51 votes
Accepted

Why put a 10k resistor between VUSB and GND?

It might appear useless, but it is not. The 5V node at the regulator input goes to an IO pad which can be used both as an input and as an output. As an output it can be used for powering external ...
  • 97.2k
11 votes
Accepted

How to electronically switch with very low current

An N-channel MOSFET will do the job as an "on-when-on" switch. The entire current draw comes from the gate "anti-float" resistor R2. Most optoisolators have current transfer ratio ...
8 votes

Why put a 10k resistor between VUSB and GND?

The 10k allows the device to detect a disconnected USB port when it its powered by an external source (think battery or wall wart). In that case your only way to reliably detect the "disconnected&...
  • 9,754
7 votes
Accepted

Capacitor discharge problem in long button press action

If you use a push-to-break switch, the solution is trivial: simulate this circuit – Schematic created using CircuitLab Here, the switch keeps capacitor Ct discharged, and only allows it to ...
  • 10.4k
5 votes

Why put a 10k resistor between VUSB and GND?

From a purely electrical point of view, without R10, the anode of the diode and the connector pin would be floating so ESD or otherwise could produce a voltage that would forward bias the diode. The ...
  • 3,649
4 votes

How to toggle two LEDs with a minimum of components?

That would be very easy if you had a resistor included: since red LEDs generally need a lower voltage (1.5-1.9V) to run compared to the green LEDs (1.9-2.4V), all you need to do is turn one of them so ...
  • 2,269
4 votes
Accepted

Calibration of MQ sensors and programming logarithmic function in MCU

I see sloppy math and poor programming practices in the links. Some #defines and/or enums would help. Here is how to do it in Excel. They are not using y = ax + b, but I prefer it also. Sometimes it ...
  • 11.2k
4 votes
Accepted

Controlling LEDs with I2C

What you want is an I2C GPIO expander IC. If you're driving LEDs from one, make sure you check the maximum current allowed per pin, as well as the total maximum current for the device. If you need ...
  • 7,499
4 votes

Why put a 10k resistor between VUSB and GND?

I remember seeing somewhere that some USB chargers will require a minimum current flow (greater than the demands of the RP2040) or they shut down the 5VDC supply being provided. I like all the other ...
4 votes

Power drain meaning

When the inverter is on, it is "stronger" than the resistor, so there would always be current going through said resistor but T1OSC will be whatever output the inverter forces it at (think ...
3 votes
Accepted

Delay power-off for a 12 V circuit powering a Raspberry Pi

I want the pi to have power for another 10-30s so it can shutdown itself I've re-written this answer with a new design that I think now does what you want without incurring the diode drops in the ...
  • 396k
3 votes
Accepted

With a standard key-matrix, how many IO pins are required for N keys?

m + n seems to work (obviously) 2x2 => 4 3x3 => 6 4x4 => 8 For a rectangular array, 2x3 => 6 4x3 => 7 1x8 => 9 Actually, for a general case, and minimum, ceil( 2 * sqrt(n) ) 8 => ...
  • 2,161
3 votes

Resistor in series with capacitor or inductor

The circuit provided is the definitive 1st order low pass filter. The unwanted frequencies are dropped across the resistor. If R is zero then there would be no filtering at all. The time constant that ...
  • 3,649
3 votes
Accepted

How to find resistors from E series for a certain ratio?

You can create such a table yourself using Excel and a macro function. I've created one for the E24 series. Formulas: F3 cell: =E3*POWER(10;$C$2) G3 cell: ...
  • 1,528
3 votes

How to electronically switch with very low current

So, I am trying to put a pretty high resistance in front of my circuit that receives that current to limit it to microamps. It sounds like you need to buffer that circuit using a MOSFET: - simulate ...
  • 396k
3 votes
Accepted

What is the function of the feedforward capacitor on the LM2596-ADJ?

A feedforward capacitor across the top resistor in the feedback divider will provide the addition of a zero and a pole to the open-loop transfer function. If the zero and pole locations are placed ...
  • 20.6k
3 votes
Accepted

What is this sheet-like part in this circuit? Can I cut through it?

It's copper, a single flat piece of copper, the technical term for a extensive area of copper plating like this is a "pour". You can cut it with a knife saw it with a saw or drill it with a ...
  • 27.9k
3 votes

Power drain meaning

If you are using the T1OSC or applying an external oscillator input then T1OSCEN must be set. if it is not in use then those internal nodes will be floating and the inverter will be off so it is not ...
3 votes
Accepted

Manual switch override for solid-state relay

Other than the oddly drawn schematic - no problems. If you connect a switch in parallel with the output of your SSR, it will work as you describe.
  • 7,814
3 votes

How can I modify a synthesizer with multiple voices to include a discrete line-out jack for each voice?

You should be able to pick up the various signals at the top (clockwise, ungrounded) terminal of the individual volume controls.
2 votes

Battery with resistor: looking for verification that my understanding is correct

Conceptually it seems like you are correct except you are saying "the other side of the resistor" which is unclear. Also you say "use a 4 Ohm resistor." But it is not clear HOW you ...
  • 26.5k
2 votes

Sensitivity characteristics of MQ-7

I think R0 is Rs in the datasheet, in other words the sensor resistance under 100ppm CO clean air is somewhere between 2kΩ and 20kΩ, depending on the unit you happen to get. You cannot extract it from ...
2 votes

Why is this not a negative half wave rectifier? What is it?

It is a negative-output filtered half-wave rectifier, or, equivalently, a negative peak detector. It is wrong to say that it is "not" a half-wave rectifier. It is, functionally speaking, ...
2 votes
Accepted

Need help with a seemingly simple RC circuit with a Schmitt trigger inverter

I would have thought that the energy consumed was independent of the size of R2. The important thing is that you keep R1 high in value to limit the current being sunk through the switch when it's ...
  • 6,969
2 votes

With a standard key-matrix, how many IO pins are required for N keys?

The required array for \$N\$ keys is some \$n \times m\$ array where \$nm \ge N\$. If you want to minimize the number of pins for a given number of keys and \$N\$ isn't a perfect square, you'll need ...
  • 38.9k
2 votes

Golf cart battery fuel gauge?

Likely, the lead-acid batteries have high internal impedance, or the connections to the batteries are corroded or loose. This statement is based on the fact that at only one volt below full charge, ...
2 votes

Photodiode detector 2-stage amplifier design

This is a difficult problem, so you want to do everything possible to maximize the sensitivity. I a building something similar to what you need. Opamps usually work better with dual supplies, a single ...
  • 11.2k
2 votes

How to figure out equations for a signal flux diagram?

Hint to get you started: - $$V_1 = KV_i + \dfrac{V_2}{Q} - V_3$$ And you can probably see that \$V_2 = \dfrac{-V_1}{sT}\$ and, \$V_3 = \dfrac{-V_2}{sT} = \dfrac{V_1}{s^2T^2}\$ You should be able to ...
  • 396k
2 votes
Accepted

BJT phase shift oscillator

Please tell me what is wrong and how can I make a circuit with such a waveform. In your diagram you had base connected to +5 volts and that would have destroyed the BJT. However, given the component ...
  • 396k
2 votes

How do I power an STM32 dev board with 9 V properly?

You can download the schematics from the manufacturer. But the 5V does not connect to the MCU. Not directly, as there is a 3.3V regulator to convert 5V to 3.3V. The board accepts 5V from multiple ...
  • 97.2k

Only top scored, non community-wiki answers of a minimum length are eligible