Technical Application Note TAN2005004
Revised August 24, 2014
Technical Application Note (TAN2005004): Buffering a GPIO pin strobe output signal using an optocoupler to drive external devices.
This TAN applies to all PGR Imaging Products that implement general purpose input/output (GPIO) functionality and the ability to output a strobe pulse. This includes:
The purpose of this Technical Application Note is to provide the user with the ability to drive an external device, such as a strobe light, LED, or other apparatus, that requires more power or voltage than the Applicable Product’s GPIO pins are capable of generating.
For example, the Flea GPIO pins are TTL 3.3V pins protected by two diodes to +3.3V and GND in parallel. The pins have almost no drive strength (less than 1mA), and are therefore unable to trigger many external devices, such as the X-Strobe™ strobe.
In order to trigger such a device, which requires a +5V TTL pulse and 20mA of current, the signal off the GPIO pins must be buffered with a transistor or driver, such as an optocoupler, to lower the impedance. This TAN describes alternatives for users who wish to:
The Applicable Product(s) is/are equipped with a set of general purpose input/output (GPIO) pins that can be accessed via the GPIO connector on the back of the camera. Different products may use different connectors; consult your camera’s Technical Reference or Getting Started manual for part numbers and specifications, GPIO connector pin layouts, and GPIO electrical characteristics.
To configure and test the information presented in this TAN:
An optocoupler with a buffered input, such as a Fairchild H11L1M, may work for a variety of situations. This part can be found at:
See the figure below for an example test circuit, showing a PGR Flea connected to the Fairchild H11L1-M, which is connected in turn to an LED.
Figure 1: Fairchild H11L1-M Test Circuit for PGR Flea Camera
For users with electronics or electrical engineering knowledge who wish to build their own circuit to attach to (for example) an LED:
See the figure below for an example circuit diagram. In the case of the X-Strobe™ device described above, the NPN can be attached directly, since the optoisolator and resistor is internal to the strobe light.
Figure 2: Example optocoupler interface for PGR Flea camera