We’ve had our tropical aquarium for eighteen months now and I find the kids like taking occasional trips to the fishy shop to help choose new residents and they seem to enjoy arguing with each other over who is going to feed the fishy fellows.

I’ve been thinking for some time about starting a cold water tank and getting some white cloud minnows to place in it. So it was that last weekend we purchased a Baby Biorb, a 21st Century goldfish bowl with integral halogen light and filtration system.

My Biorb
My Baby Biorb on some cube shelves. You can get square Biorbs but I didn't know that until after I bought mine. Bah!

While browsing the ‘net for funky accessories for my Biorb I came across some interesting lighting. The full size Biorb has a LED lighting accessory which cycles through a lighting sequence over a 24 hour period with blue ‘moonlight’ gently illuminating the tank at night gradually fading into white lighting in the day which increases in brightness before finally fading out and back to the blue for the night shift.

The Baby Biorb has a similar accessory with a single blue LED fitted into the halogen lighting unit so the halogen light illuminates the tank by day and the blue LED gives it a ‘moonlight’ glow at night.

The interesting thing is, the moonlight and non-moonlight halogen units are made from the same mould so the non-moonlight unit has a hole where the blue LED would ordinarily be fitted.

Now I can’t leave a LED size hole in anything – it’s just gonna annoy me, so I figured I’d spend my Tuesday evening converting my halogen unit into a moonlight unit.

Annoying hole
Annoying hole (arrowed). That's going to bug me until I fit an LED into it.

First thing to do was unclip the lighting module which is easy as it is made to come apart so that the halogen bulb can be replaced when it burns out. Next remove the bulb without touching it with your bare skin as the grease will shorten the bulb's life. See all those screws?? They're going to need to be removed so that we can get deeper into this thing.

Here's where things get interesting. The Biorb has a mains AC transformer that drives the air pump and halogen lamp. The output of this transformer is 12V AC however my particular LED is going to require 3V DC. We're going to need some half wave rectification ACTION for this job....

... and a resistor.

We're also going to need to prise open the inline switch using a small flat blade screwdriver. This is so we can tap off the line to the transformer before it goes through the switch allowing the LED to remain illuminated regardless of whether the halogen lamp is on or off.


So here it is. The top bit is the existing Biorb circuitry - a simple two wire halogen light with an inline switch. A wire has been spliced into the switch enclosure on the transformer side and feeds through a 220 Ohm resistor into my blue LED.


Connected to the LED is a 1N4148 diode whose job is to provide half wave rectification by blocking the reverse current. Yeah, I know the LED is itself a diode however a rectifier diode such as this has a higher reverse breakdown Voltage than a standard LED. Finally, a fuse is fitted which is always a good idea when fitting electronics near water and the circuit completes by wiring to the other side of the halogen bulb.


The rectifier diode blocks the reverse current leaving only a positive forward Voltage to drive the LED. It doesn't provide full rectification so the LED is effectively 'pulsing' on and off at 50Hz. This is just on the periphery of human vision so if staring directly at the LED it is noticeable however when it is lighting the tank the movement of the water and bubbles negates this pulsing effect.

With so few components and so little space it wasn't worth a stripboard build. Instead, the components were soldered directly together within the light housing.


You can see at the bottom in the above picture the extra third (grey) wire between the light unit and the switch. The picture below shows the unit after reassembly with the LED on and halogen lamp off....

LED on

... and this picture (below) with both the LED and halogen lamp on...

LED + Halogen

The LED uses very little power which is further reduced by the fact it is pulsing and not permanently illuminated at full brightness. My Ammeter measures 20mA as an average current. With the halogen lamp on, the LED is strong enough to illuminate the bubbles from the central filter tube of the Biorb while deactivation of the halogen lamp leaves the desired blue glow gently illuminating the tank in the dark.

Don't believe me??  Well, I wouldn't either, so here are a couple of snaps...

Biorb blue 1

A picture taken with the flash (above). The LED is strong enough to illuminate the bubbles leaving the filter tube.

Biorb blue 2

This snap is taken without the flash and shows the strength of the illumination more clearly.

Now I just need to get the kids to buy a couple of minnows to put into this thing. The kids won't appreciate my efforts but the fish might.

Like this? Well, the tropical tank didn't escape some LED action either. See here.