What is the UV Tron flame detector sensor?

A flame sensor (UV TRON) made by HAMAMATSU PHOTONICS, Japan, is an ultraviolet light detector. It has a narrow spectral sensitivity of 185 nm to 260 nm, being completely insensitive to visible and infrared light. Unlike semiconductor detectors, it does not require optical visible-cut filters, thus making it easy to use. In spite of its small size, it has wide angular sensitivity and can reliably and quickly detect weak ultraviolet radiations emitted from flame (eq. it can detect the flame of a cigaret lighter at a distance of 5 meters).


The sensor is well suited for use in flame detectors and fire alarms. Compare with the other fire alarm, such as smoke detector, I found that this flame detector sensor can not distinguish how big the fire it is. So I think it more suitable to put in the place which can not even allow a small flame, for example, paper manufacturing factory.

Here are some more common applications for the sensor may include, • Flame detectors for gas/oil lighters and matches • Fire alarms • Combustion monitors for burners • Inspection of ultraviolet leakage • Detection of discharge • Ultraviolet switching Such as the pictorial example from HAMAMATSU:

Another exemplar use of the sensor is a competition robot in Trinity Firefighting Robot Contest from Ohio Northern University. The robot is to navigate through a model floor plan of four rooms where a candle representing a flame source is randomly placed, locate the candle, extinguish it, and return safely to its starting location.


Operation Theory

The sensor’s theory of detection is actually fairly simple. When ultraviolet rays such as those from an open flame hits the cathode leg of the sensor, electricity or more specifically, photoelectrons, is generated. When the electrons flow toward the anode leg of the sensor combining the applied voltage to the sensor, an electric field is generated and grows stronger over time. This process beings to ionize the enclosed gas inside the bulb and thereby creating more flow of electronics like a tsunami wave building up its energy as it get closer to shore. And finally a discharge occurs like an avalanche effect between the cathode and anode that in turn, result in a voltage drop. The UV TRON driving circuit board essentially provides the supplying voltage to the tube, look for the discharge effect, and regulate the voltage drop. Then it conditions the signal and provides pulses signal outputs to the users. Here is the operating theory diagram I drew !

Electronics Characteristics

Detection Characteristics

The effectiveness of the UV TRON sensor lies in its ability to sense a narrow range of light source wavelength. It is this ability to sense light source between 185 nm and 260 nm below other common light sources like the sunlight and tungsten light allows the sensor to immune to such “noises”. However, there are still other sources of UV light that can cause “false” detection. For example, Halogen lights such as low-voltage home lighting, torchier lights, and lights on video cameras.

Another advantage of its ability to sense ultraviolet rays means it does not need to directly facing the light source. Ultraviolet ray bounces like other lights thus it can bounce off walls or other surfaces to come back to the sensor to be picked up. Thus a flame sensor has a great potential to detecting a flame source anywhere inside a confined space within the detecting range of the sensor.

The viewing ranges of the sensor shows how the sensor has a large range of sensitivity, or a nearly ±45o horizontal and vertical viewing range rather than a narrow focused sensing range directly in front of the sensor.

UV TRON Drive Circuit Board: Layout

The jumper lead for background cancellation is default to 3. It is set such that if UV TRON bulb generates only 3 pulses per every two seconds, the drive board ignores these signals and provides no pulse outputs. It can be set to higher step at 5, 7, or 9. The power supply input pin is for unregulated power between 10 to 30 Vdc. If a reliable regulated 5 Vdc power is available, it can be connected to a pin 0 shown as my red wire.

The above schematic diagram is a synonym of the board layout discussed earlier with more clear display of how the background noise cancellation filter, power inputs, and the output signals construct.


My orinignal idea to use this fire detector sensor is building an installation when user light up a candle, the projector will project some beautiful pattern on the wall. After I got this sensor and did the research and experinment on it, I found that the sensor is not an analog output sensor, which means it can not distinguish how big the fire it is. I got same amount of the pulse number from the same distance of a candle light and two gas stoves.

So I tested this sensor with different distance. By using the "attachInterrupt" function in Arduino, which can trigger a function when the input to a pin changes value, I got a 0-7 pulses per second depending on the distance of the fire. The first output is a LED, when it detect the fire, the LED will blink. Moreover, I connected the Arduino with processing and drawing the circles which changed the diameter accorading to the amount of the pulse per second.

Arduino code

Processing code