How to drive a transistor with alternating current

Here you see how to design a circuit with a Darlington in which you can drive a transformer, lamp, or LED by means of a (very small) alternating current (approx. 1 mV - 3 Volt or so). The capacitor decides on which frequencies the schematic ideally will react, because the "pass through" of a capacitor is frequency dependent. This is already very well demonstrated on a video on the channel of Allamericanfive radio on YouTube. You can optimize the schematic for a certain frequency band by selecting the right capacitor experimentally or with the help of an oscilloscope. If you use a NPN Darlington, the schematic will react on the positive sides of the AC, if you use a PNP darlington the schematic reacts on the negative sides of the AC. First set the schematic to its "reacting point" (threshold voltage) by means of the variable resistor in the base-circuit. You can also make this schematic with 1 transistor, it is less sensitive in that case; use a potentiometer of 25-50 K in that case. Practical applications: let a LED glow on the rhythm of the music (only positive or negative sides of the pulses), drive a lamp or relay. Or drive a transformer for instance for higher voltage applications. In that case you can drive the Darlington with a AC generator (sine-/squarewave), the ideal frequency depends on how the transformer is constructed (resonance frequency of the transformer). Important: if you drive a transformer realize that this is an inductive load. So this does not act as a Ohms resistor. Due to a reverse voltage/amp that is created in the coil as soon as the Darlington conducts, high peak voltages can appear on the collector of the power transistor of the Darlington. You can prevent this a little bit by connecting a diode over the primary coil or make the load less inductive by connecting a resistor over the coil, so that the high peaks will be muffled a bit.