A “slave flash,” alternatively referred to as a speed light or strobe, is a flash unit that remains inactive until it detects a burst of intense light. Once triggered by an external flash, the slave flash emits its own burst of light. Utilizing off-camera flash units enables photographers to achieve dynamic and captivating lighting effects that would be challenging to reproduce using other methods. In this demonstration, we present an accessory known as the “Optical Slave Flash Trigger,” which proves to be exceedingly valuable when working with a speed light. Essentially, it comprises a highly responsive light-sensitive sensor that detects light pulses, activating the associated electronics that subsequently trigger the connected speed light.
The base is to make things small, cheap, isolated, and independently powered. The best way to do this is to use inexpensive and readily available components as the heart of the circuitry. I prefer not using a microcontroller, even a cheap variant, for such a simple design (it would be an overkill). The idea behind the design is very simple:
- Sense the light impulse (from the master flash) using a standard light sensor
- Verify the light level is above a constant threshold (which can be altered using a potentiometer to tweak the detection sensitivity)
- Trigger the save flash through a galvanically-isolated (contact-free) electronic switch
This little circuit works on the principle of Wheatstone bridge. When light falls on the phototransistor (PT1), its conductivity (and current) increases which makes the driver transistor (TR1) forward biased. As a result, the opto-isolated triac (OT1) wakes up to trigger the slave flash (thanks to an optical coupling, it enables safely to use even the oldest flashgun that could otherwise damage your circuitry by high voltage ignition). However, you need to be aware that the connected slave flash doesn’t work when the ambient light is too strong i.e. during spacious daylight. Likewise, it may even misfire when you have a very bright source of artificial light in its path. Potentiometer (RP1) here helps you to set a light detection threshold the right way.
The light sensor utilized in this design is a standard 5mm round phototransistor. Specifically, it is a high-speed and highly sensitive silicon NPN phototransistor. Its clear epoxy covering renders it suitable for detecting visible light and infrared radiation.
The opto-isolated triac employed in the circuit is of the MOC3021 type. It comprises a gallium arsenide infrared emitting diode, optically linked to a silicon bilateral switch. Given that the CR2032 Lithium Manganese Dioxide battery provides a nominal voltage of 3V (with a nominal capacity of 220mAh), the entire circuit can be comfortably powered by it, without any significant challenges.
To establish a connection between the fully assembled slave flash trigger circuit and the slave flashgun, a female hotshoe connector is necessary. The simple operation required to trigger the flash when it’s placed in the hotshoe is to create a short circuit between the two trigger contacts. Therefore, you can easily extend the switch output (SW1 & SW2) of the circuit to the electrical (trigger) contacts of the hotshoe using an integrated or standalone sync cable.
Flash ‘n’ Camera
Within the realm of artificial light sources, Xenon is renowned for closely approximating the natural color spectrum of the sun within the visible range, spanning approximately 380-750nm. Consequently, Xenon is extensively employed in camera flash systems. In an electronic flashgun, a high-voltage capacitor plays a pivotal role in storing electrical energy for subsequent utilization. Upon triggering the flash, the capacitor discharges its stored energy through a flashtube. This Xenon-filled flashtube emits an exceptionally intense and brief burst of light.