Most Arduino and electronic projects are made up of one or more building blocks attached to each other. Below are a variety of building blocks I find useful:
Arduino can deliver at most 40mA, too little for almost all but the tiniest motors. By using a transistor, the 40mA of Arduino is amplified to control the much higher current of the motor.
Arduino is strictly a 5V device, but a transistor can be used to control a motor powered by a voltage higher than Arduino.
This circuit uses the popular 555 timer in a monostable configuration. The output (LED in this example) is normally off, until the switch is closed momentarily. The LED stays on for about 1 second, regardless of whether the switch is let go or not. After being on for a second, the LED will not light again until the switch is opened and closed again.
R3 and C2 determine the length of time the LED is on. The on time T(on) is:
T(on) = 1.1 * R3 * C2
Instead of the LED (and its resistor) a relay or transistor could be used to activate some other device
If you want to use an electret microphone with Arduino or with a power amplifier, you first need to magnify the signal 10-100 times. This circuit does that. R5, the 100K potentiometer, controls the gain: The larger the value, the more gain. I’ve had good results with a fixed 10K resistor.
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