In this instructable I will explain how to make a real (optical) slave trigger flas with a minimum of components.
There are many complex designs you can find on the internet, this design is a very simple one and works well in bright and dim lit environments up to a distance of 5 meters. (Even more in the dark I think, I did not try this out). You do not need a sophisticated expensive flash to make this slave unit, any flash will do, the opto coupler inside the trigger can handle flash connections up to 30 V DC. Keep in mind that the trigered flash will only work in manual mode, not in TTL mode.

Step 1: Components:

1. a flash, I used the sunpak pz40x-ne
2. a piece of printed circuit board, hole distance 0.1", wxh = 15x10 holes
3. an optocoupler IC type 4N37
4, a resistor 1000 Ohm, 1/8 Watt
5. a photo transistor type HW5P-1
6. a hotshoe with cable connection for the flash
7. a black plastic flash foot
8. a USB cable with a type A male connector
9. a piece of MDF wood 12 mm
10. 5 screws 10 mm
11. 1 tiewrap
12. black paint
13. piece of double sided adhesive tape
14. shrink sleeve insulation

Step 2: Assemble the PCB:

First drill the two 3 mm holes for the fixing screws in the PCB.
Then mount the components on the PCB as shown on the picture. Insulate both wires from the photo transistor and mount it appr. 1.5 cm above the PCB surface.
Cut the camera connector from the hotshoe and solder both wires on the PCB. The "+" must be connected to pin 5 and the "-" to pin 4. To check this you can temporally connect the flash on the hotshoe and measure the polarity with a multimeter. If needed solder additional wires to this tiny cable and insulate the connection with shrink sleeve.

Then cut one of the connectors from the USB cable (NOT the A type..) and cut away the metal screenings and the white and green wires. Isolate these green and white wires to avoid they make contact. Connect a pair of short wires to the red and black wire (red is "+" and black is "-", preferably also red and black to avoid wrong connection. Insulate the soldered wire connections with shrink sleeve.

Solder these wires onto the PCB. (connect the red one to the photo transistor and the black one to the 1000 Ohm resistor.

Finally fix all wires (from the hotshoe and the power supply) together with the tiewrap.

Step 3: Building It Together:

Put the black plastic flash foot on the piece of MDF wood and draw the silhouette onto the wood.

Cut out this silhouette with a jigsaw and bolt the black plastic flash foot on top of it with three screws.

If you like you can paint the wooden part black as I did..

Now bolt the PCB on the front side and put the hotshoe on the foot, if needed fix the shoe with a piece of double sided adhesive tape.

Step 4: Using the Slave Flash Trigger:

Swich the flash ON and set it to manual flashing with the desired strength.
(I used minimum strength 1/64 and thus shortest flash time to make high speed water droplet pictures)
Connect the USB A plug to a 5 Volt DC power supply (battery or wall wart).

Happy flashing !

Experimenting might be needed when the flash seems to be too sensitive, you can cover the photo transistor with some material. I used a little white tube from shrink sleeve material with a rubber cap..