Flashing Eyes

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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.


R1 = 10K
R2 = 1M
R3 = 1K
C1 = 4.7uF-25V
C2 = 47uF-25V
D1 = 2mm LED
D2 = 2mm LED
Q1 = BC547
Q2 = BC557
B1 = 3V Battery
SW1 = SPST Switch
MIC1 = 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 × 1.5 V batteries type: AA, AAA, AAAA, button clock-type, photo-camera type & others.
2 × 1.4 V mercury batteries, button clock-type.