Security Circuits | Electronic Project Circuits

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Over Voltage Protection

Safety devices like fuses provide protection against excess current, but do nothing for transients and short duration spikes of high voltahe on the power supply lines.This circuit uses the “crowbar” method and provides fast protection against transient voltage spikes, transients that could cause damage to sensitive components. The thyristor will trigger in a few microseconds. This is over 1000 times faster than an ordinary quick blow fuse. If the output voltage exceeds the limit set by the zener, then it will conduct. The voltage across the 4.7k resistor will rise, the thyristor switches on and the power rails are short circuited.

1 comment - What do you think? Posted by admin - June 10, 2008 at 8:59 am

Categories: Detector, Electronic Control, Power supply, Security Tags: security circuits, Voltage Protect

Ultrasonic Pest Repeller circuit

It is well know that pests like rats, mice etc are repelled by ultrasonic frequency in the range of 30 kHz to 50 kHz. Human beings can’t hear these high-frequency sounds. Unfortunately, all pests do not react at the same ultrasonic frequency. While some pests get repelled at 35 kHz, some others get repelled at 38 to 40 kHz. Thus to increase the effectiveness, frequency of ultrasonic oscillator has to be continuously varied between certain limits.

Be the first to comment - What do you think? Posted by admin - June 4, 2008 at 9:14 am

Categories: Audio diagram Tags: Insect repellent circuit, mosquito repellent circuit, security circuits, ultrasonic mosquito repellent circuit

Enhanced 4 Digit Alarm Keypad by 4081

The Keypad must be the kind with a common terminal and a separate connection for each key. On a 12-key pad, look for 13 terminals. The matrix type with 7 terminals will NOT do. The Alarm is set by pressing a single key. Choose the key you want to use and wire it to ‘E’. Choose the four keys you want to use to switch the alarm off, and connect them to ‘A B C & D’. Your code can include the non-numeric symbols. With a 12-key pad, over 10 000 different codes are available. Wire the common to R1 and all the remaining keys to ‘F’. When ‘E’ is pressed, current through D2 and R9 switches Q5 on. The relay energises, and then holds itself on by providing base current for Q5 through R10. The 12-volt output is switched from the “off ” to the “set ” terminal, and the LED lights. To switch the Alarm off again it is necessary to press A, B, C & D in the right order.