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Inverting square wave from 555

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boylesg

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Am I missing something here?

Shouldn't the BC557 be inverting the square wave from 555?

And also how can I get rid of or reduce that little voltage ramp in the square wave coming out of the BC557 emitter?

555.jpg

Channel A is on pin 3 of the 555 and channel B is on the emitter of the BC557.
 

EDITS:
We cannot see what the emitter of the BC557 is connected to. Maybe you think the collector is called the emitter?
Darlington Q4 has its emitter and collector pins connected backwards.
Please post the entire circuit showing what the emitter of the BC557 connects to and what the emitters of the darlingtons connect to (their load).
Please separate the two signal waveforms shown on the 'scope.

EDIT: Darlington Q2 also has its emitter and collector pins connected backwards.
EDIT again: The BC557 collector does not have a load resistor to ground.
 
Last edited:

Oh that was stupid, I forgot to flip them.

No wonder it is not working properly.

Thanks anyway.

- - - Updated - - -

It works a lot better now:

555_.jpg
 

What is causing the little voltage spike while the output of the BC557 is low?

The higher the value of R1 the larger the spike is.

I am trying to use a larger value for R1 to limit the amount of current flowing to GND and maximise the voltage at the junction between R1 and the collector of Q5 BC557.

The orange waveform is offset by +10mV or so so I can see it properly.

555__.jpg
 

The output of the BC557 does not "go" low. Instead the BC557 simply turns off and lets the 10k resistor pull the voltage down if it can.
I suspect the output of the darlingtons has a load that has the voltage spike and is causing the little voltage spike to feed through to their bases.
 

Audioguru said:
The output of the BC557 does not "go" low. Instead the BC557 simply turns off and lets the 10k resistor pull the voltage down if it can.
I suspect the output of the darlingtons has a load that has the voltage spike and is causing the little voltage spike to feed through to their bases.
Click to expand...
Is there any way to reduce it other than to reduce the value of R1??
 

boylesg said:
Is there any way to reduce it other than to reduce the value of R1??
Click to expand...
The transistor inverter has a high output resistance. You do not show a supply voltage but your SIM shows around 15V which is fine for using some Cmos CD4069 inverters in parallel to make a low output resistance to feed the darlingtons.
 

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