Optical Liquid Sensor Circuit

Optical Liquid Sensor Circuit

Here is one simple design idea of a self-contained optical liquid sensor, realized using generally available electronic components. The sensor is designed specifically for liquid level detection by non-contact methods in clear or very translucent pipes. The sensor, features an infrared light beam from an infrared light sender that becomes refracted and undetectable by the infrared light receiver when the liquid is present, can be mounted on a pipe that is between 3 to 13mm in diameter. This compact and non-invasive sensor device runs on 12 Volt DC. The ‘thru scan’ principle where liquid refracts light beam insures reliable detection, and the Infrared beam technique can help penetrate slurries and other solids with similar detection effect as water/liquid.

Infrared Detection Technique

(Optical Liquid Sensor – Infrared Detection Technique)

This optical liquid sensor was originally designed for a small-scale industrial application, but will also find many hobby/domestic applications. Note that the circuit is designed with simplicity as the top priority. The supply voltage of 12V is usually suitable for industrial use, and the sensor connections are protected against polarity reversal. The infrared sender is built around a common infrared light emitting diode (IRLED1), in the prototype, the 5mm-type IR333-A from EVER LIGHT, which is a high intensity diode molded in a blue transparent plastic package. Similarly, the infrared receiver (IRPD1) is realized with the help of LITE-ON’s LTR-3208E photo transistor available in a 2-pin/5mm special dark plastic package that cut the visible light. However, note that the indicated part numbers of the infrared sender and receiver (IRLED1 & IRPD1) components are not very critical!

Optical Liquid Sensor Circuit

circuit diagram

When powered, the infrared light sender (IR LED1) lights up. Resistor R1 (470R) is used here to limit the operating current of the infrared LED. In case of a no-liquid condition, light from the infrared sender falls on the infrared receiver with the result that the infrared receiver energises the electro-magnetic relay (RL1) via the driver transistor T1. The spdt (1 C/O) contacts of the relay can be used to turn off an external electric device, or to turn on an acoustic sounder like an alert beeper.

Assemble the circuit on a general-purpose circuit board and after initial testing try to enclose the whole circuit in a suitable forked-box. The 470-Ohm value of the resistor R1 works well with most liquids, but you can increase or decrease this value as per the actual demand. It is recommended that the fabricator must evaluate the water/fluid/chemical combination/mixing situation to determine which resistor value works best before finalizing the design.

Mouting Plan

(Optical Liquid Sensor – Mouting Plan)

Finally note that the detection depends on the index of refraction and the transparence rate of the pipe and the object and the thickness of the pipe.


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  • v sambath kumar

    Hi MR Krokkenoster,

    Thank you very much for your feedback and valuable information.

    Regards, V.Sambath kumar.

  • V.Sambath kumar

    Hi Hareendran,

    Excellent article good idea , can we use transparent plastic pipes .

    the break and make of the relay contacts can produce high ac voltages

    which may be higher than the voltage rating c1 i think for safer side

    it can be rated at 100v it is a suggestion from my end what is your opnion.

    Regards V.Sambath kumar.

    • Krokkenoster

      dear Sambath, I made a thermostat for a deep freezer because the old thermostat was installed in such a way that removing the sensor was made impossible. I then buiult one with a 555 i.c. and thermistors as sensor. When the temperature was reached, the motor would chatter for the back EMF retriggered the i.c.. I then fitted a transorb over the contacts and this cured the spikes on the a.c.mains.


    Who said that everything that works need plenty components? Interesting circuit that a lot of applications just make sure it must built in a lightproof box The old A77 Revox open reel tape recorder used a similar idea and the circuit stopped working and frustrated many a recordist! Too much ambient light made the recorder to stop. I got some machines from a broadcasting station and the lamps were either removed or covered with black tape. They were in use before LEDS were affordable and reliable and they had to be red and not needed to have a bright (Detectors were then only a dream……..)

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