The Darlington transistor (often called a Darlington pair) is a semiconductor device which combines two bipolar transistors in a single device so that the current amplified by the first is amplified further by the second. This configuration gives a high current gain (written β, hfe, or hFE) and can take less space than two separate transistors because the two transistors can use a shared collector. Integrated circuit packages are available, but it is still common also to use two separate transistors.
Here, we use Darlington pair of transistors to make a dark sensor.
In the figure above, transistors Q1-BC548 and Q2-BC548 make a Darlington pair, these two transistors together work as a single transistor. So, the above circuit resembles a normal automatic dark detector circuit. The following circuit also uses Darlington transistors pair.
In this case, LDR has been replaced with capacitor C1-0.01uF and resistor R3-1K has been replaced with L1 & VC1. In place of a LED, a Piezo Diaphragm has been kept.
We just need to add inductor, capacitor and a piezo diaphragm to convert a dark detector to an AM radio.
To make sound louder, we can use an audio amplifier. Connect ‘OUTPUT’ point of AM radio to INPUT of Audio amplifier.
AM RECEIVER AUDIO AMPLIFIER
Datasheet of LM386!–more–