Summary of strobe project

I'll be moving from LA to Paris (via a 2 week holiday in Ireland), and I likely won't be doing anything more on my strobe measurements until all my electronics gear arrives in Paris, assuming it doesn't sink into the Atlantic. Here's a summary of where the project is:

1. Understand the Nikon CLS protocol - not started yet.. I need to get a digital oscilloscope to work on this. I have my eye on a USB scope from the UK company Picoscope.
2. Understand the response of the Nikon strobes to TTL control signals - some measurements made. Seems like it should be possible to calibrate a controller to generate fixed power settings (1/2, 1/4 etc..) to emulate a Nikon CLS wireless flash in manual mode. This calibration would need to be changable so that it could work with any Nikon strobe.
   
3. Design and build a controller that can be connected to a Nikon strobe to make it compatible with the Nikon optical "wireless" protocol - not started.
   
4. Design and build a radio frequency controller to do CLS over radio - a long way off!

Obviously there is still a lot to do. Hopefully I will get back to it in the new year.. or sooner if I can get a DSO before then, as point 1 may not really need all of my electronics gear.

Before I leave I hope to post something about interfacing a Nikon D300 with a serial GPS module.

Flash pulse vs. quench delay

Likely the best way to measure the amount of light emitted in the flash pulse is to use a flash meter. These are calibrated devices which measure the absolute amount of light in a flash pulse. They also cost a lot! Home-made solutions, based on a cheap photo-diode, are also possible. A photo-diode is a device that produces a current that depends on the intensity of light falling on it. Assuming that the flash pulses all have the same spectral profile, i.e. that all pulses have an equal mix of colours, but differ only by the overall amount of light produced, the photo-diode will produce an electrical pulse with a charge proportional to the amount of light in the flash pulse.

A nice method too read out the photo-diode would be to digitize the flash pulse intensity vs. time profile using a digital storage oscilloscope (DSO) and integrate the amount of light in software, but again, you need an expensive DSO to do this. A cheap solution is to integrate the amount of light in the flash pulse using an electronic integrator and then read out the result with a volt meter. An integrator is a small circuit using an operational amplifier chip, which stores charge in a capacitor, producing a voltage that is proportional to the total amount of charge that flows into it, ideal for this application. Pretty much any op-amp will work, but those with FET input transistors work best, since they draw almost no input current and have very low bias and offset currents, to which integrators are particularly sensitive. A typical integrator circuit is shown below.

I used the Texas Instruments TLC27M2A1 opamp in this circuit and was very happy with the results. The charge in the pulse is integrated while switch SW2 is held closed. The pulse size is read as a voltage at OUT. Switch SW1 clears the charge on the capacitor, zeroing the integrator for the next flash pulse. Ideally D1 is a photo-diode, but any regular LED will also work. To cover a large range of flash pulse sizes it is convenient to be able to change the value of C1 - an IC socket into which the capacitor is plugged makes this easy.

The plot shows the integrated charge from the photo-diode for a Nikon SB-25 and SB-600 strobe set in manual mode. The strobes were set to full, 1/2, 1/4, 1/8, 1/16, 1/32 and 1/64 power and the photo-charge measured at each setting. The charge is plotted in stops -- a change of one stop corresponds to a factor of two change in the detected charge. With the exception of the full power setting on each strobe the amount of charge is linearly related to the manual power setting to within a fifth of a stop. For each strobe the full power setting is almost a half a stop above the expected value. I have no explanation of why this is.. my guess is that it is difficult for the manufacturer to control the discharge of the capacitor at the full power setting, and this is the best the could do.

The results of using the a time delayed TTL quench with the SB-25 and SB-600 is shown in the second graph. The plots shows the amount of photo-charge measured as a function of the delay between the trigger and quench signals in milliseconds. The flash pulse size is again shown as a fraction of the full-power pulse in stops. A few things are immediately obvious:

• The SB-600 can produce flash pulses over an amazing range of almost 14 stops. Fourteen stops is equivalent to ~1/16000! The lowest few points were hard to measures and there is considerable measurable error in the data.
  
• The SB-25 seems to flatten out at small quench delays. Presumably the electronics in the -25 are not able to stop the current flow from the capacitor quickly enough to produce very dim flash pulses.

Well, that's enough for now. Next time a summary of where the project stands, and possibly also a small diversion into interfacing Nikon cameras with a GPS.

Digital TTL quench signal generator

The aim of this first experiment is to measure of the amount of light generated by the strobe as a function of the delay between the trigger and TTL quench signal. The delay circuit is required to trigger the strobe and generate a quench signal at a precise time thereafter. The delay should be easy to configure and cover the range from a few 10's of microseconds to a few 10's of milliseconds, i.e. over a range of at least a thousand. It's also advantageous to be able to record the amount of delay easily.

Probably the best way to solve the problem would be to use a microcontroller, which could likely do almost everything required in software with a minimum of external components. I don't have experience with microcontrollers so I went with an old-fashioned discrete IC design, based around a 1MHz crystal oscillator and the 4536 programmable timer, which divides the oscillator signal by any power of two between 1 and 24, giving a period between 2 microseconds and 16.7 seconds, more than enough to cover the range of potentially interesting delay times. The only problem using the output of the 4536 directly is that the delay duration can only be doubled or halved from one setting to the next (i.e. the delays would be logarithmically separated). To give more precision a 74193 (or 40193) decade counter is driven from the output of the 4536 and used to generate the delay before the quench signal. The possible range of delays in then:

• D = 1uS * m * 2^n

where m is between 1 and 9 and n is between 1 and 24. In practice n is limited to a range of ten consecutive values whose lower value is set with jumpers on the circuit (I usually set this to 2, so that n is in the range of 2-11). This design requires only two digits of input (i.e. n and m) and two seven segment LED displays to show the selected values.

