Monthly Archives: August 2015

Infrared camera trigger experiments

Wouldn’t it be nice if I could take a picture of a bird as it passed through an invisible beam of light? The idea’s not original, these things exist, but they are quite dear, so I am experimenting with making these.

The most obvious way is a light source and a photocell, and indeed many years ago at secondary school I developed an analogue circuit 1 using OC71 transistors scavenged off postwar computer boards to make up monostable multivibrators for the delay elements  and one with the black scraped away from the housing to act as a phototransistor.

Not bad for a school project from 1976. It looks better as an animation than a sequence of stills.

This gonzo technology of 40 years ago triggered the flash for the source negatives used in the animation – you set a very slow drip, and as the drop passes the photocell it triggers the delay. By increasing the delay between the drop passing and the flash going off you get the progressive animation, assuming each drop makes a similar pattern.


OC 71

This was done with a manual camera, a new Canon AE1 ISTR that one of the other kids had. But the trick is to do all this in the dark, click the camera on Bulb and use the trigger to trip the electronic flash, which responds within milliseconds and has a short duration of about a millisecond if you reflect some of the flash back into the photocell of the flash (to turn it off as early as possible).

So there’s nothing incredibly hard about doing this, in controlled conditions, in a darkened room. If I were doing it again, I’d do it in the same basic way, using a phototransistor and a CD4538 CMOS dual monostable rather than a discrete monostable – one half to give the delay controllable with a pot and the other to make a pulse off the falling edge to go into a NPN transistor to trip the flash. There’s no need to muck about with PIC microcontrollers or Arduinos though you could do it that way if you really have to for a higher cost plus the aggravation of writing code, plus the jitter of the Arduino sampling the sensor/responding to the interrupt. In high-speed photography sub-milliseconds matter.

Everything gets harder outdoors

Outdoors you have massive and variable amounts of light from the sun, distances are longer, there’s just a whole lot more hurt all round.

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  1. people normally consider monostables as digital but mine was built using discrete transistors and resistors, and the time delay was infinitely variable, as it would be with a CMOS 4538, so I consider it analogue

Measuring paramagnetism 3 – a portable instrument

In Part 2 I described a flat coil sensor which changes inductance according to the magnetic susceptibility of what is in front of it. To make this useful in the field I need something to display the change.

the field instrument

the field instrument. The black button stores the calibration frequency with no sample, the red button takes the reading and displays the result after 4 seconds

I was going to count the high-frequency microcontroller clock over, say 500 periods of the low-frequency sensor signal. That turns out to be a terrible way to do this. I don’t have the gear to measure it, but I suspect the jitter from slicing the 1.5kHz sensor signal is too high. The result is that the third significant digit twitters a lot. By counting changes in the sliced sensor signal (thus doubling the frequency) over a fixed period I get the twitter down to one part in >12000 counts 1 over a four-second signal acquisition time. Continue reading


  1. the operating frequency is ~1500 Hz so counting transitions gives me 2 x 1500 * 4 = 12000 counts in four seconds

Wildlife camera with Raspberry Pi, Motion, and temperature sensing

I’m toying with the idea of going along to the Ipswich Raspberry Jam on Saturday 8th Aug and figure it’s be nice to have something to show. There’s of course our farm Raspberry Pi cameras which are in service and this one is riffing a bit off an idea Wildlife Gadget Man is playing with. He’s the guy with the wildlife – I only have sparrows 1 so I have to make do with a stuffed toy stoat 🙂



There’s nothing earth-shatteringly new in here, but the ability to make a box which gives you video, snapshots and a temperature plot taken from one of those Chinese waterproof DB18B20 probes is good for mammals.

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  1. I like my sparrows but they aren’t going to pose long enough for the camera, and presumably they have their heads under their wings in a hedge somewhere now, a hedgehog in a hog box is the sort of target that would work well here

Seed saving beans

The Seed Saver’s handbook says beans are easy to save, so it seems a good idea to start out with them, in this case some Sutton Dwarf beans. The idea if you leave them to dry in the pods and then save the good ones. Beans are an easy win as they adapt over the generations to the local conditions; they don’t use insects for pollination and the book says the gene pool is kept wide to allow self-pollination.
Right off the bat the book says that

The first pods to form are the best for seeds. They are to be found at the base and are larger than subsequent pods, Allow these pods to dry on the bush, and choose those from the most vigorous plants. Such refined steps cannot be taken on a large scale where a whole field is combine-harvested and threshed.

Well, we don’t have a problem picking seeds out of the combine harvester we don’t use 😉

the good bean seeds

the good bean seeds

The guys that wrote that book are Australian, and I guess they don’t have a problem with saying you need to store seeds at a relative humidity of 5%.

Closing the conservatory door at about 9 was a good move, though I am a long, long way from 5%!

Closing the conservatory door at about 9pm was a good move, though I am a long, long way from 5%!

So I am writing on the evening of what has been a reasonably warm sunny day and I see the RH starting to skyrocket to 50% by 10pm and realise that I need to close the door to the conservatory because the dew comes in the evening as the sun goes down, not in the morning. 5% is going to be a tough call in the UK, probably involving silica gel. Interestingly the Seed Saver’s Handbook says good airflow is more important that high temperature, and it should not go beyond 35C anyway.

long bean pods give the best results

long bean pods give the best results

They’re right about those lower pods – long beans are definitely the place to go for the size of the seeds. You have to be pretty discriminating about the seeds, however.

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