LM555: One Cycle Pulse Generator

jitendra rana23

Part Number: LM555
Other Parts Discussed in Thread: TLC555, SN74AHC123A

Hi all,

I want to make a one cycle pulse generator with an LM555 timer IC. The pulse timing after powering up the chip is: high (300ms) --> low (1s) --> high (continue until power off).

Help me make a circuit with the LM555 to meet the required pulse (given below).

300ms high 1 second low continuoe high till power off

As soon as the IC is powered up, the pulse should be started with the high edge.

Regards,

jitendra

over 7 years ago

0 Ron Michallick over 7 years ago

TI__Guru**** 164926 points

Jitendra,

This circuit provides low output power up then provides high output for 1s. The output goes low and stays low. Therefore the output needs to be inverted before it is useable.

LM555 can be used in place of TLC555 in schematic

D2 discharges C1 faster in case power off time is short.

D1 prevents overvoltage on trigger input.

C6 ensures U2 powers up in low output state.

This simulates well from 0 to 400ms in TI-tina. To get 0 to 1.4s to simulate, change C5, C7 to zero.

TLC555 1s high after 300ms delay.TSC

0 jitendra rana23 over 7 years ago in reply to Ron Michallick

Prodigy 100 points

thanks a lot,
but sir it is little big circuit, could you please tell me if LM555 we use as astable and monostable multivibrator, what will be status of first output pulse, it would start with positive high edge or negative edge, i think it would start with positive edge.

0 Ron Michallick over 7 years ago in reply to jitendra rana23

TI__Guru**** 164926 points

Jitendra,

Both timers are mono-stable. The U1 output start high then goes low after 300ms. The LM555 timers are level triggered so C2 & R2 block the low DC voltage from the U1 output otherwise the timing of U2 would never end.

You can reduce parts count by changing to SN74AHC123A (or similar). This is a dual monostable that is edge triggered.

0 Ron Michallick over 7 years ago in reply to Ron Michallick

TI__Guru**** 164926 points

New post for SN74AHC123A is here e2e.ti.com/.../711988

Clock & timing

Clock & timing forum

LM555: One Cycle Pulse Generator