4.2. Transistor clock

By carefully connecting the two transistors, a simple clock can easily be achieved, as shown in the figure.

Figure 4‑6 Transistor clock

What is important to notice is that the base of the Q1 transistor is connected to high voltage, as well as its collector. The difference is that the base resistor is 100 times greater. The same is true for the Q2 transistor.

The question is which of the transistors will first conduct, considering that we use the same components. The question is actually rhetorical, because the components cannot be the same, but are considered the same with certain tolerances. Therefore, one of the transistors will surely conduct first.

Let us imagine that the Q2 is cut-off and Q1 conducts. The voltage on the base of Q2 rises through the R2 resistor. When the voltage on the base of Q2 is high enough to make it conduct, the current starts flowing from the collector to the emitter of the Q2 transistor, collecting also the charges accumulated on the right side of the C2 capacitor. Since the voltage suddenly drops on the right side of the capacitor C2, the same thing happens also on its left side, causing the Q1 to cut off. Now, the Q2 conducts and Q1 is cut-off.  The process now repeats for the Q1 transistor, which is perfectly explained in Mr.Charles Platt’s book, Make: Electronics, which I highly recommend.

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References:

C. Platt, Make Electronics (Maker Media, 2015)