KICAD 4: BJT Oscillator

When beginning with electronics you’ll get introduced to alot of beginner projects. One of those probably uses the 555 timer IC to generate pulses that make LEDs flash or speakers buzz. This generation of pulses is called oscillation and is used in electronics everywhere.

A more primitive way to get oscillation is with a very ancient circuit called the ‘transistor LED flasher’. This circuit uses a very nifty way to create oscillation. Also notice how the circuit is perfectly symmetrical.


R1 and R4 limit the current through the LED and R2+C1 and R3+C2 determine the speed in which the capacitors loads and thus determines the switching speed of the transistor. The speed (frequency) at which the oscillator ‘vibrates’ goes up when R2 and/or R3 go lower in value or when the capacitors go lower in value.


The values to choose for the components depend on the supply voltage. In this case with a 9 volt power supply (like a battery) the current limiting resistor for the LED’s (20mA) should be:

R1 & R4 = U / I = 9-2.8 / 20mA = 7 / 20mA = 350 Ohm. (Subtract 2.8 volts because of the forward voltage of the LEDs and transistor internal diode!)

The frequency of the circuit where C1=C2 and R2=R3 is given by:

f = 0.72 / RC

For example we use 68k Ohm as R2=R3 and 4.7 uF as C1=C2 then the frequency is:

f = 0.72 / 68000 * (4.7*10^-6) = 2.25 Hz

If the values of C1 != C2 and R2 != R3, then the following equation applies:

f = 1 / (0.693 * (R2*C1 + R3*C2))

Applying this equation to the same values give:

f = 1/(0.693 * (68000*(4.7*10^-6) + 68000*(4.7*10^-6))) = 2.25 Hz


The circuit above has been transferred to the following PCB:


As you can see I forgot that ground planes were a thing. Using ground planes would’ve got rid of a lot of unneeded traces and would’ve made the PCB single sided instead of two sided. Also the fancy edge cutting wasn’t necessary but I like to make the boards look odd. The 3D render proves that

3D Render