The amusing circuit of electronic candle was designed to react by blowing or giving a puff to a microphone, causing the bulb or LED to turn OFF like a normal candle.
Terminology BC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipmentBC337 – a small signal NPN Silicon AF medium power transistor used for general purpose switching and amplifying applications with features such as TO-18 manufactured package, suited for AF driver stages and low power output stages, and divided into three group typesElectret Microphone – a type of condenser of capacitor microphone that utilizes a permanently charged object to eliminate the use of a power supply Circuit Explanation
The circuit employs the use of SPST push button switch P1 to start the operation of the circuit as the switch is pressed. The LED will steadily illuminate at this stage since the transistors Q2 & Q3 are forming a self-latching pair by locking to each other. A small electrets microphone is used so that a strong puff or blow by someone within the vicinity will result to a signal that will be greatly amplified by Q1. The combination of D1 & C2 will shape a rather long positive pulse that will reset the latching pair via the emitter of Q2.
A simple high pass filter will be performed by the very low and unusual value of capacitor C1. This is an essential part of the circuit since it is responsible for preventing the environmental noise or the normal speech to cause the shutting down of the device. Since the filter is very simple, it will not be more concentrated and favorable which makes the circuit still with vulnerability. This is because the microphone can still be affected by a loud shout, stroke, or clap.
A satisfactory incandescent lamp may be used in replacement of the LED which can have a value of 3 V to 3.5 V with an output power of 0.7 W at 200 mA current. If used so, R5, R6, and D2 may be neglected and the value of R4 is changed to 1K5 while the bulb is wired in place of R5. Using an incandescent bulb instead of LED, with a rating of 1.5 V at 0.3 A, can be supplied by 1.5 V battery source. Simultaneously, a reliable circuit operation will be allowed to be adjusted by replacing the resistor R4 with a 470 Ohms trimmer. During the stage where the LED or bulb are OFF, the circuit is still drawing a small amount of current. For the LED the current is approximately 1.2 mA while for bulbs, the current is 1.5 mA for the 3 V bulb and 1 mA for the 1.5 V bulb type. The accumulation of a power ON-OFF switch could be vital in some instances.
The electronic candle circuits are being used widely during special occasions like Halloween and Christmas season as it functions as a prop or part of the display. As a decorative item, they are being used in churches as altar candle and in funeral or memorial service. The incandescent bulbs however, are being utilized in advertising signs & marquees, stained glass lamps & lanterns, hobby applications, novelty applications, mood lighting effects in commercial & home applications, electric fireplaces, arts & crafts applications, model railroading accessories, haunted houses & spooky lighting, automotive novelties, doorway & garden lanterns, amusements, and miniature models like the doll houses with the use of gas and oil lamp simulation.