View Full Version : DIY: Add 6 Full Stops to Vivitar 285HV

Bill Markwick
06-06-2007, 01:21 PM
The Viv is a great flash with lots of features at a good price, which is why it's been around for 30 years. However, the manual range switch looks like an afterthought, what with the 1/8 power position missing, and 1/32 isn't there either, although the flash is capable of it. Here's a project that extends it from four settings to six.

Note: there's an older model of this flash called the 285 (without the "HV"). Some of these use 100V trigger voltages (or more) and are not suitable for this project. Be sure you have the "HV" type.

Opening the Flash and Why You Don't Have To:
It turns out that opening the 285HV is no easy task, not to mention the surprise value of meeting the main capacitor when it's charged to several hundred volts. I retired the project until it occurred to me that the sensor module pulls out easily and comes apart by removing two screws. There were the two pins I needed. The methods shown below won't damage your flash (or you). There are no voltages on the circuit until it fires, and even then they're very low. No hazard at all.

A Note on the Pin Numbering:
There are web pages that erroneously list the 285HV's numbering as starting with the center pin as pin 1. This comes from the similar Vivitar 283; the numbers I've shown are molded right into the sensor module plate.

The Switch Choice
The next problem is the switch. The deluxe method would be to replace the Viv's switch function entirely with an external 6-position rotary, but I couldn't find one that didn't stick out of the flash like a giant wart. For this reason, I added a subminiature single-pole double-throw switch with a center-off position. If you have a small rotary or don't mind a big one, see Fig. 5.

The Toggle Switch
In the upper position, the switch changes 1/4 power to 1/8. In the lower position, 1/16 power becomes 1/32. Juggling the Viv switch and the toggle is not as convenient as a rotary that does everything, but the toggle is very simple to install.

So what happens if you accidentally leave the toggle switch up or down for other power settings? Well, nothing is harmed, but there'll be an error. With the 1/32 setting, all modes become more or less 1/32, including the auto ranges. With the 1/8 setting, M and 1/2 become about 1/4, and 1/16 drops about a half stop.

Speaking of stops, the resistor values don't follow the expected divide-by-two - I suspect this is because the auto-range photocell is non-linear and needs some correction, so the manual-range resistors have to follow suit. I chose the two added resistors by shooting gray-card sequences and checking the intensity with Photoshop's eyedropper tool. This is not a precision circuit, but the stops should be accurate to +/- 1/3 EV.

Building It
You'll need a 91k resistor (or a 100k in parallel with 1M) and a 22k resistor. Any size or power rating is fine. Any small two-conductor wire will do - it doesn't need shielding. Polarity doesn't matter - it'll work fine even if you accidentally reverse something.

Pull out the sensor and remove the two screws and the pin plate. Drill a small hole in the sensor to suit whatever two-conductor wire you have (see Fig. 1). Solder the added wires to pins 1 and 2, on top of the red and black Vivitar wires. Pass the other end of your wire into whatever enclosure you're using, and then solder it and the resistors to the switch. Reassemble the module and fasten the enclosure to the flash with a mounting tab, epoxy, tape, or whatever. That's it.

You might well ask, "Wherever did you get that elegant switch enclosure?" With great effort and expense, I might add - it's the plastic container for breath mints. :p


Fig.1. The sensor module with its pin plate removed. The
added wires go on top of the existing red and black wires.

Fig. 2. The original Vivitar sensor resistors for a four-position range (which is actually five stops because of the missing 1/8).

Fig. 3. The added switch and resistors.

Fig. 4. Wiring the toggle switch.

Fig. 5. For those who like a challenge, the rotary switch method replaces the function of the Vivitar range switch (which you leave in the M position).

Fig. 6. An external view.