I have always wanted an external flash slave. That is an external flash that detects an on camera flash and will also be triggered. An external slave gives a photographer a lot of options for lighting. It will allow for nice smooth cross lighting, and will eliminate a lot of harsh shadows. There are several ways of triggering an external flash. The first way of triggering the flash is to use a camera sync cable. This will send a signal to the flash, when the camera shutter is depressed. The next way is to use radio frequencies, to trigger the external flash, and the final way is to use a light detector.
I have been playing around lately with an old disposable camera that I found lying around in my room. It was an ancient Kodak camera, but it had a flash, so I tore the thing apart. I noted how flimsy the case for the disposable camera was, and how hard it was to open it up. This would later spark my interest in finding a more suitable disposable camera for my project (Kodak Max). A film camera was necessary to use because they are so cheap. There was no reason to use a disposable digital camera because they cost a lot more.
Anyway, I decided I wanted to make an external slave. I thought about buying a slave peanut trigger. This is a little device that does exactly what I wanted to build. I would merely connect the flash switch to the peanut. I could not find one, so I decided to build my own.
I pulled out my old 200 in 1 electronics kit from when I was a kid. I found a Cadmium sulfide photovoltaic cell, and some NPN transistors, and a relay. I charged up my flash, and connected it across the relay, and gave 9 VDC to the relay coil, and it triggered the flash. I then measured the resistance across the photo cell, and found that in ambient room light, it was like 65K. With a flashlight on it, it went to like 5K. That was perfect, so I connected it to the base of an NPN transistor, connected the emitter to ground, and the collector went to one pin of my relay. The other coil pin of the relay I connected to +9v, through a 250 ohm resistor. When I turned on the flashlight, the flash did indeed work. (Now, I want to take a second to explain why I am using a relay in this project. A SCR, silicon controlled rectifier, would be much better, but I donâ€™t have one, and Radio Shack does not have one either. I used parts for this camera that I had lying around, or could easily get.)
After getting that to work, I thought that I wanted a circuit that would recharge itself. The old flash that I had made you hold the flash charge button for a minimum of 16 seconds, and that is on a brand new AA battery. I found another small 5 VDC relay from Radio Shack, and connected it to the charge switch, and found that I could make the relay charge the camera. I found some problems with it though. How was I going to detect when the camera flash was charged, and how was I going to keep the relay turned on long enough? I decided that this project was in desperate need of a microcontroller.
I will get to the microcontroller later, but for now, I decided to test my flash system with my camera flash. It was a miserable failure. The relay takes a finite amount of time to switch, and the flash on my camera is only on for 10-20 microseconds. That is not near enough time to flip the relay. I needed a circuit that would turn the relay on for at least 100 microseconds or more. This is when the microcontroller became a necessity. Note once again that an SCR does not have this problem of delay, and would be ideal, but I like the relay. I love the click that it makes, and I was determined at this point to use it.
I pulled out my new PICkit1 and started playing with the PIC12F675 (U2 in schematic). I went over an algorithm in my head. I want this micro to do several things. First, I want an LED (D3) to light saying that the camera is charged, and turned on and ready to fire. Second, I wanted it to turn on the main 9VDC (RLY2) flash relay for around Â½ of a second, and third, I wanted to control my small 5VDC reed relay (RLY1) to recharge the camera, and I wanted the LED (D3) to stay turned off for 16 seconds, so that the camera flash could not be fired while the flash was being charged. I found that a Kodak Max has an automatic rechargeable flash, and that all that was needed to charge it was to press the button once. It would then totally recharge itself. This eliminated the need for a circuit that would detect the charged voltage of the capacitor, which is around 300 V, and it would not have been easy to build a circuit like this.
The Kodak Max also has an advantage in that it has a nice sturdy case that is easy to take apart. It makes a perfect project case.
You can see the completed camera above
Here you can see the front two wires that go to the camera recharge, and the two side metal pieces are the flash switch. The recharge wires (Flash Recharge 1 and 2) go to my 5V reed relay (RLY1), and the two flash switch wires (Flash1 and Flash2) go to my 9V trigger relay (RLY2).
I soldered all of my components together on a small breadboard, which you can see in the above picture. I did not have a CDS photocell, so I replaced it with an IR phototransistor. My GP3 port on my microcontroller was my input from the phototransistor, which I set to high +5V through a 10K resistor (R2). I then connected my IR phototransistor (Q3) to +9V, through a 10K resistor (R4), and a 10K potentiometer (R3). I connected the emitter to the base of a NPN transistor (Q2).
GP0 was connected through a 100 ohm resistor (R1) to my reed relay (RLY1). I have this pin set to activate on a Low output. The other pin of my reed relay (RLY1) is connected to +5V. The reed relay then goes to connect Flash Recharge1 and 2. GP4 is connected to an LED (D3) which is also activated on a low output. This is my â€œReadyâ€ LED, which tells when the Flash is ready to fire again. GP5 is connected through a 1K resistor (R6) to the base of another NPN transistor (Q1). The emitter of Q1 is connected to ground, and the collector is connected to one pin of RLY2. The other coil pin of RLY2 is connected to +9v through a 250 Ohm resistor (R7).
I use a 7805 voltage regulator (U1) to supply the +5V needed to power the PIC12F675 (U2) chip.
You can download the code for the PIC12F75 here (Just rename to flashtrigger.asm).
Here are some photos of the camera working. You can see me taking a picture of the flash, and actually using the camera to illuminate my photographic subject. (Note, the camera should not be in the frame, I just have it in the frame so that everyone can see it working).
One final note about my camera, is that I have a Canon 10D, which has a pre-flash that fires so that the camera can calculate how long the real flash should be on. This pre-flash tripped the trigger on my flash which caused it to fire to early. I hardwired a 100 ms delay so that the disposable camera would flash at the right time, and could be seen on my digital camera. In most cases this would not be needed, and you will need to edit the timing on the PIC chip to get it to work with your own camera. I should have made it where the camera will have to flash twice to flip the switch, so I donâ€™t know for sure if the relay will work in a standard camera without a pre-flash. It may take so long to trip the relay (RLY2) that the digital camera exposure will be over before the disposable flash goes off. An SCR would take care of this, and would eliminate any time spent to flip the relay. Just replace the RLY2 with an SCR that is capable of handling 400V, and edit the code to change any delays. Also this code had a switch debounce subroutine, which I edited to make a 10ms delay. (I took the switch debounce program from the pickit1 tutorial and edited it to make my program). Be extremely careful with the capacitor on this flash. It has 300 V when fully charged. Before handling, discharge the camera, and remove the AA battery. Then you can short out the cap with a screwdriver, or a 1K resistor. I am not responsible for any injuries obtained from working on this camera.
One more use for this camera is that it could easily be changed to be used as a high speed flash. A microphone could be used instead of an IR phototransistor, and you could fire the flash with a balloon pop. You could then use a digital camera, and actually make it take a picture with the sound also.
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