Wednesday, May 18, 2022

WATCH-DOG FOR TELEPHONES

Most of the telephone security devices available in market are simple but quite expensive. These devices provide blinking or beeping type line-tap misuse indications. Quite often they do not offer guaranteed...

TELEPHONE CALL METER USING CALCULATOR AND COB

In this circuit, a simple calculator, in conjunction with a COB (chip-onboard) from an analogue quartz clock, is used to make a telephone call meter. The calculator enables conversion of STD/...

PARALLEL TELEPHONE WITH SECRECY

Often a need arises for connection of two telephone instruments in parallel to one line. But it creates quite a few problems in their proper performance, such as over loading and...

Ultrasonic Proximity Detector

We the humans can hear sound of up to 20kHz frequency only. This proximity detector works at a frequency of 40 kHz. It uses two specially made ultrasonic transducers: One transducer...

Secret Bell

Many people move through the corridors and steps in multistoried buildings. As most of them are strangers for the inhabitants of the flats, it becomes necessary to verify the identity of...

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...

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...

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...

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...
Briefcase Alarm

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...
 
 

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