This is a simple circuit for chances of a breakdown are always greatest on a cold and windy night just when you need some sort of warning to say you are laid-up at the side of the road. This circuit is based on flip-flop concept. This is the figure of the circuit.
This type of circuit is very easy to design and is almost guaranteed to work with any type of transistor. It will also accept a wide range of values for the capacitors and resistors. It is also self-starting and uses very few components. In other words, it works superbly. A number of other flip flop circuits have been invented but they have been a flop. Some required close tolerance resistors and high gain transistors to make sure the circuit self-started and others were very complex in operation. Some of the circuits looked completely different to our “symmetrical” design and it was almost impossible to see how they worked. This is exactly what the BD 679 is. It is two transistors in one package and yet it can be treated as a single transistor as far as the circuit is concerned. This device is called a Darlington transistor, named after the inventor.
The only major difference between a single transistor and a Darlington is the turn-on voltage. For a normal transistor the turn on voltage is about .65v, while the turn on voltage for a Darlington transistor is 1.3v. In our circuit this turn-on voltage does not worry us as the circuit provides plenty of voltage for the base. The only thing we are concerned about is the turn-on CURRENT and when a Darlington transistor is used, the circuit provides sufficient base current. This is the secret behind getting this type of circuit to work and is quite often over-looked by designers.
This type of circuit is very easy to design and is almost guaranteed to work with any type of transistor. It will also accept a wide range of values for the capacitors and resistors. It is also self-starting and uses very few components. In other words, it works superbly. A number of other flip flop circuits have been invented but they have been a flop. Some required close tolerance resistors and high gain transistors to make sure the circuit self-started and others were very complex in operation. Some of the circuits looked completely different to our “symmetrical” design and it was almost impossible to see how they worked. This is exactly what the BD 679 is. It is two transistors in one package and yet it can be treated as a single transistor as far as the circuit is concerned. This device is called a Darlington transistor, named after the inventor.
The only major difference between a single transistor and a Darlington is the turn-on voltage. For a normal transistor the turn on voltage is about .65v, while the turn on voltage for a Darlington transistor is 1.3v. In our circuit this turn-on voltage does not worry us as the circuit provides plenty of voltage for the base. The only thing we are concerned about is the turn-on CURRENT and when a Darlington transistor is used, the circuit provides sufficient base current. This is the secret behind getting this type of circuit to work and is quite often over-looked by designers.
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