Delay circuit for (high speed) photography
This article describes a uC based delay circuit for photographic applications
like drop or high speed photography. It can be used to control the trigger lag of
cameras or flash units. The input and output of the circuit use
standard 3.5 mm jack connectors, so the delay element can be looped into your
existing trigger circuitry like the SmaTrig for example.
As simplicity and budget components were important design goals, there
is no text display or any sophisticated analog input stage. The circuit allows
three delay ranges which are indicated by LEDs:
- 1 s, red LED
- 0.1 s, yellow LED
- 0.01 s, green LED
A short press on the button switches the circuit on and off.
The range is chosen by holding down the button when switching on the delay.
The current range is saved in EEPROM so you don't have to set it each time.
The actual delay is set by two potentiometers. The left one
is used for the coarse time setting (0-100% of the chosen range), the right one allows finer tuning
(0-10% of the chosen range) of the delay time. The coarse/fine solution proved to be very
convenient in practical use (dark room). For drop photography, the absence of a display
turned out to be unproblematic, because you keep your eyes on the drop and not the delay
while adjusting the time.
The minimum lag of the circuit in the green mode for both potentiomenters set to 0 is
about 12 us (microseconds).
The Circuit
The circuit is based on the ATtiny24 AVR by Atmel. With its 14 pins the controller is a good
compromise between the very small Tinys and the bigger ATmega types. The controller
is connected permanently to the battery, because it draws less than 0.5 uA in off-state
(sleep mode).
The potentiometer
voltages are converted into digital numbers by the internal ADC of the AVR.
I think, the other
components don't need an explanation. The second output can be omitted if not needed.
The parallel transistors at each output are necessary to avoid a permanent connection between the shutter
and the focus wire. Otherwise pressing the shutter button half-way will trigger the camera. With Canon cameras
or flashes, the focus transistor can be dropped completely. You can also use a diode instead
(cathode to shutter, anode to focus).
The chip should be mounted on a socket for easier programming. Three types of
batteries can be used with the PCB: CR24xx print type, CR20xx print type or two
AAA batteries when the battery holders are used, see title image above.
The PCB layout can be downloaded below in Eagle format or a PDF. The trace clearance is
rather DIY friendly.
C code
The AVR is programmed in C (AVR Studio). You can download the source code below. The code may serve as an example of how to:
- use ADC in continuous mode
- switch ADC ports
- use the pin change interrupt
- poll a button
- write and read EEPROM
- use timers
- use the sleep mode
on the ATtiny24. Changing fuses is not necessary.
To do
If you like to further improve the circuit I recommend to
- add button to simulate a trigger event (just pull input to ground)
- add a programming port
Download
delay.c - C source code
delay.hex - flash file for ATtiny24
delay.sch - Eagle schematics
delay.brd - Eagle board file
delay.pdf - PDF of board
partlist.txt - part list
Links
http://www.dslr-forum.de/showthread.php?t=837474 - A thread about the circuit in the dslr-forum (in German)
SmaTrig 2.1 - A 15-in-1 trigger with light barrier support for drop photography
Best regards
Don
would it be possible to by a complete buid circuit from you, or at least this programmed IC ?