UV flame sensor for VEX
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Discontinued.       VUVFL    UltraViolet Flame sensor    $139.00     

Listed here for reference only.  This part is no longer availab.e

The UV Flame sensor detects candle flame and other sources of Ultraviolet light..

The UV Flame sensor is for use with theVEX Robotic Invention System.  It is intended to help your robot find a candle and go to it, as in a robotic firefighting competition.   The sensor detects the presence of ultraviolet light.  Ultraviolet (UV) is like normal light, except you can not see it with you eye.  UV light does not pass through normal glass or most clear plastics.

DANGER!  The sensor uses an ionization tube that runs at 350 volts. This voltage is dangerous! Do not operate the sensor if it is wet or the glass tube is broken. This product should only be used with adult supervision.

Motion detector for Mindstorms.

Motion detector on RCX 


The UV Flame Sensor has a binary output. You may program it as a bumper switch or a limit switch. The output pulse is 100 milliseconds long. You should read the sensor frequently or you may miss the pulse.

The sensor is best used as an interrupt source. Use the “Optical Shaft Encoder” block to use the UF Flame Sensor in interrupt mode. See the example below.

Example code for interrupt mode.


A small tea candle 1 meter away will generate about 15 pulses in 5 seconds. The same candle at 2 meters will generate around 8 pulses in 5 seconds. Maximum detecting distance of a cigarette lighter is about 5 meters.

Tube sensitivity pattern




Technical Description

Inside the glass tube a 350 volt potential exists between the metal plate and the curved wire. When UV light hits the metal plate electrons are released. The electrons are accelerated toward the wire. As they travel they strike gas molecules and ionize the gas, releasing more electrons. The process grows quickly until much of the gas is ionized. The ion/electron mixture conducts electricity and discharges a small capacitor, causing the current to fall below what is needed to maintain ionization. The tube can not “fire” again until the capacitor recharges to about 280 volts. This takes aprox. 50 mS.

When the tube discharges it creates a pulse that is filtered through a counting circuit. If the circuit has seen 3 pulses in the last two seconds, the pulse is passed through. Otherwise, the pulse is blocked. This prevents false output due to background radiation.