Matthias' Blog

A few months ago I modified a window fan to use a light dimmer as a speed controller. I mentioned that at low speeds, a hum could be heard from the motors since the voltage fed to the fan was not a sine wave. I removed the speed control circuit built into the fan and replaced it with an external light dimmer. When the fan was running at full speed, it drew about 70 watts and had a near-100% power factor. However, as the dimmer was reduced to a lower setting, the motors got louder and the power factor dropped. In this article, I will describe a PWM-based dimmer that maintains a near-100% power factor no matter the speed setting of the dimmer. The schematic is shown below: gEDA schematic file | Link to simulation A 555 timer (U1) is used to generate the PWM signal and drive the gate drive transformer (T1). Suppose the capacitor C4 is charged at 1/3 Vcc. The 555 timer switches its output on, and the capacitor is charged through D4 and the upper half of the potentiometer. When the capc

In this article I will describe the operation of a people counter that uses capacitive sensing. This people counter uses two sense wires to detect the motion of a person through a hallway or entrance. These sense wires are mounted vertically to the wall and sense when a person is close to the sensor. These wires are placed so that the person walks past one wire and then the other when entering the room and vice versa when leaving. The control circuit and microcontroller can determine whether the person is "close" to a sense wire, and this can be used to determine the direction in which the person went. An image of the setup I used is shown below In this image, the black parts represents a wall and the red part represents the door. The green and cyan crosses indicate the positions of the sense wires. If a person walks along the blue arrow, they will first be closer to the cyan cross than to the green cross, then closer to the green cross than the