Want to add a DC offset to a 555 timer circuit

[kd4pbj]

I need to build a circuit to create a 5 volt peak to peak square wave, that is 5 volts high and 0 volts low. How can I add a 2.5V DC offset to one of the standard simple 555 timer oscillator circuits? The circuit will be operating around 500 khz (exact frequency doesn't matter).
I wasn't sure if I put a Zener diode in the ground pin to the timer chip or I floated it above ground with a voltage divider if that would work.
If this is not a workable idea, is there a circuit that I could build to feed the output of my function generator into that would allow me to add a voltage offset (can I do this with an opamp?)
Thanks for any advice.

Chris

 

[dknguyen]

You can use an op-amp buffer circuit, except you connect the input that normally goes to ground to a resistive divider. Though, the op-amp supply has to be 7.5V or greater.

 

[ericgibbs]

hi chris,
If you use a 7.5Vdc supply for the 555 and use a 2.5Vref[zener] from the 555 0v pin to the 'system' ground that should be OK, connect a cap across the 2.5VZ.

You should use this 2.5V virtual ground [junction of the 555 and the 2.5VZ] as the ref ground for all the timing components on the 555 circuit.

What are you going to drive with the 5Vsqrwave + 2.5v ?

 

[kd4pbj]

Hi DK and Eric,

I am testing a digital transceiver chip for a project and the datasheet said that it could be driven up to VCC (I am using 5 volts) but not to let the wave swing below -0.3V.
I have a spare LM358 or two lying around so I will use this as my bias generator for the 555 timer circuit.
Thanks again!

Chris

 

[Nigel Goodwin]

I don't see why you would need to?, running the 555 off the 5V supply it will output a rectangular wave from (roughly) 0V to 5V.

 

[ericgibbs]

Hi DK and Eric,

I am testing a digital transceiver chip for a project and the datasheet said that it could be driven up to VCC (I am using 5 volts) but not to let the wave swing below -0.3V.
I have a spare LM358 or two lying around so I will use this as my bias generator for the 555 timer circuit.
Thanks again!

Chris

The output from the 555 will not swing to -0.3V when using a +5V/0V supply to the 555????