For my application I have to read 12V pulses from a optical encoder into a GPIO pin of a Raspberry Pi. What could be the best solution to convert 12v HTL signals to 3.3V for RPi GPIO input? Is a level shifter, resistor+clamping diode or an opto isolator good for this application?
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\$\begingroup\$ How fast are the pulses coming in (frequency)? This will help determine whether you can use opto-isolators or not. \$\endgroup\$– evildemonicCommented Mar 17, 2020 at 15:07
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1\$\begingroup\$ First check the datasheet to see if the encoder can be run off a compatible voltage. "12V TTL" doesn't make sense : TTL is generally 5V. \$\endgroup\$– user16324Commented Mar 17, 2020 at 15:23
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\$\begingroup\$ I saw the datasheet.its 12 HTL \$\endgroup\$– kunal pCommented Mar 17, 2020 at 15:31
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\$\begingroup\$ The pulse frequency is 1 pulse every 2ms. \$\endgroup\$– kunal pCommented Mar 17, 2020 at 15:32
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\$\begingroup\$ @kunalp Ok good, this is well within the range of most opto-isolators. Easy solution. \$\endgroup\$– evildemonicCommented Mar 17, 2020 at 17:40
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2 Answers
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simulate this circuit – Schematic created using CircuitLab
I would bet that a couple resistors and a clamping diode are ok if the environment is not too hostile (big motors, long cables, high currents...).
You can also add a small capacitor in parallel with the diode, it aids in reducing spikes if any. The resistor R1 could (or probably should) be slightly bigger.
answered Mar 17, 2020 at 15:58
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Look at CD74hc4050. It is designed for level shifting. http://www.ti.com/lit/ds/symlink/cd74hc4049.pdf
"They have a modified input protection structure that enables these parts to be usedas logic level translators which convert high-level logic to a lowlevel logic while operating off the low-level logic supply. For example, 15-V input pulse levels can be down-converted to 0-V to 5-V logic levels. The modified input protection structure protects the input from negative electrostatic discharge. "
Available as DIPs also. https://www.digikey.com/product-detail/en/texas-instruments/CD74HC4050E/296-9213-5-ND/376792
Better response times (faster) than the old CMOS stuff.
answered Mar 17, 2020 at 16:55