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# Astable Flip-Flop Circuits

Have fun designing your own flasher. The circuit will need sufficient non-inverting gain to achieve oscillation which probably means at least two transistors. Make sure that the two or more stages are "alive" by biasing them away from ground or the power supply voltage. Then apply the feedback and try to figure out what happens when the circuit switches from being a linear amplifier into a switching, non-linear flip-flop. If the gain is sufficient and non inverting, something will happen!

Here are some notes from readers:

Karen mentions that, " there is a practical supply limit on the good old fashioned two transistor astable, and in fact any timing ciruit that uses a reverse-biased BE junction. Between 5V and 9V most reverse-biased BE junctions go into zener mode. Explains why they were never shown with supplies in excess of about 6V!" Good point, Karen! When one collector pulls down, the voltage on the base of the other transistor will be pulled below the emitter voltage by  nearly the supply voltage. I would recommend adding diodes in the emitters much like the LEDs in the first circuits for operation above 6 volts.

Thought I would pass this along. I was using the "Astable Flip Flop Circuit" page and setting up the first example, the "flip flop flasher". I added a potentiometer between the power supply and the two 47K resistors and came up with a single component modification that will adjust the flash rate. I removed the 47K resistors from the power supply and connected them to one end of a 500K VR. I connected the opposite end of the 500K VR to the power supply. The center tap gets connected to either side of the VR producing a 0 - 500K (slow to fast) or 500K - 0 (fast to slow) range.

Trial and error shows that the second circuit, "With negative feedback bias", can be adjustable using a 10k resistor and a 0-100k variable resistor in series between the two transistor bases.

Jay Herde Louisville KY

Thanks Jay!

Jay's first modification connects the bases together somewhat but since one is directly connected to the collector of the other transistor through the capacitor, the switching still occurs. There might be a problem if the potentiometer is much higher in resistance than the resistors, especially if the transistors do not have similar characteristics. A small capacitor, maybe 10% of the timing capacitors, connected to ground at the junction of the bias resistors and potentiometer might fix any problem. Jay's worked with a pot 10 times bigger than the resistor so perhaps the problem is minimal.