AC Voltage Monitor

The Rainbow Kits AC Voltage/DC Voltage Monitor.

In summary, this is a simple kit designed to indicate a certain voltage input level by illuminating certain LEDS in a sequential fashion. The kit can be used to monitor DC voltages as well as in-home AC voltages.

To accomplish this with the standard VDC monitor kit, one needs a simple AC to DC rectification circuit. Notice I say rectification and not regulation...the included wall-wart VAC to VDC power supply has only a simple bridge style rectification and filter capacitor circuit. Any necessary voltage regulation must take place in the kit circuit.

The wall-wart transformer steps the input VAC down (10:1), so the VDC+ripple input to the kit is around 12 Volts.

Why the above label reads "110 Volt DC monitor," I have no idea...it seems a little confusing. RainbowKits.com needs a Net Disaster

The basic part layout is shown above. The supplied parts are shown below.

The kit did not include an enclosure, so I fabricated one out of plastic.

Here is a view of the enclosure and nearly assembled board.

I made the enclosure 'on the fly,' without much thought, and due to that lack of forethought I encountered a few obstacles. Specifically, mounting the LEDs would have been easier if I used hot glue. My enclosure drilling was not precise, so some of the inserted LEDs would not remain in place. The lead length of the LEDs was another challenge; as I soldered them in one by one, I found that no matter how hard I tried to keep the lead length constant, there appeared height variations between LEDs.

I slew the birds by taking pliers and bending their leads like so: < >, staggering them left/right so as to eliminate any shorting potential. This way, the variation in height was accounted for by vertical adjustment, and the same vertical adjustment solved the LED snugness issue by angle variations on the newly "formed" LED leads. Technically, after height adjustment, the LEDs angled out in different ways, so the outward variable force of each LED against the side of the enclosure holes summed was enough to hold the whole lot of LEDs in the enclosure panel.

Upon initial power-up, the measured VDC equivalent input was 12.58 V at input. From this value I concluded that the VAC output of the testing facility outlets was a little above 120 V rms.

The kit allows and encourages calibration. Using a small variable resistor (trimmer pot), one can set the approximate voltage points that ignite the LEDs.

To calibrate for input VAC, I needed a stable and defined VAC. For this I needed a Variac, a variable transformer based AC voltage adjuster.

I figured a trade off point between voltage sag and voltage hump out of my home VAC outlets should be around 110 V rms, so I set the output of the Variac to the amount shown above.

I then connected the Variac output to the wall-wart and adjusted the trim-pot so that a certain number and color of lights were illuminated.

Shown above is my calibrated light output for 110 V rms (In). The first Red, Yellow, and Green LEDS are heavily conducting, and the middle Green is lightly flickering (not shown).

110 VAC is about as low as I want to sag in my studio. Given that there's a nuclear power plant down the road, I probably won't be starved for power. Therefore, if I look over and see that at least two Green LEDs are lit fully, I know I have at least 110 Vrms at my wall outlets.

Any loading of the outlets on a circuit will cause variations in the available outlet voltage, and thats a different story. All the monitor is doing is telling me an approximation of the wall outlet voltage for that particular in-home circuit. For example, when the load on the outlet changes (i.e. the power or voltage/current demand changes,) the voltage output will vary accordingly.

When I hooked the wall-wart back to the original test outlet, the measured value was around 121 V rms. Notice below that the first Red, Yellow, and two of the first Greens are conducting heavily, while the third Green is half-way illuminated. This shows that the device has a roughly 10 V rms difference between 110 (2nd Green) and 120(3rd Green).
The kit does do what it is designed to do.

In use I noticed that the ICs run very hot, and I speculate that a more robust design/mod should include some sort of thermal drain or heat-sink. The high amount of heat produced during constant use suggests the possibility of thermal instability and drift. I tried leaving it "on" for few days and found that the loaded and unloaded voltage levels shown are different when comparing cold measurements to warm measurements.

Though the kit does function as intended, I don't recommend this kit as a
cheap replacement for quality (always-on) VAC monitoring. I do recommend it as a simple soldering and theory project.

Thanks for tuning in-

Andrew