Both Canon and Nikon offer infrared based wireless flash controls. This type of systems, similar to your TV remotes, suffer from typical line-of-sight limitations because the infrared light cannot go through wells. In a studio environment, it is not too bad because the signals can still get bounced around by the walls and ceilings and reach the infrared sensor of the remote flash. It can be very problematic in an outdoor environment if the remote flash isn’t in the line-of-sight of the master unit.

To overcome this limitation, RadioPopper has come up with the RadioPopper PX system that transmit the signals via RF instead of infrared. It appears to be very popular despite the awkward device mounting requirements. The magic comes with a high cost though. That’s probably why many have tried to roll their own. Most recently we covered two examples from Europe: Italian version and Polish version.

The second one appears to be very interesting because the Phottix flash trigger has circuits that look similar to the iShoot PT-04 CN trigger I reviewed. After probing the circuit boards of the iShoot PT-04 CN transmitter and receiver with an oscilloscope, it became clear that the same DIY can be done as well.

iShoot PT-04 CN

The following is the PCB inside the transmitter. Each function block is marked.

The transmitter is powered by a 12V battery. The micro-controller (uC) is the brain of the device. It reads the DIP switch “channel” configuration and sends out flash firing command via the RF transmitter circuit when it senses the x-sync input (or the press of the test button). The connection between the uC and the RF circuit is shown as a yellow dashed line. The uC works at 5V but the RF circuit works at 12 volts.

The RF circuit has a fixed frequency of ~433MHz, determined by a SAW resonator. The RF circuit uses OOK/ASK modulation. The “channel” switch doesn’t configure different frequencies but rather the “codes” the transmitter sends out. When the transmitter instructs the receivers to fire the attached flash, it sends out a special signal repeatedly for a duration of 100-200 ms. Each signal consists of 3 pulses of various widths as shown in the following graph.

The following is the PCB inside the receiver unit.

The signal sent from the transmitter is detected by the receiver circuit. The signal is quite weak and polarity is inverted for some reasons. So the signal is wired through the HEX inverter chip. 3 of the 6 inverters are connected in series. The first one is used as an inverted amplifier, and the other two shape the signal to a good square wave with proper polarity and send it to the micro-controller. The micro-controller decodes the signal and determines if the signal is intended for it by checking the DIP switch configuration on the receiver. Once a match is determined, the micro-controller sends the x-sync signal to the hot shoe to fire the attached flash.

How the DIY works

It is pretty simple: the RF transmitter and receiver circuits transmits and receives amplitude modulated signals. Instead of sending the pulse modulated trigger code signals, we can send the pre-flash communication signals from the master flash unit. We just need to convert the light pulses into a suitable signal for the transmitter circuit and convert them back to infrared pulses at the sensor window of the remote flash. This can be easily done using photo diode and IR LED.

Circuit modification details

The following shows the wiring for the DIY. I removed the micro-controller from the transmitter because I think it is necessary but I could be wrong.

Here are the actual circuits boards after the soldering job. (The transmitter wiring looks slightly different compared to the drawing above but that’s just because I wanted to use an existing hole in the PCB to make the wires more secure. )

Does it work?

It does work as expected but perhaps not as well as the real thing. One thing I noticed is that the photo diode is too sensitive so it needs to be about 2 feet away from the master flash. I haven’t tried to package them up nicely so it is completely impractical at this time.


The information above is provided as is without any warranties. If you decide to try this, you do it at your own risk. I am not responsible for any damages or losses your may suffer.

Posted in: DIY Photography on May 18th, 2010. Trackback URI
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