By Stefan Kalbermatter (Switzerland)
Some time ago the author was asked about options for making a Drum Light for the Carnival festivities. The contraption should light up brightly every time a drum is slammed, providing a nice light effect during late evening and night performances of the Carnival reveler banging the drum, whether solo or in a samba band.
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The initial thought was to have a microphone followed by an amplifier and some kind of monostable multivibrator to make a couple of LEDs light. The idea got rejected because the microphone might catch too much ambient noise during Carnival and could eventually cause the LEDs to light constantly. It seemed another type of triggering device was needed to capture “the drum slam.” A solution was found in a junked alarm clock with a broken case, obviously having crashed to the floor once too many. The built-in piezo buzzer seemed a good candidate for the triggering device. It was quickly dismantled and wired up to an oscilloscope and lo and behold, even gently rapping the piezo element produced nice peaks on the ‘scope. Seemed to work!
Based on this discovery a circuit was worked out on the premise of only dead common components being used — see the schematic. The electronic function is easy: via ‘sensitivity’ potentiometer P1, the piezo element (mounted directly on to the drum) is connected to an LM555 (IC1) which is configured as a monostable multivibrator (MMV). The output pulse of the LM555 can be adjusted by a second pot, P2, within a range of around 0 - 1.1 seconds. A medium-power transistor, T1, at the LM555’s output drives the LEDs. The LEDs are modern high-power types salvaged from a GU-10 style LED lamp, of which the power supply had given up the ghost. Other types of LED may be suitable, but keep the current rating in check and adapt R3 to your LED type.