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49 votes
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Add a passive hardware token using a 3.5mm jack

Use 1-wire bus and any 1-wire chip inside the button. I wrote "any", because each 1-wire chip has its own, unique hardware address, so all that you need on RPi side is checking that the chip was ...
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22 votes
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Where Does VDD+0.3V Input Limit Come From on IC chips?

Most likely there is an ESD protection diode connected between the input pin and the VDD net on the chip, in such a way that it is normally reverse biased (A schematic showing the configuration is ...
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21 votes

Why does most test equipment have 10 MΩ voltage input impedance?

Having worked with test equipment with a 100 MΩ input impedance, I can say that it does not only have advantages. It requires much more careful handling than normal voltmeters or scopes. For example, ...
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20 votes
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Can unconnected inputs make an IC get warm?

Yes, CMOS circuits can get hot when there are floating inputs. You should always connect unused CMOS input pins to a defined voltage, usually GND or Vdd, unless the datasheet tells you otherwise (see ...
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20 votes
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Why does most test equipment have 10 MΩ voltage input impedance?

Remember that the actual ADC in the meter doesn't have infinite input impedance. It will source or sink some current on the voltage divider circuit. That means that raising the voltage divider ...
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18 votes

12V input on microcontroller pin

I would try a resistor divider solution like shown below. Select the resistor ratio so that the divided voltage is at the proper level for the MCU when the input is at its nominal voltage. The zener ...
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18 votes

Where Does VDD+0.3V Input Limit Come From on IC chips?

This is due to the input protection diodes. A typical input looks like this (CMOS inverter shown): simulate this circuit – Schematic created using CircuitLab The diodes in newer parts are ...
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17 votes
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Considerations when using internal pull-up/down resistors

Yes... You can... It's a proven feature. If you see pin structure in STM32 Reference manual, you can see that it has required components -- If this satisfies your requirements, then, you can do away ...
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17 votes

Considerations when using internal pull-up/down resistors

In general, it is perfectly fine to use the internal pull-up or pull-down resistors. Disadvantage is that you can forget to enable it from software, that can cause a little headache. The external ...
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17 votes

12 V input on 3.3 V GPIO, TVS pulled down or Schottky pull up?

This is actually an age old problem with PLCs and not nearly as simple as your intended solutions. The biggest issue you have is that as well has having a wide variety of potential logic voltages you ...
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16 votes
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What is the need for using active low inputs like EN' (read as "Enable bar") in digital electronic circuits?

Almost fifty years ago, the dominant semiconductor technology was the (usually NPN) bipolar transistor, in the form of 74-series TTL logic. This led to input stages that required some current to pull ...
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14 votes

Considerations when using internal pull-up/down resistors

Unless the pullup and pulldown resistors are active during reset (I suspect they aren't, but I'm not familiar with STM32 devices), there is an undefined pin state at reset. This is one major reason to ...
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14 votes
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PCB "Touch" Button

For reliability reasons I wouldn't go for an open-gate design and rely on the 50Hz noise. It probably might work, but your idea of using interleaved fingers should work quite well. The resistance of ...
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14 votes
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12V input on microcontroller pin

Use a circuit like this: simulate this circuit – Schematic created using CircuitLab R1 and R2 determine the voltage range, and perform the initial division. These resistors must be capable of ...
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14 votes

What is the need for using active low inputs like EN' (read as "Enable bar") in digital electronic circuits?

Brian's "legacy" answer is quite fine. There is another reason why active low is somewhat more reliable than active high in modern CMOS designs, and why it shouldn't disappear soon. This ...
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14 votes

Why does most test equipment have 10 MΩ voltage input impedance?

Infinite input impedance would be ideal. 'High enough for most people' turns out to be commercially more practical. It's relatively straightforward and cheap to make practical amplifiers with 1 MΩ and ...
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  • 142k
13 votes

Measuring 230V from Raspberry Pi

The industry-standard way of doing this is to use an optically-isolated coupler. The proper industrial method is to use a modular system where you can select a module for mains power input and logic-...
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13 votes

PCB "Touch" Button

It is possible to use resistive connection pads like you show, but capacitive pads are generally better. Resistive pads leave a direct connection to the circuit open to the outside. They are ...
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11 votes
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3 input XNOR gate operation

The misunderstanding is that, given XOR as a logic gate, XNOR is defined as being always its negation. Having defined your XOR-3 as an odd parity checker (by accepting the minterm \$xyz\$ - ...
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  • 171
11 votes

Identify Vintage Power Cord for 1980s Organ

I'm reasonably certain you need an IEC 60320 C9 plug to mate with that connector. Wikipedia has this to say: "This kind of coupler is used by Roland Corporation for a couple of synthesizer and drum ...
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11 votes

Where Does VDD+0.3V Input Limit Come From on IC chips?

The 0.3V drop comes from the Schottky clamping diodes used to protect the pins of the chip. These diodes typically connect between each pin and the two power rails. If they are forward biased by more ...
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  • 165k
11 votes

Why does most test equipment have 10 MΩ voltage input impedance?

It's a happy middle ground for most users that allows satisfactory accuracy while still allowing sloppiness in use. Too low and your signal source gets loaded and distorted more than you can tolerate. ...
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10 votes

Considerations when using internal pull-up/down resistors

As others have said, internal pull-ups and pull-downs are fine as long as: you remember to configure them, they are strong enough, you do not end up fighting them with an external pull, and the ...
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  • 341
9 votes

How CRT allows use of ligh-guns or pens etc?

The quintessential example of a light gun is Duck Hunt with the NES Zapper. When the trigger was pressed, the Nintendo would trigger a special frame at CRT refresh rates. The screen would draw a ...
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9 votes

12V input on microcontroller pin

If the signal levels are GND and 12V (or >5V), the most simple and 100% safe way is this: simulate this circuit – Schematic created using CircuitLab If it really serves your purpose depends on ...
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8 votes
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How to make clamping circuit to clamp exactly at 3.3 volts

The reason that people don't care about the precise clamping voltage is because in most cases, it isn't important enough to take special precautions. Unless you have some really unusual circuit ...
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8 votes

Diode Clamp failing with LDO Circuit

Alternative design concept: simulate this circuit – Schematic created using CircuitLab You may be able to eliminate R2 if the minimum draw from other circuitry is similar (a few hundred uA). ...
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8 votes

12V input on microcontroller pin

I would use a resistor divider and then protect the uC with a 5.1v Zener If you put the zener between the pin and ground in parallel with, say, a 10k pull down resistor, then feed your voltage ...
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8 votes

Add a passive hardware token using a 3.5mm jack

I would make each "token" an I2C device. Using a tip-ring-ring-shank style jack would give you 4 conductors -- ground, power, data, and clock. Each token would need to have it's own I2C address, and ...
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8 votes
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What will happen if the input voltage of the buck-boost converter goes below the recommended input?

Most DC-DC chips have an input undervoltage lockout, so if the input voltage drops too low, it's probably going to shut down. Then, since it is no longer drawing current, the input voltage will rise, ...
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