Flagpole light with burned out bulb sensor.

As you can see by my banner I am a Veteran. I’m proud to have served my country. When I purchased this house it was the last “spec house” in the development. The developer was anxious to sell. As added incentives I was offered an automatic garage door opener, a seeded back lawn, a fireplace and a refrigerator. I suggested that I had always wanted a flagpole so the developer told me that they could add a flagpole. Therefore I have the only 25 foot commercial flagpole in the development. Some of my neighbors refer to me as the “Flagpole Guy”. I’m not always home at sunrise and sunset so instead of raising and lowering my flag daily I have it illuminated at night as required by protocol. The light the developer installed was installed in the ground with the switch inside the front door. I automated the light with a X-10 light switch module to go on at sunset and off at sunrise daily. This turns on the 110 volt to 12 volt transformer that powers the light. The light the developer installed in front of the pole was damaged several times by vandals. Here in Arizona many of us have desert landscaping in the front with stones and rocks. They would use the rocks to break the glass and the bulb. This year I bought a new light system that mounts on the corner of the garage roof. It’s a 12 volt, 50 watt quartz-iodine bulb floodlight. Within two weeks of installing it the bulb burned out. I didn’t know how long my flag was un-illuminated and that embarrassed me. So I decided to build a monitoring system with an indicator LED inside, to tell me if the light was powered and if the bulb was burning. I used a dual color red and green LED where green meant the light was powered and the flag was illuminated and the red LED to tell me the light was powered but the bulb was burned out. Next to the inside door to the garage I have a control to open and close the garage door. I have a large green LED mounted on it to tell me if the garage door is open or closed. I decided to mount the flagpole sensor on the same housing. I glance at it when I go to bed and two green LEDs mean the garage door is closed and the flagpole is lighted.

Remote indicator - Green.

Both LEDs are green.


Remote indicator - Red.

Light is powered on but bulb is burned out..

To just tell if the flagpole light was turned on would be simple enough. Just rectify the AC from the 12 volt transformer powering the light and light a green LED. The flaw is that it won’t tell me if the bulb has burned out. I needed a circuit that monitored the current going to the bulb. Burned out bulb, open circuit, no current flow. Insteon makes a nice module for Home Automation called the I/O Module.

Insteon I/O Module.

Insteon I/O Module.

The module integrates the burned out bulb sensor with my home automation system. It makes the status of the flagpole light available on the home automation system where I can use it to to control other devices if I want. For instance I could make a failure of the bulb cause the home automation system to flash a house light. For now I’m content with it operating a LED. The module contains a form “C” contact, that consists of a common lead with a normally closed contact and a normally open contact. The contacts can be operated in either of two ways. You can address the module through the power line of the house and remotely turn it on or off or you can apply either a ground or 5 volts to the sense contacts on the module to switch the form C contact. I decided to effectively apply a ground to the sensing input by way of an opto-isolator. The opto-isolator has an internal LED which shines on a photo transistor. The light turns the transistor on and that transistor effectively shorts the sense terminal to ground. The voltage to energize the opto-isolator LED is derived from the .75 ohm shunt in series with the floodlight illuminating the flag. A 50 watt bulb powered by 12 volts creates a  4 amp current flow. I put a .75 ohm resistor in series with the floodlamp and this would generate a 3 volt drop across the .75 ohm resistor. The power dissipated by the .75 ohm resistor will be 3 volts times 4 amps or 12 watts. That’s pretty hot. I bought some .5  ohm, 5 watt resistors on eBay. By wiring six of them in series parallel I got the equivalent of .75 ohms with a wattage rating of 30 watts.


The six physical resistors that constitute R1.

Six 5v watt resistors mean 30 watts of dissipation.

To increase power dissipation six 5 watt resistors are wired in series parallel.

Now I have a source of 3 volts when the light is turned on and the bulb is not burned out. I also want to know if the light is turned on. For that I will need additional circuitry. This is where the Insteon I/O module comes in. We will use the form C contact in the module to control the red and green LED. This is a two color LED. The LED has three leads, the common anode and a cathode lead for the red and another for the green.

The Insteon module has three states:

1) X-10 off so no power to the transformer and everything is off.

2) X-10 on and the transformer is energized. The bulb is not burned out. The opto-isolator sees the 3 volt sense input and turns on the photo transistor. The Module sense terminal sees the ground causing the I/O module to close the normally open contact, lighting the green LED.

3) X-10 0n and the transformer is energized. The bulb is burned out so there is no voltage to the opto-isolator. The photo transistor is off so there is no I/O module sense input and the I/O module normally closed contact lights the red LED.Schematic of the Flagpole light monitoring system.


Schematic of Flagpole Light Sensor

Schematic of Flagpole Light Sensor

The schematic is also available as a pdf.

Sorry, I did not document the build so I have no photos of the circuit I built. I do not want to tear it apart at this time. Suffice it to say that all the parts except for the shunt current sensing resistor array, R1 are wired on a piece of perf board. The resistors are mounted outside the box to allow them to dissipate heat. It has been working for  over a year now without any problems. Perhaps the resistors in series with the bulb and the resulting lower voltage to the light bulb gives me longer life for the bulb. The first bulb only lasted two weeks.

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