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Forum Index : Microcontroller and PC projects : level shifter with J-FET's
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| Volhout Guru  Joined: 05/03/2018 Location: NetherlandsPosts: 4238 |
This is pure hardware related.... To interface 3.3V logic with older 5V peripherals (such as a PS2 keyboard) we use level shifters These are typically based on MOSFET's (such as 2N7000 or BSS138). An alternative could be to use a J-FET (Junction FET). This is an older FET type, typically a depletion FET (meaning: you need to drive its G (gate) negative to turn if OFF). But this can be used to your advantage.  Just tested above circuit using a BF245, and it works. This may not be of any interest to others, especially since it may be more difficult to buy a J-FET than it is to buy a MOSFET. Volhout PicomiteVGA PETSCII ROBOTS |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6795 |
MOSFETs seem to be cheaper than J-FETs from my usual supplier. I have no J-FETs anyway. :) Very interesting though. . Edited 2023-12-15 02:31 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 864 |
Many thanks, Volhout. I love tidbits like this. Reasonable deal on Aliexpress |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
Nice, and as simple as possible for active high inputs. For pull-up inputs such as PS2 devices just a Schottky diode to 3.3V works, limiting the input to 3.5V. Although the Pico isn't strictly "5V tolerant" as that would imply it can accept at least 5.5V the inputs can and do survive 5.0V. The official 3.6V limit, like the 133MHz clock limit is very conservative. |
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