# Is an ADC always necessary between 8051 and zero crossing detector?

I am making a SCR(Thyristor) trigger by using 8051 microcontroller. I have a here is a zero cross detecting (ZCD) circuit which detects the zero points and this is conveyed to the 8051 which will do further action.

The ZCD circuit is made up of BJT transistor(s) and gives a pulse of 5 V . So is it necessary to add an ADC in between the ZCD and 8051?

PS:I am trying to first simulate in multisim, and its not working (I have directly connected the output of ZCD to pin p1.0 of 8051).

edit(updated problem):sorry for not clarifying the problem.I mistakenly used led word for probe thinking both are same. You can see one round thing connected to pin P2.0(bottom right). It is the probe. What I was trying to do is to detect the pulse from ZCD and convey it to 8051. 8051 will take the input and turn on the probe, and then again go for checking it. SCR(with optocoupler) will come in place of probe, but thats second part. I am just trying to make the first part work correctly.The probe blinks at first -90 to 0 while it should only blink when the pulse is given to p1.0 port.

\$MOD51  ; This includes 8051 definitions for the Metalink assembler
ORG 0000H;
loop:MOV P2,#00H;//make probe off
SETB P1.0;//make the p1.0 as input port
AGAIN:JNB P1.0,AGAIN;//loop here until p1.0 is high
MOV P2,#0FFH;//turn on the probe
sjmp loop;//go for checking the pulse again-infinite loop
END;


• In general, if your circuit is generating a pulse you should be able to feed this directly in as a digital input to your 8051. You just need to make sure that it meets all the requires of the digital pin and does not in turn stress it (over voltage). Your circuit may be different however, posting it here will help you get a concise answer. – placeholder Feb 27 '13 at 17:00
• I am trying to add image but it is not allowing me giving reason of 10 rep.. – Santosh Feb 27 '13 at 17:08
• just post a link and either I or someone else will place it up there for you – placeholder Feb 27 '13 at 17:10
• Where is the LED? Is that your entire code for the 8051? – Oli Glaser Feb 27 '13 at 18:46