Circuits designed by David Johnson, P.E.
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List of Dave's Circuit Designs
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Emitting Diodes) Circuits,
Sound Operated Circuits
LED Night Light
June 12, 2013
back, I received a schematic from a guy by the name of Karl. He described a
situation he and his wife found themselves in aboard a cruise ship in the Caribbean.
They were staying in a room toward the inside of the ship, which did not have any
port holes, windows or a balcony. At any time of the day, when the room lights
were turned off, they found themselves in absolute total darkness. The complete
lack of any light was a bit unnerving. They didnít have a night light and could
not one for sale at the shipís stores.
They completed their cruise but Karl decided to build a sound activated
nightlight, for his next cruise. He admitted that his circuit was a bit more complex
and it demanded more power than he liked but he used what parts he had on hand. I
scratched my head and came up with my version of his circuit.
To keep the overall current to a minimum, I selected a cheap crystal
earphone to be used as a microphone. These devices are quite sensitive and draw zero
power, unlike electrets microphones. However, their peak frequency response tends to
be higher than normal speech, somewhere between 1KHz and 2KHz. I found that a short
whistle or a hand clap was all that was needed to turn on the light for 2 minutes using
the circuit shown below. Two minutes would provide enough time to the light
switch or a pathway to the toilet. For a light, I selected 4 super bright white
LEDs made by Cree. The circuit was powered by a 9v battery, housed inside the
plastic box shown below. The box has a nice 9v battery compartment.
The signal from the microphone is AC coupled using capacitor C2 and
connected to a X22 audio Amp, using one half of the dual op Amp A1. The Amplified
signal is then routed to a voltage comparator circuit, using the second half of A1.
Whenever the audio signal exceeds the threshold of the voltage comparator, the output of
the device charges the capacitor C7. The capacitor slowly discharges through R8.
The time constant of the two components is such that it takes about 2 minutes for the
capacitor to discharge. The capacitor voltage is connected to a Schmitt trigger
inverter A2. When the circuit is timing, the n-channel MOSFET transistor Q1 is
turned on, activating the four LEDs. In standby mode, the complete circuit draws
just 35ua. Assuming a 500ma-hour typical capacity of a 9v battery, the battery
should power the unit for more than a year.
on Drawing Below to view PDF version of Schematic