This is an experimental circuit that allows you to study the operation of a blocking oscillator. In this circuit, the signal feedback is done through a transformer as follows: when the power is turned on, the potentiometer P1 and the resistor R1 drive the transistor base bias current, bringing it close to saturation. In this way, a current pulse appears in the collector, causing the LED to light and at the same time energizing the primary winding of the transformer.
With this, an induced voltage appears in the secondary that passes through C1 and momentarily polarizes the transistor in order to cut it. In these conditions, for a moment the LED will turn off and the primary current disappears. The circuit then returns to the initial condition and again R1 and P1 lead by bringing the transistor near saturation,
The cycle continues indefinitely, causing the circuit to oscillate. With the values of the indicated components the frequency can be adjusted between 1 and 5 Hz, or 1 to 5 pulses per second. In figure 1 we have the complete diagram of the oscillator.
In Figure 2 we have the arrangement of the components on a terminal strip.
The transistor can be any NPN of general use and the transformer can have secondary of 5 to 12 V with current of 50 to 300 mA. The other components are common.
After assembly, switch on the unit. If there is no oscillation, reverse the connection of one of the transformer windings. If you want to change the frequency, change the value of C1.
Material list
Q1 - BC548 - NPN general purpose transistor
LED1 - Common LED of any color
T1- Transformer - see text
P1- 100 k ohm - potentiometer
S1 – On-Off switch
B1- 6 V - 4 batteries or source
C1-470 nF at 1 uF - polyester or electrolytic capacitor
R1 - 10k ohm x 1/8 W- resistor - brown, black, orange
R2 - 220 ohm x 1/8 W- resistor - red, red, brown
Miscellaneous: battery holder, terminal bridge, wires, solder, etc.
