Astable Flip-flops

Posted on Feb 5, 2014

The familiar astable flip-flop circuit is a handy configuration for making flashers or generating squarewaves. Here is a typical alternating LED flasher with the LEDs in the emitters instead of collectors as is normally done. (There is another good reason to put them in the emitters - see Karen`s note below. ) The bias resistors are directly conne

Astable Flip-flops
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cted to the supply and are chosen to have a value about 100 times the collector resistor for ordinary gain transistors. The flashing period is approximately the product of this resistance and the capacitance which is about 1 second for the circuit as shown. The 470 ohm resistors set the LED current and may be reduced for lower battery voltage but remember to also reduce the bias resistors. If no LEDs are desired, the emitters may be directly connected to ground and two out-of-phase voltage squarewaves are available on the collectors. This is another version of the circuit that uses negative feedback for the bias. This technique is generally more desirable because the feedback ensures that both transistors are in a high-gain, linear mode when power is applied. In actual practice the first circuit will often work "better" with ordinary bipolar transistors since there is no negative feedback slowing the switching. The feedback makes the circuit more immune to parameter variations due to temperature changes, gain variations, or even component substitution. This version will work with just about any NPN darlington transistor. The bias resistor may be much larger due to the high gain of the darlington so much lower value, non-polar capacitors will give a suitable flash rate. Of coures, other applications may require different oscillation rates which are easily achieved by changing the capacitor value. Other voltages and currents...

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