In this tutorial, we are going to learn about Best Tutorial for The automatic bedside lamp.
It has many found many applications. The most well-known might be the automatic light switch off delay in cars. Here the electronics is responsible for the delayed switch off the interiors lights. This prevents effectively a flat battery incase one forgot to switch off light. But also automatic stairwell lighting or air conditions are based on this principal circuit. If a switch off delay is required somewhere, this circuit can be used. The application here is an automatic bedside lamp. If the button is pressed, the LED lights up for about 2 minutes and then goes off automatically. If the device is placed on a bedside table, one can read the current time of a watch. In this case it is only necessary to press the button once. After two minutes the light is switches off itself without any further action.
How does the circuit work ?
Via the button, the 100 muF elco is charged up. In this way the two transistors start to conduct and the LED lights up. The 1 Mohm resistor discharges the elco slowly. If the elco voltage falls below a certain value, the transistor will block the electric current again and the LED will become dark. As per below image for the introduces also the circuit diagram of the electrolytic capacitor[c1]. Please note the small plus sign in the symbol. As mentioned already in chapter 5, the polarity of a capacitor in a set-up always has to match it orientation in the circuit diagram.
If the 1M ohm resistor is replaced by a 100K ohm type the LED is already dark after about 20 seconds.
Why? If the on-time shall be adjustable, the 1 MOhm resistor can be replaced by a so called potentiometer or “pot” for short. This device is kind of variable resistor that can be adjusted via a rotary knob. Potentiometers are used for example at the volume control in radios and audio amplifiers. In order to protect the pot from over current, a 1K ohm resistor in series to the pot has to be used in this circuit.