# LM555: two signls with phase shift

#### DedeHai

Joined Jan 22, 2009
39
Hi there

I want to connect two 555-timers, such that I get two signals with 50% duty cycle that have about 90° phase shift to each other. The Frequency should be between 50 and 100Hz.
The first signal is easy to create by using the timer in astable mode configured such that the output is at the desired frequency and with a duty cycle of 50% (I already have that).
But how do I get a second timer chip to copy the signal of the first one but with a time delay?
The circuit can operate at a fixed frequency, although adjustable frequency would be a nice to have.

#### simeonz11

Joined Apr 29, 2008
98
you can use a xor gate I believe .

run a d flip flop divider faster frequency and a slow freq and xor both ?

You will get that slowe one to be lagged by 90% I believe . Or is that 45 degress ?

#### Wendy

Joined Mar 24, 2008
21,840
You need 2 timers for the 90 degree shift, one for a delay, the second for pulse.

Simeonz11 is close, though a straight gate won't work. Think 2 flip flops counting. Then two AND gates feeding off them would give you want you want (or two NOR gates).

#### DedeHai

Joined Jan 22, 2009
39
can i not use a certain threshold voltage (since the 555-timer has a thresholt input, but i don't really understand how it works) and run the square wave signal of the first timer through a low pass filter and connect the filter output to the input of the second timer?
so instead of using another timer as a delay, just use a low pass instead`?
can you give me an example of how you would connect the second timer in such a way (if there is one)?

#### jpanhalt

Joined Jan 18, 2008
7,697
A couple of suggestions:
1) Remember that the 555 is triggered by a negative going iput. Thus, you will need an inverter after you delay the signal with the filter. A small-signal transistor (e.g., 2N2222 or 2N3904) in common emitter (emitter grounded) configuration should work.
2) I have found that using a coupling capacitor (0.01 uF) between 555 stages helps .
3) This method will not give a fixed offset in phase angle (e.g., 90°) as you vary frequency. The offset will be in units of time, which is determined by the RC constant.
4) Since you want them at the same frequency, you might want the second 555 configured as a one-shot that gets re-triggered from the first 555 with each cycle.

John