Low Cost Transistorised Intercom

Several intercom circuits have appeared in www.circuit-lab.com using integrated circuits. The circuit described here uses three easily available transistors only. Even a beginner can easily assemble it on a piece of veroboard. The circuit comprises a 3-stage resistor-capacitor coupled amplifier. When ring button S2 is pressed, the amplifier circuit formed around transistors T1 and T2 gets converted into an asymmetrical astable multivib-rator generating ring signals. These ring signals are amplified by transistor T3 to drive the speaker of earpiece. Current consumption of this intercom is 10 to 15mA only. Thus a 9-volt PP3 battery would have a long life, when used in this circuit. For making a two-way intercom, two identical units, as shown in figure, are required to be used.

Output of one amplifier unit goes to speaker of the other unit, and vice versa. For single-battery operation, join corresponding supply and ground terminals of both the units together. The complete circuit, along with microphone and earpiece etc, can be housed inside the plastic body of a toy cellphone, which is easily available in the market. Suggested cellphone cabinet, with the position of switches, speakers and mike etc is shown.

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Build a 500W Low Cost 12V to 220V Inverter

Attention: This Circuit is using high voltage that is lethal. Please take appropriate precautions

Using this circuit you can convert the 12V dc in to the 220V Ac. In this circuit 4047 is use to generate the square wave of 50hz and amplify the current and then amplify the voltage by using the step transformer.

How to calculate transformer rating
The basic formula is P=VI and between input output of the transformer we have Power input = Power output

For example if we want a 220W output at 220V then we need 1A at the output. Then at the input we must have at least 18.3V at 12V because: 12V*18.3 = 220v*1
So you have to wind the step up transformer 12v to 220v but input winding must be capable to bear 20A.

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Low Cost Universal Battery Charger

Low cost solution for charging of both NiCd and NiMh batteries

Here is the circuit diagram of a low cost universal charger for NiCD - NiMH batteries. This circuit is Ideal for car use. It has ability to transform a mains adapter in to a charger . This one can be used to charge cellular phone, toys, portables, video batteries, MP3 players, ... and has selectable charge current. An LED is located in circuit to indicate charging. Can be built on a general purpose PCB or a veroboard. I hope you really like it.

Picture of the circuit: 

 A Low Cost Universal Charger Circuit Schematic

Circuit diagram:

A Low Cost Universal Charger Circuit Diagram

Parts:

R1 = 120R-0...5W
R2 = See Diagram
C1 = 220uF-35V
D1 = 1N4007
D2 = 3mm. LED
Q1 = BD135
J1 = DC Input Socket

Specifications:

• Ideal for in car use.
• LED charge indication.
• Selectable charge current.
• Charges Ni Cd or NiMH batteries.
• Transforms a mains adapter into a charger.
• Charge cellular phone, toys, portables, video batteries …

Features:

• LED function indication.
• Power supply polarity protected.
• Supply current: same as charge current.
• Supply voltage: from 6.5VDC to 21VDC (depending on used battery)
• Charge current (±20%): 50mA, 100mA, 200mA, 300mA, 400mA. (selectable)

Determining the supply voltage:

This table indicates the minimum and maximum voltages to supply the charger. See supply voltage selection chart below.

Example:

To charge a 6V battery a minimum supply voltage of 12V is needed, the maximum voltage is then 15V.

Voltage selection:

Voltage Selection Chart For Low Cost Universal Battery Charger

Determining the charge current:

Before building the circuit, you must determinate how much current will be used to charge the battery or battery pack. It is advisable to charge the battery with a current that is 10 times smaller then the battery capacity, and to charge it for about 15 hours. If you double the charge current , then you can charge the battery in half the time. Charge current selection chart is located in diagram.

Example:

A battery pack of 6V / 1000mAh can be charged with 100mA during 15 hours. If you want to charge faster, then a charge current of 200mA can be used for about 7 hours.

Caution:

The higher charge current, the more critical the charge time must be checked. When faster charging is used, it is advisable to discharge the battery completely before charging. Using a charge current of 1/10 of the capacity will expand the lifetime of the battery. The charge time can easily be doubled without damaging the battery.

Note:

• Mount the transistor together with the heatsink on the PCB, bend the leads as necessary. Take care that the metal back of the transistor touches the heatsink. Check that the leads of the transistor do not touch the heatsink.

Source : http://www.ecircuitslab.com/2011/08/low-cost-universal-battery-charger.html

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Low Cost Digital Code Lock

Many digital code lock circuits have been published in this magazine. In those circuits a set of switches (conforming to code) are pressed one by one within the specified time to open the lock. In some other circuits, custom-built ICs are used and positive and negative logic pulses are keyed in sequence as per the code by two switches to open the lock.

Digital Code Lock Circuit diagram :

A low-cost digital code lock circuit is presented in this article. Here the keying-in code is rather unique. Six switches are to be pressed to open the lock, but only two switches at a time. Thus a total of three sets of switches have to be pressed in a particular sequence. (Of these three sets, one set is repeated.) The salient features of this circuit are:

1. Use of 16 switches, which suggests that there is a microprocessor in-side.

2. Elimination of power amplifier transistor to energise the relay.

3. Low cost and small PCB size.

An essential property of this electronic code lock is that it works in monostable mode, i.e. once triggered, the output becomes high and remains so for a period of time, governed by the timing components, before returing to the quiescent low state. In this circuit, timer IC 555 with 8 pins is used. The IC is inexpensive and easily available. Its pin 2 is the triggering input pin which, when held below 1/3 of the sup-ply voltage, drives the output to high state. The threshold pin 6, when held higher than 2/3 of the supply voltage, drives the output to low state. By applying a low-going pulse to the reset pin 4, the output at pin 3 can be brought to the quiescent low level. Thus the reset pin 4 should be held high for normal operation of the IC.

Three sets of switches SA-SC, S1-S8 and S3-S4 are pressed, in that order, to open the lock. On pressing the switches SA and SC simultaneously, capacitor C3 charges through the potential divider comprising resistors R3 and R4, and on releasing these two switches, capacitor C3 starts discharging through resistor R4. Capacitor C3 and resistor R4 are so selected that it takes about five seconds to fully discharge C3.

Depressing switches S1 and S8 in unison, within five seconds of releasing the switches SA and SC, pulls pin 2 to ground and IC 555 is triggered. The capacitor C1 starts charging through resistor R1. As a result, the output (pin 3) goes high for five seconds (i.e. the charging time T of the capacitor C1 to the threshold voltage, which is calculated by the relation T=1.1 R1 x C1  seconds). Within these five seconds, switches SA and SC are to be pressed momentarily once again, followed by the depression of last code-switch pair S3-S4.

These switches connect the relay to out-put pin 3 and the relay is energised. The contacts of the relay close and the solenoid pulls in the latch (forming part of a lock) and the lock opens. The remaining switches are connected between reset pin 4 and ground. If any one of these switches is pressed, the IC is re-set and the output goes to its quiescent low state. Possibilities of pressing these reset switches are more when a code breaker tries to open the lock.

LED D5 indicates the presence of power supply while resistor R5 is a cur-rent limiting resistor.

The given circuit can be recoded easily by rearranging connections to the switches as desired by the user.

Source : www.ecircuitslab.com

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