741 Astable Timer
Circuit :  Andy Collinson
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Description
An astable timing circuit made from the ubiquitous 741 op-amp.



Circuit Notes:
In this circuit a standard op-amp is wired as an astable multivibrator. The output is non-symmetrical but has the advantage that the timing is controlled by only 1 resistor and capacitor, in this case, the 100k variable resistor, U2 and 100u capacitor C1. Because of the high loop gain the output rise and fall times are very low, slope times better than 30uS can be acheived even with a 5V supply. Waveforms are shown below:



The green trace is the output voltage at Pin 6 of the op-amp, the blue trace is the voltage at Pin C and also the charge and discharge of C1, and the voltage at the op-amps pin 3 is shown in red. The pinout for the 741 is shown below:



Please note that the the inverting input is pin 2 and non-inverting input is pin 3 on the in the schematic. The timing is shown below. The "on" time is for a positive half cycle, "off" time is the negative cycle, although being run from a single supply, the off voltage is slightly higher than 1 Volt. The frequency is the reciprocal of one half postive and one half negative cycle.

On Time Off Time Freq
VR1 = Max 10.31s 10.21s 48.7mHz
VR1 = Min 0.223s 0.222s 2.25Hz
741 Astable Timer
Downloadable Circuit:
The circuit file is available to download in LTspice, click here.

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