Software and Hardware to control the power of old Canon EOS TTL flash/speedlite.
##Rationale
Digital EOS cameras uses E-TTL/E-TTLII flash system which requires compatible E-TTL/E-TTLII flashes also known as speedlites in Canon world.
Film EOS cameras uses older TTL/A-TTL flash system with requires TTL/A-TTL compatible speedlites.
Newer E-TTL speedlites, with model ending in "EX", are backwards compatible - with a few exceptions - with A-TTL film cameras. Older A-TTL speedlites are only usable in digital EOS in manual power, with no automated TTL metering/exposure.
Few film-era speedlites does not have dedicated user interface to control flash power, as is the case of Canon Macrolite ML- 3, firing always in full power with non-TTL cameras and rendering the unit pratically useless in real situations without workarounds such as the use of ND filters on flash head as shown in "Adapting a Canon ML3 Ringflash for Digital".
This project aims to build a small and simple power controller for EOS TTL speedlites.
##The Concept
Canon A-TTL/E-TTL/E-TTLII speedlites have a hotshoe pin called "quench" that shutdowns the flash light before full flash duration is reached. This mechanism provides a way to control the amount of flash power iluminating the scene as shown in "Adding manual power control to the Canon 200E" (Rudy's Rants blog).
The general idea of this project is to control the quench time to generate the right amount of flash duration corresponding to a user selectable power.
Long story short, we need to detect the flash trigger signal (comming from the camera) and react (almost) instantly by controlling the quench pin as a real EOS film camera would do in TTL mode.
##Proof of Concept
A simple test using Arduino UNO was made as a proof of concept and after a few tweaks the following code arised:
int newPotValue = 0;
int potValue = 0; // value read from the pot
int flashDelayTime = 0;
void setup() {
Serial.begin(9600);
pinMode(2, INPUT_PULLUP);
pinMode(8, OUTPUT);
pinMode(9, OUTPUT);
attachInterrupt(0, flash, FALLING); //external push-down button to trig the flash
digitalWrite(8, 1);
digitalWrite(9, 1);
}
void loop() {
//simple loop to calculate new delay time when pot value changes and to print it on serial
newPotValue = analogRead(A0); // Analog input pin that the potentiometer is attached to
if (newPotValue != potValue) {
potValue = newPotValue;
flashDelayTime = potValue * 10;
Serial.println(flashDelayTime);
}
delay(100);
}
void flash() {
noInterrupts();
digitalWrite(9, 0); //quench pin start (active-low)
digitalWrite(8, 0); //trigger pin start (active-low)
delayMicroseconds(flashDelayTime); //flash duration delay
digitalWrite(9, 1); //quench pin end
digitalWrite(8, 1); //trigger pin end
interrupts();
Serial.println("Flash");
}
##The Prototype