All Sky Camera

[Gina]

An idea for the focus control.  Two rings run on the lens barrel.  The top one moves the focus lever whilst the bottom one, which is connected to the top one, could be turned in some way - probably by making it a spur gear and mounting a corresponding spur gear on a servo motor (or possibly stepper motor).

With a servo motor driving the shutter and probably another driving the focuser I may well completely redesign the camera casing.  I think it will need to be quite a bit bigger that the present casing.

[Gina]

Teeth added.

[Gina]

Here's a sketch of how I see the shutter and servo motor.

[Ajohn]

I'm pretty sure I would use a single rail op amp or comparator for that sort of thing. Probably an op amp as I'm a cheap skate. The type to look for has pnp transistor inputs. Some should be very cheap as the automotive industry uses buckets of them mostly quad versions but duals will be available as well. They can be so cheap the fact that one is spare hardly matters. Afraid I am not up to date with the current flavour of the month so any number I quote is likely to be more expensive than it needs to be.

They are pretty vice free but can miss behave if used in an analogue fashion such as a low drop voltage regulator where there will be other semi conductors in the feed back loop to it's inputs. Max input currents are usually specified so that a series resistor can be used to limit it if the input goes over the rails. That is one factor that makes them popular plus the input range includes ground.

If some one is into solving simultaneous equations comparator type 3 resistor feed back networks can be solved by setting one of the resistors to 1. The others will come out as fractions of 1 so can be used to select suitable actual values. For some reason not many people seem to know this and just add the feedback resistor by trial and error and see what happens. There may be resistor ratio tables about on the web. These can help a lot with many resistor network solving problems.

John

-

[Gina]

Thank you John   Yes, I have some very cheap MOSFET op amps already.  I could use one in conjunction with an LDR with appropriate feedback to form a Schmitt trigger and feed the servo through a suitable resistor network to provide two appropriate positions for the shutter.  Wired remote focus can be arranged with a servo and a pot.  Unlike AP, focus is unlikely to vary and once set should stay put.

[Ajohn]

The sort of thing I would use would be the LM358N, £1.93 for 5 off ebay. There is a data sheet here

www.jameco.com/jameco/products/prodds/839367.pdf

As OP amps go they aren't spectacular but are suitable for all sorts of things. The LM258N would be a -45 to 85C part but to be honest they make so many of these types I doubt if there is really any difference. There are other variants as well, most manufacturers produce several types.

There is usually a matching comparator as well so if some one is unsure of the circuit arrangements it's likely to be shown in the data sheet for those. National Semi often give the biggest range of circuit ideas and OP amps can be used in the same arrangements but will probably switch a mite slower.

One thing to be aware of when looking at op amp circuits is differential arrangements. They look great but often require really closely matched resistors to function as they should. For single ended use inverting or none inverted they are fine. If some one wants a true differential amp the best bet is analogue devices, gain will usually be programmed with a single resistor. I have used that sort of thing in data loggers without any problems providing the signal levels are suitable.

John

-

[Gina]

I've used lots of LM358Ns in the past and I should have a number of those too   I'm well familiar with op amp circuit design.

[Ajohn]

The matching comparator is the LM393, data sheet here

www.solarbotics.net/library/datasheets/LM393.pdf

but it looks like the LM2903 is cheaper and the same style part. The op amp variant of that is more expensive than the 358 for some reason. No opp amp prices I can find on these but the same circuits can be used with an op amp.

There's no harm in using others providing they are single rail types. With these I would generally aim for currents a hundred uA and up through the networks but they can be used at lower values. That is where they score over bipolar circuitry. Most small signal transistors are designed to run at 2 to 5 ma. A few much lower.

John

-

[Gina]

I'll bear that in mind John

I've found a small meter that I could possibly use for the shutter but for daylight use with the solar film I think there would be too much stray light getting round the film as the meter movement wouldn't be strong enough to use with flock paper (or whatever) to keep stray light out.  The filter needs to touch the flock to keep light out.  I could perhaps use it with aluminium foil to save the sensor from the sun and give up on daylight imaging. 

One possibility I thought of was having two all sky cameras, one for day and another for night though I don't know how to make a daytime camera work well enough.  The problem is getting the exposure right.  No problem until the sun comes out then either the whole frame goes white or, with auto exposure, everything goes dark and there's just a pin point of white where the sun is.

Anyway, ATM I'm only building one camera that uses the QHY5 and Fujinon fisheye lens and I'm thinking that I might be best advised to give up on daylight imaging to start with and just get a night-time camera working.  The meter movement operated shutter has the advantage of being fail-safe as well as much simpler.  The unpowered position would be covering the sensor and only with power AND low light would the shutter be moved out of the light path.

[Ajohn]

  I thought this might still be about - it's pretty unique

https://ia600809.us.archive.org/25/items/FerrantiELineTransistorApplications/Ferranti-ELineTransistorApplications.pdf

Of interest to budding analogue fiends who want to use bi polar. It's unusual in that the sums they suggest actually work. Some of the ZTX E-Line parts are also unique most especially the higher powered ones but I have no idea if they are still about.

John

-

[Gina]

Goodness me - that takes me back a bit

[Gina]

This is all that's required to drive the meter circuit.  When dark the LDR (light dependent resistor) goes to a very high resistance allowing current to flow from the +ve supply through the other resistors and the meter coil moving the shutter out of the light path. When daylight arrives, the LDR conducts more (lower resistance) and shunts the current to ground, removing it from the meter coil causing the pointer to return to the unenergised position and the shutter to block the light to the sensor.

The addition of a zener diode improves the shutter control.

Photo of the meter movement with dial cut off and a piece of kitchen foil glued to the pointer.