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Calculation of the blink period
for A-stable multivibrator (TR type)


The period which repeats a blink is fixed by the value of capacitor (Cx) and resistor (Rx), and capacitor (Cy) and resistor (Ry). The half period time (t) is possible to calculate by the following formula. The half period is the time which is made H condition or an L condition.
It uses a calculation formula on the side of TR1 as an example.



t : second, Cx : Farad, Rx : ohm


A repeat period is decided by the combination of Cx and Rx, and Cy and Ry.
In case of Cx, Cy=47 F, Rx, Ry=22 k-ohm, it is as follows.

The time that TR1 becomes OFF condition (t1)
t1 = 0.69 * Cx * Rx
= 0.69 * 47 * 10-6 * 22 * 103
= 0.713 seconds

TR2(t2) is similar.

The frequency (f)
f = 1 / ( t1+t2 ) = 1 / ( 0.7 + 0.7 )
= 0.71 Hz


The measurement frequency of the circuit which was made this time was to 0.78 Hz be.

The error with the calculation value is for the error of the part and the characteristic of the transistor and so on. The measurement value of Cx, Cy was to approximately 47 F be. The measurement value of Rx and Ry was to about 20 k-ohm be. When calculating with this value, the frequency becomes about 0.77 Hz. The calculation value and the measurement value are an near value.

Don't expect a correct period with this circuit. The period changes with the temperature around. So, this circuit isn't possible to use for the circuit with strict period.