In this instructable I will explain how to make a real (optical) slave trigger flash 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. 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 triggered flash will only work in manual mode, not in TTL mode.

I upgraded my first slave flash, earlier published in this instructable:

https://www.instructables.com/id/Slave-Flash-Trigg...

The new version (Mark II) has only one USB power connection for the optical sensor and the flash itself.
It no longer needs batteries....
And it now has a very sleek mounting foot with all wiring and electronic parts hidden inside.
During startup the flash needs appr. 400 mA, so you will need a strong wall wart USB power plug. Do not connect the flash to an USB connection on your PC or laptop, it will need more power than available there. It could even damage your PC or laptop. Be careful.

Step 1: Used 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 USB cable with at least one A-male connector

7. a piece of MDF wood 12 mm and 12 mm

8. 7 screws 10 mm

9. 1 tiewrap

10. black paint

11. shrink sleeve insulation

12. 3 diodes, rating minimum 1 Amp

13. a 2amp USB wall wart

14. 2 round wooden sticks, dimensions the same as an AA battery

15. wires 1 sq mm (preferably red and black)

Step 2: 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.

Step 3: The Mounting Foot and the Flash

Make the mounting foot as shown on the picture from pieces of 6 mm and 12 mm MDF wood. Drill the holes for the 4 bottom screws in the bottom part. Drill the holes for the flash trigger wires, the flash power connection and the light sensor in the top cover. Finally drill the hole for the USB power wire in the side of the 12 mm middle part. Glue the 12 mm middle part and the 6 mm top cover together. Sand the outside if neccessay.
The hotshoe of my flash was damaged, so I removed it, carefully cutting the little wires inside the flash. I checked which wires were used for triggering the flash. The black and blue wires were used so I mounted longer wires to these wires, leading inside the wooden mounting foot. When the foot is ready, mount the flash on the top cover with strong 2-component glue.

Step 4: The Flash Power Connection

The flash I used could not be completely disassembled. To connect power wires to the flash instead of the two AA batteries, I made two "fake" batteries from two wooden sticks with a diameter of 10 mm and the same length as the batteries. I drilled a hole through these sticks for a wire. Then I soldered these wires each on the sharp end of a 10 mm screw and mounted these screws inside one end of each stick. Finally I drilled a hole in the side cover of the battery compartment of the flash for the wires.

Step 5: Final Wiring

Fix the PCB inside the wooden foot (with one screw and a little piece of 6 mm MDF wood) with the sensor sticking through the top cover. Wire all components together inside the mounting foot according to the diagram and the picture. Use shrink sleeve for insulation and take care of the diodes not touching any wires (they might get a little hot). Use some shrink sleeve for mechanical protection on the inlet of the USB power wires and mount a tiewrap around these wires on the inside so they cannot be pulled out. .

Step 6: The Final Touch

Paint the wooden foot in a nice matt black color. Cover the sensor with a little white plastic shrink sleeve (not shrunk) and a black cover on top to make it less sensitive (needs experimenting for the best result).

Swich the flash ON and set it to manual flashing with the desired strength.
(I used minimum strength 1/64 so it has the shortest flash time to make high speed water droplet pictures)

That's it.....