I do not understand the NE556 circuit depicted in this data sheet:
on page 6 in figure 1.

NE556 datasheet - figure 1, circuit for astable operation

More specifically I do not understand why they loop 'Out' back to 'Vcc'. I also don't understand how 'RL'´s resistance will influence the circuit.

I am sorry if this question is trivial in any way. I just don't know where to ask apart from here. Thanks

  • 2
    \$\begingroup\$ "RL" is the "load resistance" which represents the circuit you are attaching the 555 timer to. \$\endgroup\$ – Ron Beyer Aug 14 at 20:32
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    \$\begingroup\$ That schematic really should show a connection between the Vcc node and the Vcc pin. \$\endgroup\$ – Hearth Aug 14 at 20:48
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    \$\begingroup\$ If 'Vcc' was actually connected to a voltage source I guess attaching the load in-between 'Vcc' and 'Out' rather than 'Ground' and 'Out' would just invert the signal. But 'Vcc' is exclusively attached to 'Out', isn't it? \$\endgroup\$ – Lukas Schmale Aug 14 at 20:49

The "Vcc" terminal of the IC should be connected to the Vcc(5-15V) point at the upper left, as shown in Figure 2 on the datasheet.

Since the 556 output can source or sink current, RL is not needed (but is perhaps shown to suggest how you could connect an external load.)


Connecting RL between VCC and Output pin improves the output high voltage parameter VOH .
Reference from TI datasheet below
About 1kOhms is mentioned in the datasheet for the specifications.

Choose RA and C so that RA× C > [maximum normal input high time]. RL improves VOH, but it is not required for TTL compatibility.

VCC should definitely be connected to the 5V - 15V source which is missing in the circuit diagram.


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