Soft Latch Power Switch Circuit

Dаvid Jones through his Youtube channel EEVblog described in detail how to design a cheap soft latch power switch circuit, using one push button switch to toggle your circuit power on and off with the following design requirements:

  1. Zero power when off.
  2. One on/off switch.
  3. Standalone(no MCU).
  4. General components only (Diodes, Transistors, ..ect).
  5. Minimal parts.

First Basic Circuit


A passing transistor is between the input and the output, with another latching transistor.

When we initially power on the circuit, it will be off because the passing transistor is off and so the latching transistor is off (like the egg and chicken). When we turn on the “ON” switch, the passing transistor is on, so the current flows from input to output and therefor the latching transistor is on. To turn it off, press the “OFF” button and the latching transistor is off, and the passing transistor is off.


First design uses 2 switches and we are looking forward a circuit with one switch only, so let us get a look to the next circuit design.

Second Circuit


When we push the button, the transistor connected to the gate of passing MOSFET will be on, and therefor the passing transistor will be on, on the right side the BJT will be on driving the line down to ground. So next time when you press the the button the transistor connected to gate will be off.

The capacitor connected to base of right side transistor is used to prevent the oscillating on/off while the pressing of the button needs milliseconds and the transistors works much faster.

To see an experiment of the design and to learn more details, check David’s video bellow:

New Alcohol Monitoring System from University of California, San Diego

A new alcohol monitoring system from UCSD consists of a temporary and one-time use tattoo that clings to the human skin and a flexible electronic board that attaches to the tattoo magnetically transfers the results of sensing the blood alcohol concentration via Bluetooth to a smart device paired with it.

The UCSD researchers tried to design the system in a less bulky and less expensive approach. The device uses an electrochemical sensor to measure ethanol molecules emitted through our skin.


To know more about the results of using this device and how this electrochemical sensor works, read the paper published in ACS Sensors journal.


OpenCellular – An Open Source Wireless Access Platform From Facebook

Promised projects from Google and Facebook may change our world connectivity. We saw previously Loon project from Google and the Internet drone Aquila from Facebook.

Facebook this time is trying to find a general solution to improve the connectivity infrastructure. The project called OpenCellular.

This new project is a software-defined wireless access platform, and this gives it the ability to support everything from 2G to LTE networks.


The system consists of general-purpose and base-band computing subsystem with integrated power and housekeeping system and a RF with integrated analog front-end. According to the announcement, Facebook plans to open-source the hardware designs to public, along with necessary firmware and control software.

OpenCellular designed to have various power sources: PoE (power-over-ethernet), solar, DC, and external batteries and internal battery (lithium-ion).


The problem which Facebook try to solve in this new project is to extend network access in both rural and developed communities. The ecosystem cellular networks still constrained, where the infrastructure is very expensive, making it difficult to deploy it everywhere and for smaller organizations or individuals.


OpenCellular still under lab tests; “we are able to send and receive SMS messages, make voice calls, and use basic data connectivity using 2G implementation on our platform.” according to the announcement.

Via: Facebook

16 Channel Tiny InfraRed Remote Controller – NEC Code


16 Channel Infra-Red remote controller is built around PIC16F73 Microcontroller from Microchip. The receiver part follows NEC Code Format. Tiny receiver provides 16 latch outputs. All outputs are TTL and can drive Relay board or solid state relay. The circuit uses TSOP1738 Infra-Red receiver module which provides high degree of noise immunity against interfering light source.

For Philips RC5 code check this project: 16 Channel InfraRed remote controller – RC5 Philips Code


  • Supply Remote Transmitter 3V CR2025 Battery
  • Supply Receiver 7V to 12V DC
  • Modulation 38Khz
  • NEC Code Format
  • Operating range up to 10-15 feet
  • 2 Pin Screw Terminals for Supply Input
  • On Board Power LED
  • Onboard VT (Valid Transmission) LED
  • All 16 Channels are Latch Outputs
  • All Outputs TTL Level provided with Header Connector

16 Channel Tiny InfraRed Remote Controller – NEC Code – [Link]

An Atmega library for multiple HD44780 LCDs


Davide Gironi has posted an I2C multiple HD44780 AVR Atmega library:

This library implements a driver for HD44780 lcd connected through PCF8574 port expander.
Data is transmitted using only 2 wire over i2c with the PCF8574.
This library can drive up to 8 LCD concurrently.
Lcd driver is based upon Peter Fleury’s lcd driver
HD44780 to i2c library its based upon this library

An Atmega library for multiple HD44780 LCDs – [Link]

DIY PCB inspection microscope


Saulius made a DIY PCB inspection microscope and wrote a post on his blog about it:

Despite how good microscope you have, stand is still very important part of final assembly. And while there are many commercial ones made of cast iron, they might cost more than a microscope head itself. I decided to make stand from wildly available material laying around – it’s MDF (Medium Density Fiberboard). Advantages – stable over time if not exposed to water, dirt cheap, easy to process.

DIY PCB inspection microscope – [Link]

The DIY diabetes kit that’s keeping us alive



Frustrated with traditional monitoring and its risks, Mr Samuelson and George have since joined a growing group of T1 sufferers who are building their own solutions to manage their diabetes – even though they come with their own uncertainties.

The DIY diabetes kit that’s keeping us alive – [Link]

How to interface Keypad 4×4 with Arduino


In this tutorial we are going to learn some basics on using Keypad 4×4 on Arduino. by mybotic @

This keypad consists of 4 rows x 4 columns of buttons. It is ultra-thin that it provides an useful human interface component for microcontroller projects. Convenient adhesive backing provides a simple way to mount the keypad in a variety of applications, such as menu selection, security systems, data entry for embedded systems and so on.

How to interface Keypad 4×4 with Arduino – [Link]

A New Record for Single-molecule Diode

Diodes are one of the most common used electronic components in our designs used to direct the current flow. Single-molecule electronics is a “branch of nanotechnology that uses single molecules, or nanoscale collections of single molecules, as electronic components.” according to Wikipedia definition, and shrinking the size of components is one of the biggest target researchers are looking forward.


The concept of Single-molecule diode was introduced in 1974 by Aviram & Ratner, and the single-molecule diode consists of an asymmetric molecule which includes a donor–bridge–acceptor architecture to mimic a semiconductor p–n junction.

The performance of single-molecule diode still need improvement, compared to the performance of the conventional ones, and that’s what researchers are trying to do. Recently, Researchers at TU Delft in the Netherlands made a single molecule diode with a rectification ratio of 600 much higher than the value of 15 measured in earlier research.

Via: elektormagazine


A Bench Power Supply using computer PSU

As we know, PSU in computers provides various DC voltages, 3.3V, 5V and 12V. These 3 levels are the most common ones needed in our labs, so usually PC PSUs are used by a lot of makers and hackers as their personal bench power supply. The PSU has a standard connector called ATX, you can reach all of the voltages levels using it.

[Supercap2F] hacked his own PSU into a bench power supply and published the project details. Supercap2F’s power supply design include:

  • PIC18F1220.
  • 16*02 LCD as a user interface.
  • Three switches for user control (select/move).
  • Relays controlled by PIC18F1220 to switch outputs on/off.
  • Fused Outputs: +3.3V, +5V, +12V and -12V.


[Supercap2F] published the circuit designs (PCB&SCH) and source code for PIC18F1220 over Github.


Via: Hackaday