DIY light-activated switch

couldabin · Oct 31, 2011

I am considering a project that involves setting up a 4x5 to take an (unattended) early morning photograph once the light reaches a certain level. This would be dawn, but pre-sunrise. I would appreciate any feedback/advice on the approach I am considering.

I have some experience building devices that use microcontrollers to monitor temperature, and so want to do the same with photoresistors. My thought was to create a diffuser (would half a white ping pong ball work?) that has a photoresistor placed under it to read the brighness. I would then do some tests on location -- wait for the desired light level, use my hand-held light meter to see what the exposure would be, and at the same time record the resistance value from my DIY setup. The idea is that the microprocessor would watch for the desired resistance, and when it reaches the threshold value take the photograph. Would a lens to focus the diffuser shell onto the photoresistor make it more accurate? Would it be necessary?

On a related front, any thoughts on devices to press the shutter release cable?

Thanks.

AgX · Oct 31, 2011

As a child I built such a switching device out of a Philips electronics teaching kit. Don't remember the circuit, but sure the kit did not contain microprocessors basck then...

Once there were solenoids with push-pin and male shutter threading. At least my gives quite a bang, so use a long extension cable release inbetween (in case you could even get the latter...)

What about activating a cable release by an electrical motor and an excenter-wheel?

John Koehrer · Oct 31, 2011

I doubt the need of a lens behind the diffuser, I don't believe any incident meter uses them.
If you're going out in the woods at sunrise(ish) wouldn't it be just as easy to adjust the sensitivity of the device while you're out there?

ndrs · Oct 31, 2011

After considering several options I'd suggest an alarm clock that activates a human who's programmed to execute the following code:

if light_level = desired
exec press_shutter_release
else
wait 60 seconds; retry

couldabin · Oct 31, 2011

re: adjusting while I'm out there ... that's the thing -- I won't be there. I need to be able to set up the camera to take the photograph in situations where I can't be on-hand. (But perhaps NDRS has a programmable human he can donate to the cause ...)

re: lens ... the more I think about it, the more I think a condensor is needed. The diffuser is going to be at least a half-inch across, maybe twice that. The photoresistor is roughly .12" in diameter. Without a lens, most of the light passing through the diffuser won't hit the photoresistor. Yes?

Steve Smith · Oct 31, 2011

ndrs said:
if light_level = desired
exec press_shutter_release
else
wait 60 seconds; retry

For those of us old enough:

10 RELEASE = 156
20 INPUT LIGHT
30 IF LIGHT => RELEASE THEN 50
40 GOTO 20
50 PRESS SHUTTER
60 END

Steve.

Chan Tran · Oct 31, 2011

couldabin said:
re: adjusting while I'm out there ... that's the thing -- I won't be there. I need to be able to set up the camera to take the photograph in situations where I can't be on-hand. (But perhaps NDRS has a programmable human he can donate to the cause ...)

re: lens ... the more I think about it, the more I think a condensor is needed. The diffuser is going to be at least a half-inch across, maybe twice that. The photoresistor is roughly .12" in diameter. Without a lens, most of the light passing through the diffuser won't hit the photoresistor. Yes?

Most incident light meter has the dome diameter approximately 3/4 inch and yet the diameter of the photocell is 1/2 inch or less and there is no lens.

DWThomas · Oct 31, 2011

I've no personal experience with them, but the servos used in radio controlled models might offer an actuator that could run the shutter release.

I suspect you won't need a lens unless you're trying to trip at extremely low light levels and need to concentrate the received light -- on the other end, getting too carried away with that could potentially fry the sensor in bright sun!

DaveT

vyshemirsky · Oct 31, 2011

I would have probably used a phototransistor instead, they usually have somewhat better sensitivity. An analogue comparator is the only device you need to implement the rest of the logic.
There are a number of IC comparators available, in the worst case most 16 bit PIC processors have at least one comparator on the chip.

But in your pre-dawn low light situation even that wouldn't be enough I guess. The problem would be that dark current would be very similar in the magnitude to your light signal. You will be detecting noise rather than the required amount of light. I know what I am talking about, because I tried building a low-light light meter last year. I would recommend using TSL230R Light to Frequency Converter from The Texas Advanced Optical Systems - it is the only thing that worked for me in the end for that amount of light.

PS: PM me if you need more details - my light meter worked in the end

polyglot · Oct 31, 2011

You don't need a lens behind the diffuser, in fact you really don't want one at all because that will introduce an angular-dependence to the meter's response. You don't need "all" the light from the diffuser, just a constant fraction thereof. Ping-pong ball is perfect unless you can source a white acrylic dome, which will be a bit more transmissive.

Certainly you can make a light-triggered circuit, but photoresistors are notoriously wonky and variable in their output (temperature dependent among other things) so it won't necessarily trigger at exactly the light level you want. It'd probably be OK for a B&W shot if you aim to overexpose a little, but I wouldn't trust it at all for colour. Some experimentation would be required!

A (much) more accurate option would be to use a phototransistor, i.e. the same devices present in a real light meter. IMHO you need to admit that what you're building is a light meter and since you have a microcontroller involved, actually build a proper light meter that will compute exposure. That would allow you to (assuming B mode on the shutter) accurately set the exposure time, or if the shutter time is fixed, the light meter can wait until the measured level is exactly the required level. Beware though that phototransistors are very IR-sensitive, so you either want a hot-mirror to cut off radiation below 700nm or you need to accept that you will have an exposure error that is dependent on the time of day.

