'NE555 P' Reset pin problem

Thread Starter

vinodquilon

Joined Dec 24, 2009
234
I have set up the 555 astable MW to generate AF signals as shown in the attachment.
The intended function is that When StD control signal goes logic HIGH AF signals are permitted
to appear at + output. When StD is logic LOW, AF signals get suppressed at output.

But in practical when StD is LOW, AF signals not suppressed at output.
So I manually connect 4th pin to GROUND, then the signals get suppressed at + out.

So I am going for an OR gate as a replacement of Q4 & R9. One pin is always LOW, other one
connected to StD control.

Does have any idea about the problem ? I think Q4 open condition doesn't bring GROUND
condition at active low RESET pin.

Attachment- http://www.scribd.com/doc/27308469
 

Attachments

SgtWookie

Joined Jul 17, 2007
22,227
I have set up the 555 astable MW
What is an MW?
to generate AF signals
Do you mean audio frequency signals?
as shown in the attachment.
The intended function is that When StD control signal
What's the StD control signal?
goes logic HIGH AF signals are permitted
to appear at + output. When StD is logic LOW, AF signals get suppressed at output.

But in practical when StD is LOW, AF signals not suppressed at output.
So I manually connect 4th pin to GROUND, then the signals get suppressed at + out.

So I am going for an OR gate as a replacement of Q4 & R9. One pin is always LOW, other one
connected to StD control.

Does have any idea about the problem ? I think Q4 open condition doesn't bring GROUND
condition at active low RESET pin.
Pin 4 of the 555 timer is the RESET pin. If brought high, the 555 timer will be enabled. If brought low, the timer will not function.
 

Ron H

Joined Apr 14, 2005
7,014
Reset needs a current sinking control signal. Your NPN emitter follower cannot sink current. It needs 0.4V and 0.4mA worst case, so a 1k resistor from pin 4 to ground should work, unless R9 is excessively large in value.
Why can't you connect StD directly to pin 4?
 

Wendy

Joined Mar 24, 2008
23,076
Pin 4 is a direct digital input. I've found it can be as low as .7V and still work, if it is grounded the 555 stops.

Have you looked at my notes on this site about 555s? The cookbook in particular covers several different oscillators.

My Cookbook

Multiwibrator!?! That made me laugh! :D
 
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