Capacitance-Multiplier Power Supply

High-quality audio amplifiers require a very efficient power supply with negligible ripple, hum and buzz. An ordinary bridge-type power supply has a drawback in that it drops to 3 to 4...

Variable Power Supply Using a Fixed-Voltage Regulator IC

A voltage regulator (also called a ‘regulator’) with only three terminals appears to be a simple device, but it is in fact a very complex integrated circuit. It converts a varying...

Telephone Operated Calling System

Dual-tone multiple-frequency (DTMF) receiver IC is commonly used in telephone equipment. One common DTMF receiver is Holtek HT9170 used in electronic communication circuits. The Holtek HT9170 series comprises DTMF receivers integrated...

Friendly Charger for Mobile Phones

Most mobile chargers do not have current/voltage regulation or short-circuit protection. These chargers provide raw 6-12V DC for charging the battery pack. Most of the mobile phone battery packs have a...

Medium-Power FM Transmitter

The range of this FM transmitter is around 100 metres at 9V DC supply. The circuit comprises three stages. The first stage is a microphone preamplifier built around BC548 transistor. The next...

Micro-Power Flasher

With this micro-power flasher you can baffle the intruders trying to break into your home. The unit continuously emits flashing light both during day and night giving the impression that the...

A Fourth-Order Speech Filter (Based on Texas Instruments Application Note)

Human speech generally occupies an audio spectrum of 300 to 3400 Hz. There is a requirement, especially in telephone circuits, to limit the frequency response to this range. The ‘Digital Speech...

Hearing Aid

This low-cost, general-purpose electronic hearing aid works off 3V DC (2x1.5V battery). In this circuit, transistor T1 and associated components form the audio signal preamplifier for the acoustic signals picked up by...

Remote-Controlled Fan Regulator

Using this circuit, you can change the speed of the fan from your couch or bed. Infrared receiver module TSOP1738 is used to receive the infrared signal transmitted by remote control. The...

Briefcase Alarm

This miniature alarm unit protects your valuables from theft by sounding an alarm when somebody attempts to pick up your briefcase. It is a battery-operated gadget that can be hidden in...