I built an audio alarm circuit for my DuraTrax IntelliPeak Twin Pulse Charger. The alarm tells me exactly when both battery packs have peaked. Even if I'm not in the same room, I immediately know when it's time to change to another set of batteries. I find this to be a great time saver because the alarm doesn't let me forget that I have batteries on charge. This is a simple and inexpensive (about $30.00) optical detection circuit that merely monitors the status of the red and yellow LEDs on the charger. No modifications to the charger itself are required. You'll need the following items from Radio Shack:

2 phototransistors 900-6133 2x$.49

1 N-channel MOSFET 276-2072 $1.99

1 12 volt dc mini buzzer 273-055 $2.99

1 1 mega ohm resistor 271-1356 $.99

1 project box 270-1806 $4.79

1 120ac/12dc power supply 273-1773 $12.99

1 ON/OFF switch 275-624 $2.99

Radio Shack does not stock the phototransistors, but they can be ordered. You'll also need some styrene tubing with an inside diameter of about 1/4 inch. Maybe you already have some of these parts or similar substitutes in one of your scrap boxes.

The idea is to have two styrene tubes projecting horizontally from the side of the project box. The project box will be placed in front of the charger with each tube covering an LED and fitting flush against the face of the charger. Each tube will contain a phototransistor.

The project box is oriented with one of the long sides down. That is, the original long side will be the bottom of the box for this project. The original bottom will be the side of the box facing the charger and the lid will be the side facing away from the charger. As said above, the original bottom will be the side facing the charger so this is the side from which the two styrene tubes will protrude. Drill two holes in the project box, one for each tube. The tube holes should be about 2 1/2 inches from the tabletop and about 1 3/4 inches apart. I suggest that you put some rubber feet on your project box. Don't forget to account for the height of the feet when you take your measurements. For a tight fit, drill holes smaller than the outside diameter of the styrene tube then use a reamer to precisely make the holes exactly the same size as the outside diameter of the styrene tube. At this point you can also drill holes for inserting the power cord and for mounting the switch and buzzer.

Cut two pieces of styrene tube about 3 inches long. One end of each tube needs to be beveled such that it will fit flush against the sloped face of the charger. Insert each tube into one of the holes in the project box and glue with liquid styrene solvent. Prop up the beveled ends of the tubes around the LEDs. This will hold the tubes is proper alignment while the glue sets. I made several right triangle shaped braces from sheet styrene. After the tubes were set in the box I glued the braces between the tubes and the box for extra strength. Imagine the tubes are rockets and the project box is the launch pad. The braces look like fins at the base of the rocket.

Given the small number of components, I dispensed with any sort of circuit board and just soldered the components together using scrap hookup wire and shrink tube. The lead closest to the flat side of the phototransistor is the collector. The other lead is the emitter. Hookup wires to the phototransistors should be long enough such the phototransistors can be pushed all the way in to the tubes. The emitters of both phototransistors are connected to ground. The collectors of both phototransistors are connected to one end of the resistor. The other end of the resistor is connected to one end of the ON/OFF switch. The other end of the ON/OFF switch is connected to the positive supply voltage. The source of the FET is connected to ground. The gate of the FET is connected to the junction between the phototransistors and the resistor. The drain of the FET is connected to the negative lead of the buzzer. The positive lead of the buzzer is connected to the junction between the resistor and the ON/OFF switch. You'll, of course, want to mount the buzzer to the outside of the project box before connecting it to the rest of the circuit. I just chopped the dc connector off the power supply cord and hard wired it to the rest of the circuit. Use a voltmeter to determine the polarity of the wires from the power supply.

From inside the project box, push a phototransistor into each styrene tube. Push the phototransistors all the way in such that they will almost be able to touch the LEDs. A blob of silicon sealer on the project box end of each styrene tube will secure the wires and hold the phototransistors in place. You may want to glue some weights to the inside bottom of your project box to give it more heft. That's it for the electronics. Put the lid on your box and you're almost ready to go.

I found that the phototransistors are very sensitive to red light, but not very sensitive to yellow light. I had to use a high value resistor (1 mega ohm) in order to make the circuit sufficiently sensitive to reliably detect yellow light. Unfortunately, the high resistance value caused the circuit to be overly sensitive to ambient room light. I took a few steps to minimize the effects of ambient light. First, make certain that the beveled ends of the tubes exactly match the slope angle of the charger face. Brush several coats of black paint over the white styrene tubes and braces. Don't paint the inside of the tubes. Get a package of self-stick foam body washers (Traxxas #4915). Stick two washers on the face of the charger, one washer around each LED. The inside diameter of the washers is larger than the outside diameter of the LEDs. Thus a circular gap is formed around the LEDs into which the styrene tubes fit almost perfectly. The washers make an effective light gasket between the face of the charger and the styrene tubes.

Place your alarm in front the charger with each styrene tube covering an LED and fit flush and tightly against the face of the charger (here's where the extra weight and rubber feet help). Test the alarm by applying power to the alarm with no batteries connected to the charger. If the alarm does not sound, there may be too much ambient light in the room. Turn down the room lighting or pull down the shades. Having taken the light insulating precautions noted above, I've found that the alarm will work in all but the most brightly lit rooms. Now connect a pair of batteries to the charger. The alarm will not sound so long as one or both of the LEDs are continuously ON. When both LEDs begin to blink (indicating that both batteries are charged) the alarm will sound. The alarm will also work if you are charging only a single battery pack.

The phototransistors are ON in the presence of light and OFF in the absence of light. The FET inverts the state of the phototransistors and amplifies the signal to the buzzer. When one or both of the phototransistors are continuously ON, the FET and buzzer are OFF. When both phototransistors are OFF, the FET and buzzer are ON. Both phototransistors are only OFF when both LEDs begin to blink. Since the two LEDs do not blink in synchronization, the buzzer will make a random pulsing sound.

Disclaimer... I am an electronics amateur. I designed this circuit by way of tinkering and trial and error. There is no CAD system. There is no laboratory. There are no research assistants. There is no UL approval. I’ll not be responsible for you blowing up your charger, burning down your house, electrocuting yourself or any other havoc you may cause in your attempts to duplicate this project. Hope someone out there finds this useful.

Update

The charger is fried. I got about a year's use out of my Twin Pulse before it went up in smoke. Looks like three or four components on the circuit board burned up. I may try to fix it, but for now I'm relying on my four Super Brain chargers.