Most point-and-shoot digital cameras do not have hot shoe or PC connector for wired external flash. Their built-in flashes are a bit too weak. To shoot a distance more than, say, 10 feet, slave flash is the only choice. Unfortunately, slave triggers of film camera age are not compatible with digital cameras because digital cameras use a double flash. The first "pre-flash" is used to calculate white balance information. The second flash is the "real" one that illuminates the exposure. Those older slave triggers will fire the external flash on the pre-flash, leaving the exposure dark.

Having seen some so-called digital-camera-compatible flash units available in Hong Kong market, I can’t find any in Vancouver here. So, I spent some time to study and design slave trigger for the cameras I own. I have 4 point-and-shoot digital cameras, namely, Canon PowerShot A540, Fujifilm FinePix A610, Panasonic DMC-FZ5 and Pentax Optio mx. So far I manage to build one that works with Canon PowerShot A540.

The circuit is pretty simple and can be modified from an older slave trigger. The origin circuit has one SCR only. By adding 3 more parts (shown in red) and removing the resistor, the trigger will fire on the second flash from Canon PowerShot A540.

Principle of Operation

Pulses are generated when a camera’s built-in flash light fall on the solar cell. Those pulses will reach the gates of both SCRs, but not exactly at the same time. The upper SCR will receive them slightly earlier than the lower one does. On receiving the pre-flash, the upper SCR will not be turned ON because the lower SCR has not yet received the pulse. When it does, the lower SCR turns ON; but, by that time the pre-flash seen by the upper SCR has been over. The lower SCR will remain ON for a little while, and before it turns OFF, the 2^nd flash are received. This time both SCRs turn ON and fire the external flash. The timing of this simple circuit happens to match Canon PowerShot A540’s built-in flash.

The modified circuit has much lower sensitivity than the original circuit does. The solar cell needs to face the built-in flash within a few inches. It adds the merit that it won’t be miss-triggered by flashes of other cameras at the same scene.

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End of the project

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