The other option is to just build a timer with your micro. If you know what time of day will have the lighting you want, a timer will suffice and is much, much easier to build.

Steve Smith · Nov 1, 2011

polyglot said:
The other option is to just build a timer with your micro. If you know what time of day will have the lighting you want, a timer will suffice and is much, much easier to build.

I was going to suggest that too.

Steve.

paul ron · Nov 1, 2011

Why not use one of those backyard auto light fixtures? They are cheap n will do exaclty what you want.

You'll have to install a relay so when the light goes off in the morning it will power your trip circuit instead of going dead. On the other side of the relay you can now power using batteries instead of wall juice to activate the motor or solenoid.

To adjust the amount of light that will trigger, they do come with a sensativity adjustment. Otherwise you'll just have to positioning it for a darker part of the sky or ground so it won't turn off too early, by covering the sensor you can also limit the light hitting it to eactly when it shoudl take the picture.Total cost... you may already have everything you need right in your back yard.

That timer idea is excelent, didn' t see it till after I posted.

.

couldabin · Nov 1, 2011

With a 3/4" dome, the area of the photocell is less than 3% of the area of the dome. Wouldn't a person want to measure all the light hitting the dome?

couldabin · Nov 1, 2011

Great advice. I looked up the TSL230R, and it looks to be perfect for this application. Thanks much.

couldabin · Nov 1, 2011

I had not looked into the temperature dependence. Good point. I think I'm going to use the TSL230R chip suggested above. It's temperature stable and can be configured for optimal sensitivity. Yes, I plan to have the shutter in B mode, since the exposure will be on the order of 8 sec. @ f22. Someone mentioned a servo for the cable release -- I agree that a stepper motor of some kind would work well. I like the idea of being able to move it incrementally, so as to minimize vibration. I'm thinking the flash sync would be a good way to detect when the shutter has actually opened; at that point stop pushing and start timing, and then release after 8 sec.

vyshemirsky · Nov 1, 2011

TSL230R has three different versions with 5, 10, and 20% precision depending on the price. Make sure you select the one which is sufficient for your application. The cheapest 20% still must be within 1/3 of a stop, or am I wrong?

Steve Smith · Nov 1, 2011

couldabin said:
I agree that a stepper motor of some kind would work well. I like the idea of being able to move it incrementally, so as to minimize vibration. I'm thinking the flash sync would be a good way to detect when the shutter has actually opened; at that point stop pushing and start timing, and then release after 8 sec.

Not really an issue with an eight second exposure.

Steve.

couldabin · Nov 1, 2011

I would think that +/- 20% would be close enough for this. I'll be shooting FP4, exposing it at EI 80. I quickly scanned the info sheet on the chip; do I understand correctly that the output is a pulse frequency proportional to the light level? If so, that should be fairly simple to implement ...

couldabin · Nov 1, 2011

Good point.

paul ron · Nov 1, 2011

OH be sure to compensate for the difference in day light from day to day as it changes with the seasons. Sun rise if not exactly the same every day but maybe close enough.

couldabin · Nov 1, 2011

I'm thinking that the backyard auto light fixtures I've seen are 120V; there will be no external power available where the photo will be taken. Also, I'm not at all sure what the specs are for the sensors -- how repeatable they are, or what it would take to modify them so that they switch at the light level I want.

As to the timer: the sun rises almost 3 hours earlier in June than it does in January. That schedule is pretty well known, but with weather variabilty affecting brightness, it's really not possible to know on any given day when the light level will first reach the point where the photo must be taken.

couldabin · Nov 1, 2011

The difference between winter and summer sunrise extremes is 2 hours and 45 minutes. I need to take the picture when the light has reached a particular level, which on a clear morning would be about 15 minutes before sunrise. But changing weather conditions would affect the exact time. Measuring light levels, rather than time, seems the more practical approach.

greybeard · Nov 2, 2011

An interesting thread; I can't quite visualize how you are going to plan the picture without having been there at the right time of day, which would mean that you could presumably return the right number of minutes later the next day......

One reason for using a lens would be that if you used an opaque barrel you could limit the angular field of view of the photosensor and do a Zone-System-type exposure for the key area of the scene!

Leigh B · Nov 2, 2011

10 RELEASE = 156
20 INPUT LIGHT
30 IF LIGHT < RELEASE THEN 20
40 PRESS SHUTTER
50 END

- Leigh

Leigh B · Nov 2, 2011

You don't need a diffuser or a lens. A soda straw will work fine to exclude extraneous light.

Measure the light level at the desired time of the day with a light meter.
I __assume__ you can get somewhere in the vicinity close enough to make the measurement, since sunrise covers a large area of sky.

Then set up the light meter in broad daylight, pointed at the sky (with a snoot to limit its field of view and exclude the sun) and read the light level. Put a combination of ND filters over the snoot until you get the same reading as sunrise.

Place that filter set over your sensor, pointed skyward under the same conditions, and use that level to calibrate the trip point.

- Leigh

DIY light-activated switch

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