I came across this single coil, two transistor circuit on Youtube and found it interesting because it uses a touch pad. This circuit is the same as many other two transistor V booster circuits, often mistakenly called Joule Thiefs (see the schematic in the video). But with one exception: it uses a 0.2uF capacitor that is very much larger than the typical circuit. The large capacitor is two thousand times higher value than the typical 100 pF capacitor that is typical for this circuit. The higher value capacitor pushes the circuit into “squegging” which makes it look like it is flashing.
The circuit’s two transistors have enough current gain so that if a high resistance is substituted for the 2k resistor, it will still work. A typical pair of fingers touching the touch pads will have a resistor from tens of thousands of ohms when moist to hundreds or thousands of kilohms (more than a megohm) when very dry. The few microamps through the fingers is amplified up to tens of milliamps at the collector of the PNP transistor.
The author used an expensive 3W white LED, but the circuit is severely crippled and unable to deliver more than ten or so milliamps to this high power LED due to the BC557 output transistor. A considerable increase in LED current could be obtained by using a BC327-40, or better yet use a seriously high current transistor such as a 2SC2500D, KSC5041, or NTE11. But with the BC557, he might as well remove the 3W LED and put a regular 5mm white LED in its place.
Whatever transistor is used, the coil should have a DC resistance of less than 1 ohm, preferably less than 1/4 ohm at very high currents. When the NPN is used for the output transistor, the other transistor must be switched to a PNP, and the polarity of the battery, LED and capacitors (if they’re polarized) must be changed.