LM555: LM555

Yanni Li

Intellectual 1960 points

Part Number: LM555
Other Parts Discussed in Thread: REF3433

Hi team，

We have a customer inquiring about the LM555, here are the inquiry from the customer in verbatim:

“I am using the LM555 in astable mode as an LED driver. I will be using nmos transistor as a switch, connected between Vcc and Reset pins of LM555, in order to control the signal into LM555. for this purpose, I will be interfacing the LM555 with a pic microcontroller. the only issue is that I need to change the trigger level of the LM555 to 3.3V, and the datasheets don't provide information on how to do so. Could you please provide me with a solution? Also, please note that the supply is 10.3 ~ 14.8 V battery charged through a solar panel."

Thanks and Regards,

Jenny

over 4 years ago

0 Chris Featherstone over 4 years ago

TI__Mastermind 25210 points

Jenny,

The control pin can be used to adjust the trigger and threshold levels. Below you can see the schematic I put together with a control voltage of 3.3V and the supply I set to 12V. I have attached the simulation at the bottom of the post as well.

In the below waveform you can see that the timing is controlled by the voltage on the timing capacitor reaching the control voltage of 3.3V and 1/2 of the control voltage or 1.65V.

Astable With Control Voltage.TSC

0 Yanni Li over 4 years ago in reply to Chris Featherstone

TI__Intellectual 1960 points

Thank your for your help.

After a lot of researching, I found that level shifters or voltage level translators could be used to achieve the desired trigger level. but it wasn't an application on an LM555. Would you please tell me if this is applicable to my application? If yes, I would really appreciate it if you can tell me how to choose an appropriate level shifter.

0 Chris Featherstone over 4 years ago in reply to Yanni Li

TI__Mastermind 25210 points

Yanni,

You may consider using a voltage divider slightly down and using a voltage reference such as the REF3433. It can accept a power supply up to 12V so your supply should be divided down to not exceed 12V but not go below the minimum supply. The reference will output the 3.3V.

https://www.ti.com/lit/ds/symlink/ref3433-ep.pdf?ts=1617125267364&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FREF3433-EP

0 Mohammed Alsubhi over 4 years ago

Prodigy 10 points

Hello,

I appreciate all your efforts!
Unfortunately non of the provided solutions worked for me. However, after a lot of experimenting, using an open collector-collector circuit connected to the trigger pin on LM555 and PIC microcontroller did the job! That helped to change the triggering level of LM555 to the desired value of 3.3V.

Thank you.

Mohammed.

0 Chris Featherstone over 4 years ago in reply to Mohammed Alsubhi

TI__Mastermind 25210 points

Mohammed,

I am glad you were able to get your circuit to work as desired. I will keep your inputs in mind if this comes up again. Thank you!

Clock & timing

Clock & timing forum

LM555: LM555