In this article we are going to see what is a light to frequency converter circuit, how it works, how to use it in a project, and its specifications.

By Girish Radhakrishnan

No matter which category you belong to, professional, hobbyist, engineer or student, modular components always reduce half of our headache while designing circuits.

They eliminate the need for designing special circuits and reduce cost effectively. One such modular component is TSL235R light to frequency converter.

What is light to frequency converter (TSL235R)?


This modular component is basically an IC which converts light intensity into frequency with 50% duty cycle.

The light intensity and frequency are proportional.

When ambient or any external light intensity rises, the output frequency increases and vice versa.

TSL235R is three legged device looks pretty much like a transistor with a translucent casing.

It comes in two forms, one is surface mount and other is common PCB mount type.

The main advantage of this IC is that no external component is required to generate frequency; it can be directly interfaced to any microcontroller or microprocessor.

It has tiny bulged lens in front of the module for focusing the light and back side is flat. It is very sensitive that it detects tiny changes in light.



Specification overview:


TSL235R can be powered from 2.7 V to 5.5 V (5 V nominal).

It has wide range of light response from 320nm to 1050nm which covers from ultraviolet to visible light. It has working temperature ranging from -25 degree Celsius to +70 degree Celsius.

It has temperature coefficient of 150 ppm per degree Celsius. The maximum frequency it can deliver is 100 KHz and minimum frequency is in the range of few 100 Hz.

The output duty cycle is strictly calibrated 50%. It measures 19.4mm in length including terminal and 4.6mm wide.

A capacitor ranging from 0.01 mfd to 0.1 mfd must be connected from its power supply terminal and the capacitor and TLS235R must close as possible.

How it works?



It combines two components, one is a silicon photodiode and the other is current to frequency converter (CFC). The CFC is a circuitry which converts current parameter to frequency parameter.

The current flow through photodiode is proportional to light intensity.

The current to frequency converter (CFC) measure the amount of current flow through the photodiode.

When the current flow through the photodiode increases; CFC raises it frequency and vice versa is also true. Thus we get an indirect conversion from light to frequency.

How and where to use it?

You may use TSL235R where you are working with any light based project such as:

· You may use it for measuring ambient light intensity such as lux meter.

· You may couple a LED and TSL235R for feedback circuit in inverter where output need to be stabilized regardless of the connected load.

· It can be used in motion detector, where any change in light intensity can be detected.

· It can be utilized in security system.

· It can be utilized in automatic street light system, where fall in the frequency can be detected by a microcontroller and trigger the output.




Here is an illustration how to interface it with microcontroller

The applications are unlimited when start play with it and understand in right way.

Light to frequency converter using IC 555


A similar circuit can be achieved by using the IC 555 wired in an astable mode with one of its resistor replaced with an LDR, as shown below:




The capacitor C1 can be replaced with other values for obtaining other sets of frequency ranges, as per the application specs.

The pin3 of the IC 555 can be integrated to any desired external load or circuit, in case a TTL compatible output is required make sure to power the IC 555 with a precise 5V.

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