Halloween Flashing-eyes Badge

Posted in LED, Light and music, on 2015-04-01

Circuit diagram:

Halloween Flashing-eyes Badge


  • R1______________10K 1/4W Resistor
  • R2_______________1M 1/4W Resistor
  • R3_______________1K 1/4W Resistor (See Notes)
  • C1_______________4µ7 25V Electrolytic Capacitor
  • C2______________47µF 25V Electrolytic Capacitor
  • D1,D2_________LEDs (2mm. preferred - See Notes)
  • Q1_____________BC547 45V 100mA NPN Transistor
  • Q2_____________BC557 45V 100mA PNP Transistor
  • MIC1___________Miniature electret microphone
  • SW1_____________SPST Miniature Switch
  • B1________________3V Battery (2 x 1.5V AA, AAA Cells in series etc. - See Notes)

Device purpose and Circuit operation:

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.


  • 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.
  • 1 x 3 V Lithium cells.

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