The schematics for the circuit, drawn with the fabulous (and free for non commercial use) Eagle layout package are shown below:

The schematics and board layout files are also available in their original Eagle format. The circuit is relatively straight forward. The heart of the interesting functionality was described above. The final component of the primary functionality is a set of S-R latches which drive high voltage transistors (KSP44) that short the strobe trigger and quench signals to ground. These outputs are connected to the strobe using a Nikon AS-18, as described last time. The rest of the circuit (which makes up a lot of the board area) allows m and n to be selected (using up/down buttons) and drives two LED displays to show the values. An input transistor and a switch activates the circuit when triggered by an external device or by the user. I had the board fabricated by Custom PCB in Malaysia (www.custompcb.com) and was very happy with how it turned out. They take the Eagle board layout files directly so it saves you having to do the CAM processing yourself.

Next time the results of using the delay generator on an SB-25 and SB-600.

The magical AS-18

OK.. before I get on to the TTL timing circuit, a quick note on how the old-style TTL system works and some cheap ways to interface an electronic circuit with on of Nikon's TTL strobes. Basically the camera sends two control signals to the strobe, a trigger pulse to start the flash pulse and a quench pulse when the camera determines that enough light has been produced. The strobe charges an internal capacitor up to a high voltage (using a voltage multiplier). When the trigger signal is received the stored charge is dissipated through the flash tube producing a flash of light. The quench signal stops this discharge, stopping the light pulse. In TTL-metering mode the camera monitors the amount of light reaching the focal plane and generates the quench signal when the correct exposure has been reached. The flash calibration board that I will describe in the next post generates the quench signal a fixed time after the trigger signal and a flash meter can then be used to calculate the amount of light produced as a function of the delay before the quench signal is generated.

Modern Nikon speedlights have 4 contacts on their feet to allow communication with the camera. The foot itself (or two prongs below the foot on older speedlights with a plastic foot, such as SB-25) connect the grounds of the camera and strobe. Many speedlights also have additional connectors to allow off-camera flash photography, for example the SB-25 has a standard sync port which has a single contact (and ground) wired up to the trigger circuit of the strobe and a 3-pin TTL connector which allows the trigger and quench signals to be sent from the camera to a remote flash using a 3-wire cable sold by Nikon (SC-18 and SC-19 for example). Nikon make a bunch of converters and cables to help in wired off-camera applications, such as SC-17, AS-10 etc. Most of them are expensive to buy new and can be found on eBay for $20-$30.

For home experimentation the Nikon AS-18 is a great option. It is described as a "TTL Multi-flash Adapter for 950 990 995", and is possibly the cheapest product available from Nikon, selling at $3 on Amazon as of August 2008. Possibly this is because it is technically for use with the Coolpix range of cameras, which have been largely overtaken by the range of DSLRs. The AS-18 has a hot-shoe on the top and a 3-pin TTL socket on the front - it is very similar to the more expensive AS-10 ($40 on Amazon) which has multiple 3-pin sockets to allow multiple flashes to be chained using Nikon's expensive cords. You can see from the picture that the AS-18 has plastic tabs where the AS-10 would have extra 3-pin connectors. The AS-18 can easily be opened and modified to accept any signals you like.

The three contacts of the hot-shoe used by the strobe in TTL mode are shown in the picture to the left. The trigger signal is on the large pad in the center of the shoe, and the quench signal on the smaller pad to the bottom right of the shoe. The rails of the shoe itself are the ground for the camera. The camera fires the strobe by shorting the trigger pin to ground (hence you can fire your strobe by shorting the trigger pin to the foot), and stops the flash by shorting the quench pin to ground. These pads on the AS-18 are simply connected to the 3-pin connector on the front, the idea being you connect the Coolpix to the AS-18 using a cable (such as SC-18) and this allows you to use a standard Nikon speedlight.

It's relatively simple to solder a 3-wire cable onto the pins inside the AS-18 and to route the cable out by removing one of the plastic inserts. On the inside of the AS-18 the:

• white wire is connected to trigger pad, the
  
• red wire to the quench pad, and the
  
• black wire to the ground of the hot shoe.

In conclusion, the AS-18 is a cheap way to interface home-brewed electronics to Nikon's range of TTL flashes. Get 'em while they are still available!

First post

Welcome to the 2nd most boring blog ever! I'm using this blog to record some of the things I am working on at the moment. For the first while it will probably concentrate on some experiments I have been doing with an old Nikon SB-25 speedlight I bought on ebay a few weeks ago. I had the idea that it might be possible to make a controller for Nikon's old speedlights that would make them compatible with the wireless CLS protocol, or at least partly compatible. For example with CLS you can use the camera (e.g. a Nikon D300) to remotely control CLS compatible flashes, such as the SB-600 and SB-800. One of the simplest things you can do is to fire them remotely and set the power of the strobes from the camera or from an external CLS controller (SB-800 or SU-800). At a more advanced level you can do i-TTL metering wirelessly and invoke the model light. It should be possible to do something similar with Nikon's older TTL compatible strobes with an appropriate controller. For example, the controller could monitor the signals (flash or IR pulses) generated by the commander, decode them and fire the connected flash generating an appropriate light pulse.

Producing such a controller will be a long term project that will require learning a lot about strobes and the Nikon CLS system. Also my electronics knowledge is quite rusty,since I have not done anything serious in many years. To start with I decided to produce a circuit that will generate a flash pulse of a fixed duration in time using a TTL compatible flash (my SB-25). Specifically it will trigger the flash then quench it at a fixed time later. The time delay should be generated digitally, so that it is accurate and repeatable. The delay must range from 10's of microseconds to 10's of milliseconds. My next post will describe the circuit to do this.

Thanks for reading the first post on the second most boring blog ever!