Cellphone-Based Device Control With Voice Acknowledgement

Here is a circuit that lets you operate your home appliances like lights and water pump from your office or any other remote place. So if you forgot to switch off the lights or other appliances while going out, it helps you to turn off the appliance with your cellphone. Your cellphone works as the remote control for your home appliances. You can control the desired appliance by pressing the corresponding key. The system also gives you voice acknowledgement of the appliance status.

Circuit description
Fig.1 shows the circuit for cellphone based device control with voice acknowledgement. It comprises microcontroller AT89C51, DTMF decoder MT8870, voice recording/playback device APR9600 and a few discrete components.

Versatile 200KHz Function Generator using ICL8038

ONE of the most useful facilities in the electronic experimenter’s workshop is an ability to generate a.c. test signals of various waveforms, frequencies and amplitudes. This is where this Function Generator project comes in, the “function” in the name referring to the waveform of its output signal. 

The Generator can be used for testing or driving many circuits, from below audio up to a couple of hundred kilohertz, and may even be used as a variable speed clock for logic circuit testing. It has sine, square and triangle-wave outputs plus a separate 0V to +5V squarewave output for logic driving. This may also be used with a “sync” input when, for example, inspecting low-level signals on an oscilloscope.

Circuit Detail:

The full circuit diagram is shown in Fig.1. Starting with the power supply, this is of the dual rail type. The very low output frequencies require d.c. coupling in the output circuit, which in turn makes a circuit using separate positive and negative supplies about a central 0V or ground much simpler to design.

Versatile Bench Power Supply

The power supply described here provides an output of up to 20V with current in two ranges of 100mA or 1A. Ideally a workbench supply should have an output voltage that can be adjusted right down to zero as it is occasionally useful to be able to power a circuit gradually from this when fault-finding. There should also be a fast and effective current limiting facility, again adjustable from zero as this also provides valuable protection when testing circuits.

If the supply can be set to deliver a constant output current instead of a voltage it can also be used as a charger for the many types of re-chargeable battery that are available nowadays, both alkaline and  small sealed lead-acid types. The latter are normally charged to a constant voltage but until this is reached a current limit is often required to prevent an excessive charge rate.

Preferably the supply should have two meters so that the voltage and current supplied to the load can be seen at a glance, and most users would probably prefer analogue meters to the digital type for this.

High Efficiency LED Torch

THE high forward voltage of l.e.d.s is a challenge for efficient battery use. In the circuit shown in Fig.1a, a 74HC14 inverter is used to “double” the voltage of a 6V rechargeable ex-mobile-phone battery and drive a string of three 5mm l.e.d.s. The oscillator around IC1a and IC1b generates a square wave at about 2·8kHz, and its output is buffered by IC1c and IC1d used in parallel to maximise current to the doubler.

PROGRAMMABLE TIMER FOR APPLIANCES

This programmable timer is usefu for domestic, commercial as well as industrial applications. It automatically turns the appliance on/off after a preset time. The time period can be varied from 8 seconds to 2 hours with the help of rotary switches S2 and S3. The circuit works in two modes: off mode and cyclic mode. Slide switch S4 is used for mode selection.

In the off mode, the appliance turns on after a preset time (set by rotary switch S2), remains on for another preset time (set by rotary switch S3) and then turns off. In the cyclic mode, this process repeats again and again.

The circuit is built around three quad two-input NAND gate ICs CD4011 (IC1, IC3 and IC5), two 14-bit binary ripple counters CD4020 (IC2 and IC4) and a relay driver transistor (T1). It works off a 12V DC, 500mA power supply. You can also power the circuit from mains by using a 12V DC, 500mA adaptor in place of the 12V DC power supply.