Secret Diagram

There are some applications, such as double-ended sensors and audio amplifiers that require a negative voltage for operation. With limited space on today’s system boards, creating a dedicated negative supply rail would add to the cost and space of the PCB. Hence, it makes sense to generate the required negative voltage from existing positive supply rails in the system.

One such solution using a traditional synchronous step-down regulator is provided by Texas Instruments in an application note entitled “Creating an Inverting Power Supply Using a Synchronous Step-Down Regulator”1. It shows you how to generate a negative voltage from a positive input voltage to the synchronous buck regulator. [ ]

Looking for a unique gift? Here’s an antique typewriter which has been modified to function as a USB Keyboard for PC, Mac, or even iPad. That’s very cool, isn’t it?

In the world of obsolescence, this USB typewriter is a groundbreaking innovation. It does not change the outward appearance of the typewriter and is easy to install since there is no messy wiring. The 3 components of the USB typewriter are the Sensor board, the USB switches, and the Reed Switches.

It works like a regular typewriter with all letters, numerals, and punctuation marks as well as shift, space, and return carriage. It’s a better addition to your home office.

There are many approaches to the problem of activating a light when it becomes dark, and a recent Design Idea covers this topic (Reference 1). Some approaches require a dc power supply and an electromechanical relay, but a better approach involves feeding the device directly from the ac line, minimizing the number of components 

The heart of the device is a light-sensitive cadmium-sulphide resistor, PR, with a resistance of approximately 200 kΩ in the dark and decreasing to a few kilohms in the light. PR and capacitor C1 form an ac-voltage divider. In daylight, the voltage across PR is too low to generate the required gate-trigger current to turn on bidirectional ac switch Q1, thus keeping the load – usually a lamp – off. When it becomes dark, PR’s resistance rises, resulting in an increase in the TRIAC’s gate current that triggers the TRIAC and lights the lamp.

The circuit uses inexpensive, off-the-shelf components, including the VT90N1 photoresistor; a 0.1-μF, 275V capacitor; and an L2004F61 TRIAC with a load current of 4A rms, a peak blocking voltage of 200V, and a gate-trigger current of 5 mA. The exact specifications of these components are not critical; you could use others instead.

Editor’s note:
Attributes worth mentioning include the fact that the capacitor introduces a phase shift, which places the peak of the gate voltage close to the zero crossing of the load’s sine wave for optimum turn-on timing. Another benefit is thermal hysteresis, which occurs due to the reduction of the required triggering voltage and current as the TRIAC warms up after the initial turn-on.