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!