Slave Flash Trigger Circuit

This is the Richard R. Bucciarelli Slave Flash Trigger circuit, which can be built entirely from Radio Shack parts. It senses a camera flash and will trigger another flash that has some sort of input conection that one shorts to trigger a flash.

This circuit is designed to sense a sudden increase in light and does so extremely well. A few of these were actually built and tested and found to not trigger from normal room light. They sometimes malfunctioned if the phototransistors were aimed at fluorescent lamps at close range. They have been known to sense even small camera flashes very reliably from 200 feet away and fairly reliably from 500 feet away. Your performance may vary depending on the qualities of your phototransistor and the brightness of the triggering flash.

It is recommended to mount the phototransistor in a small, narrow tube to shield the phototransistor from bright light.

This circuit was typed up very late at night Tuesday night 2/24/98 and is subject to error correction (mostly done late 2/25/98) and additional construction until mid-March 1998.

The four comparator stages are a LM324 quad comparator.
B+ is 9 volts.  A 9 volt battery works.  This circuit works approximately
from 6 to 12 volts.
Power supply conections for the 324 are not shown in this schematic.

    B+       B+                               |
     |       |  .047uF 2.2K    220K           >220K
   |/C       >  +-||-\/\/-+---/\/\/-+         >
PT |    220K >  |         | |\      |         >
   |\E       >  +gnd      +-|- \    | 1N914   |    |\
     |.0015uF|              |   >---+--|>|----+----|+ \
     +---||--+-----/\/\/----|+ /              |    |   >--+--------+
     |       |     2.2K     |/                >  +-|- /   |        |
     |       >     B+                    100K >  | |/    === 22uF  |
     |  100K >     >       +-------+          >  |        |        |
     |       >     >100K   | |\    |          |  |        >        |
     |       |     >       +-|- \  |        gnd  |        > 22K    |
     |      gnd    |    2.2K |   >-+-----/\/\/---+        >        |
     |             +--+-/\/\-|+ /  |      10K    C\| NPN  |        >
     +----+        |  |      |/    | |\            |---+--+        >2.2K
     |    |        >  |            +-|- \        E/|   |  |        >
     >    |   180K > ===.68uF        |   >-+     |     |  >        |
10K  >    |        >  |            +-|+ /  |     +-|>|-+  > 22K    |
     >    |        |  |            | |/    >     | 1N914  >        |
     |    |       gnd  gnd         |       >     +gnd     +gnd     |
    gnd   +------------------------+  3.3K >                       |
                                           |      +--|<|---+       |
                                           -  B+  | 1N914  |       |
                                      LED1 V   +--+-UUUU---+       |
                                           -       relay   |       |
                                           |       coil    C\| NPN |
                                         gnd                 |-----+
                                                           E/|     |
                                                           |       |
                                                           >       -
                                                   68 ohms >  LED2 V
                                                           >       -
                                                           |       |
                                                          gnd     gnd

Schematic is copyrighted (c) 1998 by Richard R. Bucciarelli.


1.  The relay is a Radio Shack 275-232 5 volt reed relay.  Use the contacts
(not shown above) to short the trigger contacts of the flash to be
triggered.  This is for use with a flash that is triggered by shorting the
trigger input of the flash.

2.  The phototransistor, labelled "PT" in the upper left portion of the
schematic, can be an infrared or visible sensing type.  If stray light is
bright enough that enough current flows through the phototransistor to
make the voltage across its associated 10K resistor exceed 2/3 of the
suupply voltage, then you should shield the phototransistor.  You can
mount the phototransistor in a small tube aimed at the flash that this
circuit is triggered by, or aimed at something illuminated by the
triggering flash.

3.  LED1 can be any type that works well at low currents but in Rich
Bucciarelli's embodiment, it is a high brightness GaAlAsP type red. The
dropping resistor can be reduced if power consumption requirements are not

  LED1 glows if bright light is sensed by the phototransistor, regardless
of whether this light increases in brightness rapidly enough to trigger
this circuit.

  If the background or room light causes LED1 to glow, then the
phototransistor is saturated or nearly saturated and this circuit's
sensitivity to flashes may be impaired. Should this happen, it is
recommended to shield the phototransistor from this light.

4.  LED2 is a high brightness orange InGaAlP type.  If any substitution is
made, care must be taken to have a similar LED voltage drop for proper
relay operation. If an LED with a higher voltage drop is used, then the
a higher value emitter resistor should be used for the associated NPN
transistor.  Use a lower emitter resistor if an LED with a lower voltage
drop is used.

  LED2 glows when this circuit triggers.  This circuit triggers due to a
large and rapid enough increase in light being sensed by the
phototransistor.  LED1 does not necessarily glow.
  (This circuit has been known to sometimes trigger unintentionally from
sudden dimming or shutoff of a bright light.)

5.  It is recommended to have a non-electrolytic capacitor of a few tenths
of a microfarad to one microfarad across the power supply leads of the 324
quad comparator, and in close proximity to it.  Ceramic and "monolithic"
types are best.  In addition, it is recommended to have at least 10
microfarads of capacitance across the power supply anywhere in this
circuit between the 324 supply leads and the battery, but on the board
this circuit is constructed on.  This capacitor can be electrolytic.

6.  The 1N914 diodes can also be any similar type such as 1N4148.

This circuit was a project of and mainly developed by Richard R. Bucciarelli with some assistance by Don Klipstein. All questions and suggestions should go to Don Klipstein however.

Document was written by Don Klipstein.

Please read my Copyright and authorship info.
Please read my Disclaimer.