NE555: output pull up resistor

Liu Yang

Genius 10135 points

Part Number: NE555

Hi Team,

DISCH pin is Open collector type.

So is the pull up resistor necessary for it and why?

Best Regards,

Tom Liu

over 3 years ago

+1 Chris Featherstone over 3 years ago

TI__Mastermind 28055 points

Tom,

The discharge pin is connected between the timing capacitor and resistor and is used to discharge the timing capacitor to a logic low. The discharge transistor gets activated when the capacitor voltage level rises to 2/3 Vcc. The discharge transistor will discharge the current stored in the timing capacitor to ground. For the output the pull up resistor is not necessary for operation.

Best Regards,

Chris Featherstone

0 Liu Yang over 3 years ago in reply to Chris Featherstone

TI__Genius 10135 points

Hi Chris,

Thank you for your reply.

What type of OUT of Pin 3, is it open collector too?

Best Regards,

Tom Liu

+1 Chris Featherstone over 3 years ago in reply to Liu Yang

TI__Mastermind 28055 points

Tom,

The output pin is of the push pull type. It isn't open collector. Only the discharge pin (Pin 7) is open collector.

Best Regards,

Chris Featherstone

Clock & timing

Clock & timing forum

NE555: output pull up resistor