Slave Flash for Digital Cameras: Don’t throw away your old flashlight （1）

Published:2011/8/4 2:41:00 Author:Amy From:SeekIC

Udo Burret

Although most digital cameras have a built-in flash, a connection for an external flashlight is rarely seen. Still, on some occasions it’s useful to have just a tad more light and that’s why we propose a simple add-on.

As many of you will be able to avow, the light capacity of the flash facility on low-cost digital cameras is often marginal if not insufficient. Problems typically occur where the object to be photographed is too far away, or if a (very) short shutter time is required. In those difficult cases, the slave flash we’re about to describe may come in really useful.

The idea is simple: detect the flash from the camera and use it to trigger a powerful external flashlight with the aid of a thyristor.

A cursory look at the circuit diagram in Figure 1 reveals that the practical side of things is rather more complex than you might have expected. In many cases, a digital camera supplies a short ’pre-flash’ that serves, among others, to adjust the white balance of the internal CCD (charge-coupled device) chip. Lots of cameras also fea­ture ’red eye correction’ where several pre-flashes cause the pupils of humans and certain animals to close to some extent. Some cameras supply just one pre-flash, others a whole series. Of course we do not want our slave flash­light to respond to these pre-flashes, else it would be triggered too early! This problem has been taken into account in the circuit design, which is in stark contrast with many off-the-shelf slave flash controllers.



Counting flashes

The author of the circuit employed an almost antediluvian design from Elektor’s 1979 Summer Circuits issue. With reference to Figure 1, phototran-sistorTl detects the camera’s flashes. The resulting pulses across C3 are applied to the clock input of decade counter IC1.

The circuit around T2 operates as a kind of zener diode, with C2 prevent­ing instability owing to too rapid fluctu­ations of the ’zener’ voltage. Without this precaution, the counter would run the risk of missing the odd pulse. The capacitor also eliminates any 100-Hz hum that may be picked up.

Switch SI enables the circuit to be reset, causing output QO to go high and transistor T3 to switch on LED D2. With the LED on, the slave flashlight is ready for use. T3 also ensures that capacitor C4 is discharged and the reset input of IC1 is returned to Low. At each light pulse on Tl, the next out­put of IC1 will go high — Ql on the first flash, Q2 on the second, and so on. A jumper or wire link on Kl allows you to select the flash that should actually trigger the slave flashlight. The LED in optoisolator IC2 is then switched on and C5 causes T4 to conduct briefly. This in turn causes thyristor Thl to be fired and the terminals of connector K2 to be short-circuited briefly.

The optoisolator ensures that the flash­light voltage remains isolated from the (battery) supply of the rest of the cir­cuit. The connections of older flash­lights may well carry voltages of 150 V to 200 V. These are reduced to safe lev­els by voltage divider R12-R13. Modem flashlights use much lower voltages (5-10 V), in which case R12 may be replaced by a wire link.

Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/04/Slave_Flash_for_Digital_Cameras__Don’t_throw_away_your_old_flashlight_（1）.html

Print this Page | Comments | Reading(1387)