Electronic Candle Blow Out

LED or bulb switch off with a puff

Funny gadget - 3V Battery supply

Circuit diagram:

Audio Clipping Indicator


R1______________10K  1/4W Resistor
R2_______________1M  1/4W Resistor
R3_______________1K  1/4W Resistor
R4_______________4K7 1/4W Resistor (See Notes)
R5______________10K  1/4W Resistor (See Notes)
R6_____________100R  1/4W Resistor (See Notes)

C1_____________100pF  63V Ceramic Capacitor
C2______________10µF  25V Electrolytic Capacitor
C3_____________100nF  63V Polyester or Ceramic Capacitor

D1___________1N4148   75V 150mA Diode
D2______________LED (Any suitable type)

Q1____________BC550C  45V 100mA Low noise High gain NPN Transistor
Q2____________BC337   45V 800mA NPN Transistor
Q3____________BC327   45V 800mA PNP Transistor

MIC1__________Miniature electret microphone

P1_____________SPST Pushbutton Switch

B1_______________3V Battery (2 x 1.5V AA, AAA Cells in series etc.)


This design was developed by request of a correspondent having made a sort of LED candle and needing to switch off the LED with a puff.
This simple, easy to build gadget can be useful as a prop for Halloween and Christmas season, shows and the like.
Q2 & Q3 form a self-latching pair that start operating when P1 is pushed: in this way the LED (or bulb) will illuminate steadily. When someone emits a strong puff in the vicinity of the small electret microphone, the resulting signal will be greatly amplified by Q1 and a rather long positive pulse (shaped by D1 and C2) will reset the self latching pair through the Emitter of Q2.
The very low (and unusual) value of C1 acts as a simple high-pass filter, in order to prevent that normal speech or environmental noise shut off the device. Obviously, such a simple filter cannot be very discriminating, therefore, not only a strong puff will reset the circuit but also a loud shout, blow, clap or stroke.