Avon Blacklight  modifications

Avon ultraviolet unit

The following modification applies to the Avon Derma Spec Skin Imager and the fluorescent driver board contained within. Both items can be obtained from Electronic Goldmine as listed below. With a little work, you can build a really nice portable ultraviolet light you can use out in the field. 

If you can't find them there, here's another source at a slightly higher price. Naturally, the complete unit is the better deal, especially if you want a case and 12 volt wall wart power supply. The 12 volt replacement circuit board might be useful if you want to practice the mod before doing it on the complete unit (which is exactly what I did, since I had no idea how the circuit worked at first) or for parts if your original unit should stop working, but I prefer the new 6 Volt circuit board now being offered. The 12 Volt unit runs for less than a minute and automatically turns itself off, as do the 12 volt replacement boards. The purpose of the modification is to allow the unit to run continually. The reason it doesn't is poor design - it draws too much current in order to make the lamp operate a little brighter. This causes the driver transistor (Q4) to run too hot and also draws more current from the power supply than it was designed to handle.

Since I first wrote this, the Electronics Goldmine is offering a newer Derma-Spec driver board that runs on 6 volts rather than 12 and does not shut off after one minute. If you can get this board, you can replace the board in your Derma-Spec unit and have a unit more suited for battery power (uses half the power) and without the annoying one minute turn off feature. The back of the Derma-Spec unit even has a stash box big enough for 7.2 volt rechargeable lithium  ion batteries (which will run the 6 Volt driver board just fine) such as those sold by Electronics Goldmine (G4580). I've tried this and it works great. The 1.1 amp hour batteries will run the lamp for about two hours--not as great as most camping lanterns, but a lot smaller, plus it can be charged by solar power in areas without electricity. Below are two photos to help you identify the two driver boards. 

12 Volt driver board 6 Volt driver board
The original 12 Volt board  ( with Q3 removed for mod)
The newer 6 Volt board
The 12 Volt replacement boards I bought from Electronic Goldmine were minus the white push button, but the newer six volt boards seem to come with it. Since I have just gotten the newer board, I don't understand the circuit operation as well as the 12 Volt board, only that it is a vast improvement. I hope to figure a mod for the 6 Volt board to make it auto-start rather than having to push the button, which would make it more useful for solar lighting, where you want it to turn on at dusk and off at dawn. You are by no means limited to using this unit as a blacklight. All you have to do is replace the ultraviolet lamp with a white one.

All the Derma-Spec units I ordered had the older 12 volt boards, but that doesn't mean there are not any 6 Volt units for sale. I just haven't seen any. They may have discontinued the product before the 6 Volt units were actually built and only have the circuit boards for them.

There are basically two options for upgrading your Derma-Spec unit, modifying the existing 12 volt driver board as detailed  below, or replacing it with the 6 volt board, which in my opinion is the better option. If you replace the driver board, you will also need you woll also need to install a voltage regulator in the unit or get a 6 volt wall wart or other power supply that can supply 500 mA, and will probably have to replace the plug on it with a 5.5x2.5mm plug to fit the jack on the unit (more on this below). In either case, you will probably want to tie the lamp socket contacts together as mentioned below to prolong the life of the lamp.

The following modification to the 12 Volt board does the following:

Another modification you should make even if you replace the 12 volt board with the 6 volt one is to install a jumper on the lamp sockets so the connection goes to both pins at the end of the lamps as shown below.

To make the modification, you will need the following:

Now for the steps to make the modification for the 12 volt board:
Lamp socket mod
transistor jumper
The lamp will operate a little dimmer now, but neither Q4 or the wall wart will overheat. I left my unit on all night to make sure there were no overheating problems and it passed the test. A 4-watt fluorescent lamp has a rated life of 6000 hours and I have no idea how this mod affects that. Only time will tell.

Adding rechargeable batteries to the unit: As I mentioned above, there is enough room in the hidden compartment in the back of the unit to add rechargeable lithium ion batteries, such as Goldmine (G4580). After removing the batteries from the cell phone module, I mounted them on a perf board and attached that to the unit with 4-40 bolts. I also added a switch to disconnect the batteries when I wanted to run the unit on AC. Here is a picture of the unit with the batteries installed.

Batteries installed in unit

A note about the wall wart connection: The plug on the wall wart is a 5.5mm x 2.5mm power plug. You can probably find one of those at Radio Shack (274-1569).

Now for a few notes about the circuit. Most of the circuitry is part of the auto turn off feature, if you can call it a feature (I call it a bummer myself.) This includes the MC 14538 IC, which is a monostable multivibrator. I'm not sure why they used that IC rather than a 555 timer but it has the same function. When you push the button on back of the unit, it turns the unit on and starts the timer counting down. The output of the IC seems to drive Q1 and Q2, which in turn drive Q3, which is the transistor that switches the current on and off to the actual inverter circuit (and the red LED under the button).

The inverter circuit is a transistor oscillator that converts the DC current (supplied by the wall wart) to high frequency AC that the transformer steps up to the high voltage needed by the fluorescent lamp. Since the intended application (skin care) does not require battery power (it runs on a wall wart, which requires line voltage), I'm not sure why the inverter circuit is used in the first place, rather than a conventional fluorescent lamp ballast that operates directly on the AC line voltage. Also, if they had spent the money on the inverter circuit rather than on the auto turn off circuit, they could have made it so it could handle the extra current without overheating in the first place. Well, as the old saying goes, their loss, our gain. Even in the olden days, a battery powered blacklight would set you back at least $30 and required expensive 90 Volt or two 45 Volt batteries. With a little work, the Avon unit makes a nice UV field unit or camping lantern and is much brighter than those UV keychain LED lights that Electronics Goldmine had for $10 and the UV pen lights that Edmond Scientific charges $150 for. 

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Copyright 2002, Colin Pringle
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