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This circuit is a simple form of the commercial UPS , the circuit provides a constant regulated 5 Volt output and an unregulated 12 Volt supply. In the event of electrical supply line failure the battery takes over, with no spikes on the regulated supply. This circuit can be adapted for other regulated and unregulated voltages by using different regulators and batteries. For a 15 Volt regulated supply use two 12 Volt batteries in series and a 7815 regulator . There is a lot of flexibility in this circuit. TR1 has a primary matched to the local electrical supply which is 240 Volts in the UK. The secondary winding should be rated at least 12 Volts at 2 amp, but can be higher, for example 15 Volts. FS1 is a slow blow type and protects against short circuits on the output, or indeed a faulty cell in a rechargeable battery. LED 1 will light ONLY when the electricity supply is present, with a power failure the LED will go out and output voltage is maintained by the battery. The circuit bel...

The most simple polarity protection tehnique is to connect a series diode to the power line input. The diode conducts only when the power supply protection is correct. But the incovenient is that at higher current levels, the voltage drops and power loss of the diode affects the power level adversely. This polarity protection circuit is dimensioned for 12 V power supplies and avoids the voltage and power loss problem. By correct polarity, the current flowing to the D1 and the relay coil causes the relay contacts to activate. The NO contact closes powering the electronic device. The NC contact opens and the current supplying the relay coil is reduced to a low level just enough to maintain relay activation. source [link]

Have you ever needed to power a 12-volt relay in a circuit but only had 6 or 9 volts available? This simple circuit will solve that problem. It allows 12-volt relays to be operated from 6 or 9 volts, or 24-volt relays from 12 volts. While most normal relays require the manufacturer-specified coil voltage to reliably pull the contacts together, once the contacts are together you only need about half that rated voltage to hold them in. This circuit works by using that principle to provide a short burst of twice the supply voltage to move the contacts and then applies the available 6 or 9 volts to the relay to lock the contacts in place. With reference to Figure A., when the main supply is applied to the circuit the 220-µF capacitor, C1, charges quickly to +6 volts through resistor R3. The circuit is now awaiting voltage on the control input. When a control voltage (can be as little as 3 volts) is applied to the control input, transistor T1 switches on. The other transistor, a BC558, is a...

The interlock contacts installed in the previous section’s motor control circuit work fine, but the motor will run only as long as each pushbutton switch is held down. If we wanted to keep the motor running even after the operator takes his or her hand off the control switch(es), we could change the circuit in a couple of different ways: we could replace the pushbutton switches with toggle switches, or we could add some more relay logic to “latch” the control circuit with a single, momentary actuation of either switch. Let’s see how the second approach is implemented, since it is commonly used in industry: When the “Forward” pushbutton is actuated, M 1 will energize, closing the normally-open auxiliary contact in parallel with that switch. When the pushbutton is released, the closed M 1 auxiliary contact will maintain current to the coil of M 1 , thus latching the “Forward” circuit in the “on” state. The same sort of thing will happen when the “Reverse” pushbutton is pressed. The...

Types of transistor Transistor circuit symbols There are two types of standard transistors,   NPN   and   PNP , with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. This page is mostly about NPN transistors and if you are new to electronics it is best to start by learning how to use these first. The leads are labelled  base  (B),  collector  (C) and  emitter  (E). These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels! A Darlington pair is two transistors connected together to give a very high current gain. In addition to standard (bipolar junction) transistors, there are  field-effect transistors  which are usually referred to as  FET s. They have differen...

In many circuits, it is necessary to use MOSFETs for switching. In many cases, the MOSFET drive signals are generated by microcontrollers. In other cases, they are generated by ICs – PWM controllers, timers or any IC in fact. However, MOSFETs cannot always just be connected to the drive signal and be expected to work properly. Due to the construction of the MOSFET, driving it is not the simplest of tasks, especially for beginners. There are many users who regularly ask for help on MOSFET drive related issues or problems on different blogs, websites and forums. So, here I will show some MOSFET drive techniques/methods for MOSFETs configured as low-side switches. Before I head on to MOSFET drive, let me just tell you what a low-side switch is, in case you don't know. When the MOSFET (that you're using as a switch) sinks current, it is a low-side switch. The load will be between the drain and +V supply. The source will be connected to ground. Gate will be driven with respect to gr...

In recent years, following CD's introduction, vinyl recordings are almost disappeared. Nevertheless, a phonon preamplifier is still useful for listening old vinyl discs from a well preserved collection. This simple but efficient circuit devised for cheap moving-magnet cartridges, can be used in connection with the audio power amplifiers shown in these webpages , featuring low noise, good RIAA frequency response curve, low distortion and good high frequency transients behavior due to passive equalization in the 1 to 20KHz range. Transistors and associated components provide ±18V supply to the op-amp, improving headroom and maximum output voltage. Phono Preamplifier Circuits Diagram  Notes: R2, R3, R4, R7, R8, C4 & C5 should be low tolerance types. Schematic shows left channel and power supply. For stereo operation R1, R2, R3, R4, R7, R8; J1; C1, C4 & C5 must be doubled. Numbers in parentheses show IC1 right channel pin connections. Technical data: Sensitivity @ 1KHz : 2....

This is a one watt home stereo amplifier module project using the KA2209 IC from Samsung, which is equivalent to the TDA2822. It operates from 3-12V DC & will work from a battery since the dormant current drain is low. It requires no heat sink for normal use. The input & output are both ground referenced. Maximum output will be obtained with a 12V power supply & 8 ohm speaker, however it is suitable for driving headphones from a supply as low as 3V. The Specifications of the home stereo amplifier : D.C. input : 3 – 12 V at 200 – 500 mA max Idle current : approx. 10 mA Power output : > 1 Watt max. 4-8 ohms, 12V DC Freq. Resp. : approx. 40 Hz to 200 kHz, 8 ohm, G=10 THD : < 1 % @ 750 mW, 4-8 ohm, 12V Gain : approx. x10 (20 dB) OR x100 (40dB) S/N ratio : > 80 dB, G = 20 dB Sensitivity : < 300 mV, G = 20 dB Input Impedance : approx. 10 k ohm Description   The gain is adjustable from ten to 100, i.e. twenty to 40 dB. Start with feedback resistors R1 and R3 of 1k ...

Binary numbers Logic states  True  False 1 0 High Low +Vs 0V On Off   Seen on a T-shirt: There are 10 kinds of people - those who understand binary, and those who don't. Electronic circuits count in binary. This is the simplest possible counting system because it uses just two digits, 0 and 1, exactly like logic signals where 0 represents false and 1 represents true. The terms low and high are also used for 0 and 1 respectively as shown in the table. Counting  one ,  two ,  three ,  four ,  five  in binary: 1, 10, 11, 100, 101. Binary numbers rapidly become very long as the count increases and this makes them difficult for us to read at a glance. Fortunately it is rarely necessary to read more than 4 binary digits at a time in counting circuits. In a binary number each digit represents a multiple of two (1, 2, 4, 8, 16 etc), in the same way that each digit in decimal represents a multiple of ten (1, 10, 100, 1000 etc)...