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How to build Flashing Eyes

2017-02-01 15:35  
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This circuit was purposely designed as a funny Halloween gadget. It should be placed to the rear of a badge or pin bearing a typical Halloween character image, e.g. a pumpkin, skull, black cat, witch, ghost etc. Two LEDs are fixed in place of the eyes of the character and will shine more or less brightly following the rhythm of the music or speech picked-up from surroundings by a small microphone. Two transistors provide the necessary amplification and drive the LEDs.

Circuit Diagram:

Flashing Eyes-Circuit Diagram


R1 = 10KR2 = 1MR3 = 1KC1 = 4.7uF-25VC2 = 47uF-25VD1 = 2mm LEDD2 = 2mm LEDQ1 = BC547Q2 = BC557B1 = 3V BatterySW1 = SPST SwitchMIC1 = Electret Mic


Any general purpose, small signal transistor can be used for Q1 and Q2, but please note that R3 could require adjustment, depending on the gain of Q1. For medium gain transistors, the suggested value should do the job. High gain transistors will require a lower value for R3, i.e. about 390 - 470 Ohm. You can substitute R3 with a 1K Trimmer in order to set precisely the threshold of the circuit.Any LED type and color can be used, but small, 2mm diameter, high efficiency LEDs will produce a better effect.No limiting resistors are required for D1 and D2 even if this could seem incorrect.Stand-by current consumption of the circuit is about 1.5mA.Depending on dimensions of your badge, you can choose from a wide variety of battery types:2 x 1.5 V batteries type: AA, AAA, AAAA, button clock-type, photo-camera type & others.2 x 1.4 V mercury batteries, button clock-type.

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