## Wednesday, February 22, 2023

The operation of this circuit is based on the principle of the transistor connected in reverse bias. The emitter connected to the positive pole of the capacitor has a relatively low reverse breakdown voltage. If this breakdown voltage is exceeded, the emitter-collector junction of the transistor becomes conductive and the transistor begins to conduct in the emitter-collector direction. That's why we have to use a relatively large voltage of around 12V. If the voltage is too low, say 5V, for example, the transistor will not conduct in reverse and the LED will not light up. This phenomenon is due to the fact that the breakdown voltage of the collector is much higher than that of the emitter.

For example, the datasheet for the 2N3904 transistor tells us an emitter breakdown voltage of 6V while it is 60V for the collector.

The other particularity of this assembly is that the base does not have to be connected.

The flashing is based on the time constant (rate of charge and discharge of the capacitor) of the RC circuit connected to the emitter of the transistor. This time constant determines the period "t" of the signal and consequently the frequency of the flashing since f = 1/t.

So to increase the frequency, therefore decrease the time constant, it will suffice to decrease R or C. Conversely, an increase in R or C will reduce the frequency by increasing the time constant.

Of course, the resistance R can be a potentiometer which will allow the easy modification of the frequency.

The shape of the signal being linked to the charge and discharge rate of the capacitor, it can be seen on the oscilloscope that it is a kind of sawtooth whose ramp is not linear.

The signal amplitude is dependent on the supply voltage.

Thank you Christian D for your help.

The operation of this circuit is based on the principle of the transistor connected in reverse bias. The emitter connected to the positive pole of the capacitor has a relatively low reverse breakdown voltage. If this breakdown voltage is exceeded, the emitter-collector junction of the transistor becomes conductive and the transistor begins to conduct in the emitter-collector direction. That's why we have to use a relatively large voltage of around 12V. If the voltage is too low, say 5V, for example, the transistor will not conduct in reverse and the LED will not light up. This phenomenon is due to the fact that the breakdown voltage of the collector is much higher than that of the emitter.

For example, the datasheet for the 2N3904 transistor tells us an emitter breakdown voltage of 6V while it is 60V for the collector.

The other particularity of this assembly is that the base does not have to be connected.

The flashing is based on the time constant (rate of charge and discharge of the capacitor) of the RC circuit connected to the emitter of the transistor. This time constant determines the period "t" of the signal and consequently the frequency of the flashing since f = 1/t.

So to increase the frequency, therefore decrease the time constant, it will suffice to decrease R or C. Conversely, an increase in R or C will reduce the frequency by increasing the time constant.

Of course, the resistance R can be a potentiometer which will allow the easy modification of the frequency.

The shape of the signal being linked to the charge and discharge rate of the capacitor, it can be seen on the oscilloscope that it is a kind of sawtooth whose ramp is not linear.

The signal amplitude is dependent on the supply voltage.

Thank you Christian D for your help.