Page 1 of 1

Quick question for you camera makers

PostPosted: Wed Oct 31, 2012 1:23 am
by Panrock
The only mechanical camera I have to hand is a colour one and uses photomultipliers. And anyway, it's stuck in the store room and is so big and heavy I won't be shifting it for a good few years.

For those of you who have made/are making a mechanical Nipkow camera that uses a solar cell or photodiode, can you tell me what typical working signal levels are, as measured at the photodiode?

I'm expected this will be a very small figure. In this connexion, any info about the noise floor of such devices would be most interesting too...


Steve O

PostPosted: Wed Oct 31, 2012 5:11 am
by M3DVQ
It surely varies greatly depending on the device used?

When I was experimenting with the uv-eraseable eproms as detectors (see last newsletter) I was getting pretty substantial signal levels (tens of mV iirc), however this was just shining modulated LEDs towards the device (directly and reflected). There was no nipkow in the way :shock:

PostPosted: Wed Oct 31, 2012 2:28 pm
by gary
Steve (O), generally these sensors produce an output large enough for a simple 2 transistor amplifier to produce a signal easily recorded by PS sound card (say 1v p-p).

Of course, as has already been mentioned, it depends a great deal on how much light is available, and in particular (of course) this has a great effect on the SNR.

It's a little hard for me to discuss SNR as one is never quite sure of where the source of the noise is - it can be light leakage, the detector, RF, PSU noise, etc. One simply tries to reduce all of these sources as much as possible, but patently, as Graham Lewis' examples show - the SNR is large enough to produced quite acceptable pictures.

PostPosted: Wed Oct 31, 2012 4:39 pm
by Steve Anderson
I would suggest experimenting with a BPW34 photo-diode for a number of reasons. Firstly they are widely available and at a reasonable cost. Their response time is something like 20ns, circuit arrangement allowing rather than the lethargic response of solar cells or LDRs.

The thing to consider is whether they would fit into your optical arrangements. The sensitive area is and it is square (approx. 2.8mm x 2.8mm). So if you can arrange for your projected image to fall within these bounds it may be worth considering.

In addition Chris Long has used them for his long-range optical communications, it's worth a look at his (and others) site, there are pre-amp circuits which are developed for extreme low-noise without requiring liquid Nitrogen...

These should be adaptable to NBTV requirements.

If you do consider using these make sure you get the clear-packaged version, the plain BPW34, not the black IR version, BPW34F or BPW34FA. There is also a blue-enhanced version, BPW34B or BPW34BS.

As for Steve's question r.e. signal levels, it depends on the configuration. Used with reverse bias a photo-diode produces a current output. The higher the load resistance the greater the voltage out....but watch stray capacitance. This gone into detail on Chris's site.

Datasheet for the visible version attached.

Steve A.

PostPosted: Wed Oct 31, 2012 10:46 pm
by Panrock
Thanks Gary and Steve.

The BPW34 looks a good choice. I see it is cheap too! For the application I have in mind its size and spectral response look ideal.

I'll doubtless be back later looking for advice on how best to connect it for maximum sensitivity, whilst maintaining excellent hf response (at lowish output impedance). All to be done with the minimum of components.

Thanks again!

Steve O