If you’ve read old textbooks on electronics basics you may recall how it’s possible to create a multivibrator from just a neon lamp and a capacitor. The circuit of the simple multivibrator shown in Figure 1 works in exactly the same way but using an NPN transistor instead of a neon lamp and at a much lower voltage. Anyone can check this out because the function is so basic. But why? The author explains the circuit function like this: In inverse operation (emitter positive with respect to the collector) the NPN transistor has a negative characteristic (which can easily be checked) between its emitter and collector. At around 9 V the base-emitter diode displays the well-known avalanche effect. When this occurs the charge carriers in the junction (barrier) layer are so thick and fast that they release further charge carriers.
The number of charge carriers grows just like an avalanche and with them so does the current. This corresponds exactly to the same effect in a 9 V Zener diode. The internal resistance of this diode remains positive, however. The inverse transistor now adds to this effect. The emitter and collector do indeed exchange roles but the symmetrical principle of its construction means that the transistor functions equally in inverse operation. We can measure a slight current gain from about 3 to 10. The transistor still functions due to fact that the charge carriers pass through the thin base layer to reach the junction barrier. And now comes the salient point: it’s precisely in this barrier layer that the avalanche effect takes place. There are still more charge carriers, which liberate yet more of them, producing an avalanche squared (so to speak). Once this avalanche is triggered, a weaker voltage is all that’s necessary to maintain the effect. The collector current thus amplifies the avalanche effect and assures the negative characteristic.
The strength of the discharge current is sufficient to drive an LED (see Figure 2). For this, we need nevertheless a voltage greater than 9 V. The circuit functions adequately with two almost dead (discharged) 9 V batteries. The LED will still flash for a long time, right until the very last drop of energy in the batteries. The flashing frequency will slow down as the battery runs down.
For mechanical reasons and to simplify construction, the charge resistor is fitted between the batteries.