy is occupancy, and can be determined by multiple methods at the same time. I am figuring on at least two types - current sense and an optical - in each protected block, and some situations may just work better with all three. What I do know from experience is that using just one system isn't sufficient.

But speaking of IR, last time I shopped around IIRC I found both emitters and detectors in 0603. That's small enough to hide in things like trackside clutter, so there can be more physical detection where a lot of guys normally use just one or two pairs of discretes.
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Ian MacMillan

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Re: Smallest Photo Cells out there?
« Reply #4 on: February 16, 2014, 01:00:09 PM »
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What if you decide to have a night-time operating session?

I wont. I can't stand night-time ops.

But speaking of IR, last time I shopped around IIRC I found both emitters and detectors in 0603. That's small enough to hide in things like trackside clutter, so there can be more physical detection where a lot of guys normally use just one or two pairs of discretes.

I was not aware of this. using 0603's could make things much easier.  I could put the detector between the ties and the emitter in the upper fascia. Or, as you pointed out easier to had in track side items.
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DKS

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Re: Smallest Photo Cells out there?
« Reply #5 on: February 16, 2014, 01:56:15 PM »
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I could put the detector between the ties and the emitter in the upper fascia.

Too far. Usually these devices need to be within a couple inches of one another for reliable operation. Best to stick with trackside objects; a relay box would be ideal.

Ian MacMillan

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Re: Smallest Photo Cells out there?
« Reply #6 on: February 16, 2014, 03:42:50 PM »
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Too far. Usually these devices need to be within a couple inches of one another for reliable operation.

Ok good to know. I dumbly figured it was like TV remote distance.
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C855B

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Re: Smallest Photo Cells out there?
« Reply #7 on: February 16, 2014, 05:03:45 PM »
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Yeah, data transmit/receive pairs are modulated and the emitter would be very, very bright. But that's not to say your idea doesn't have merit - it might be worth trying a high-output IR device (usually a discrete leaded device) in the soffit. That might be close enough.

In another quick search, I found 0402 (!!!) and 0603 IR LEDs at Digikey, and 0805 IR phototransistors at Mouser.
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DKS

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Re: Smallest Photo Cells out there?
« Reply #8 on: February 16, 2014, 05:31:36 PM »
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I had a time trying to get detectors to respond to high-output emitters at more than 6 inches--but these were older devices, so performance may have improved enough. But using 0402 pairs, you could embed them in a flangeway and detect passing wheel flanges!

Ian MacMillan

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Re: Smallest Photo Cells out there?
« Reply #9 on: February 16, 2014, 07:39:59 PM »
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But using 0402 pairs, you could embed them in a flangeway and detect passing wheel flanges!

Tie this to a counter and then have it broadcast a defect detector recording with accurate axle count over FRS and it's epic.

Anways. Here is the PDF of the flasher module.
https://www.logicrailtech.com/gcp_inst.pdf

I dont need it just yet so I guess we have some time here to tinker.
« Last Edit: February 16, 2014, 07:55:44 PM by Ian MacMillan »
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railnerd

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Re: Smallest Photo Cells out there?
« Reply #10 on: February 16, 2014, 08:25:24 PM »
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I had a time trying to get detectors to respond to high-output emitters at more than 6 inches--but these were older devices, so performance may have improved enough. But using 0402 pairs, you could embed them in a flangeway and detect passing wheel flanges!

A guy in our HO club built an infrared detector used it for train speed and axel counter calibrated for 33" wheels. He had a single board computer that had a 1980s style "bitching betty" voice synthesizer that pretty well replicated the talking defect detectors.

He was about to productize it, but someone beat him to the punch: http://www.bouldercreekengineering.com/trainboss.php

In terms of "SMT" detectors, this looks promising, as it should fit between the rails: https://www.sparkfun.com/products/10901  but it will require some form of micro controller to do detection— it is an I2C device.

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Re: Smallest Photo Cells out there?
« Reply #11 on: February 17, 2014, 01:19:33 AM »
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Tie this to a counter and then have it broadcast a defect detector recording with accurate axle count over FRS and it's epic.

Anways. Here is the PDF of the flasher module.
https://www.logicrailtech.com/gcp_inst.pdf

I dont need it just yet so I guess we have some time here to tinker.

Look like the circuit is designed to use either photocells or IR LED/phototransistor pairs. So, you can you the very small SMD components mentioned earlier (and not depend on ambient light).
--- Peteski de Snarkski

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