ASK AN EDUCATOR! – “How can I convert an analog value to a digital one?”

Nicolas asks:

I’d really like to know how to “”convert”” an analog value to a digital one.
In a word : I have an Arduino, a photoresistor, with a pull-down resistor.
I want to know if the light is above or below a given threshold.

I know how to read the value with analogRead(photoResPin), and compare it to my threshold (in code), but I’d like to do that without software (only using digitalRead), handling that threshold in hardware.

Can you help me ?
I guess I can use a transistor, but don’t know how to “”precisely”” set the threshold (by changing the pull-down resistor value ?).

What a great question and the answer should be pretty easy to implement! As long as the current you need to control is relatively low, a simple comparator like the LM393 should do the trick. You can configure the comparator to act as a Schmitt Trigger, you can set a high and low threshold voltage using positive feedback as shown in the diagram below. This basically relies on two voltage dividers (R1/R2 & R2/R3) to set the high an low values, then uses R4 as a logic pull-up. By using this positive feedback you help to eliminate multiple transistons between a HIGH/LOW output when your input voltage approachs and hits you threshold, as illustrated in the second diagram. This feedback also helps to speed up the time in which the comparator changes state.

This Schmitt Trigger Calculator should help you determine the values for your resistors and R4 is commonly a 1k.

Here is another comparator calculator that might help as well.

I hope this has helped to answer your question, and the topic of switching from microcontrollers to stand alone hardware is certainly on my lists of things to research further! So thanks to you!

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  1. If you want to get fancy, the arduino/AVR has AIN0 and AIN1 going to an internal comparator, so if you set the registers right, you can just use a potentiometer for one and a resistor with the photoresistor for the other. It can even cause an interrupt.

  2. George Graves

    Great post!

  3. The right answer here would have been to use the internal comparator of the arduino, although reading it is a bit different.

    Also, should’t draw schematics with the ground up.

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