A simple light / dark activated relay switch circuit, suitable for many applications like the automatic switching of the lights in a shop window or a room according to the ambient light level. The circuit uses a light dependent resistor (LDR). A light/dark option has been incorporated. The term 'light/dark' is used because the circuit has a PCB-mounted switch on board. In one switch position a light-to-dark transition will activate the relay. In the other position a dark-to-light transition is required. So you can use the falling light on the detector to switch on a normally off circuit or switch off a normally on circuit. The relay is on when LDR uncovered and relay off when LDR covered. Adjust VR1 for light sensitivity. LED will turn on at the same time with relay.
Posted on Thursday, May 12, 2011 • Category: Phone Circuits
This circuit detects the dial tone from a telephone line and decodes the keypad pressed on the remote telephone. The dial tone we heard when we pick up the phone set is call Dual Tone Multi-Frequency, DTMF in short. The name was given because the tone that we heard over the phone is actually make up of two distinct frequency tone, hence the name dual tone. The DTMF tone is a form of one way communication between the dialer and the telephone exchange.
A complete communication consist of the tone generator and the tone decoder. In this article, we are use the IC MT8870DE, the main component to decode the input dial tone to 5 digital output. These digital bits can be interface to a computer or microcontroller for further application eg. remote control, phone line transfer operation, LEDs, etc...
Posted on Thursday, May 12, 2011 • Category: Power Supplies
Simple, low cost and easy to build power supply. Ideal for applications that doesn’t require too much power. It can provide power to circuit that uses less than 100mA without any problem. The disadvantage of this circuit is the danger of an electrical shock, so it cannot be used if the circuit is in contact with the user. The voltage supplied by this is determined by the zener diode.
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