Best Way To Cancel 1 Shot 555 Timer?

Thread Starter

NFA Fabrication

Joined Aug 12, 2012
110
What is the best way to cancel a 1 shot monostable 555 timer circuit? I have a timer built that goes on for about 2 minutes when triggered, but needs to be cancelled by a voltage (Basically the same voltage as the positive rail).

I have never needed to do this before, so I just connected to the + on the timing capacitor, which worked for the most part, but altered the time of the 555 because the trigger source to cancel the timer was a source that turned on some lights, but the lights were manipulating the capacitor because of the resistance to ground when the bulbs weren't lit. Any ideas?
 

Thread Starter

NFA Fabrication

Joined Aug 12, 2012
110
There are two ways, discharge the cap with a transistor, or use the reset pin (4).

OK. Because I was originally pulling the capacitor high to "Run out the timer". And I was thinking about the reset pin, but wasn't sure how I could do anything with it since it was already connected to the positive rail.
 

crutschow

Joined Mar 14, 2008
24,952
.................
And I was thinking about the reset pin, but wasn't sure how I could do anything with it since it was already connected to the positive rail.
It's only connected to the positive rail when you don't use it. You pull it to ground for the Reset function (you can use a resistor to the positive rail to pull it high otherwise).
 
I realise that this is an old thread but I am in the exact same situation and wish to clarify the advice with a circuit diagram. Is the circuit diagram below the correct interpretation of advice given for using pins four or five to cancel the timer? Thanks!

 

crutschow

Joined Mar 14, 2008
24,952
Yes.

But a 50Ω base resistor is much too small a load for the pin 3 OUT terminal.
To fully turn on the transistor, the base resistor value should be selected to allow a base current of between 5-10% of the maximum collector current.
Look at the 555 data sheet for the allowed OUT load current.

In the future please draw your schematics in the conventional manner with the power rail on top and the ground/common rail on the bottom.
;) .left to right from written English read to trying like is reverse in it Doing
 

dl324

Joined Mar 30, 2015
10,735
Another point about the schematic style. Using "pin order" components tends to make schematics less readable. Functional blocks in well drawn components typically have inputs on the left and outputs on the right (with some exceptions).

The timer component in Eagle wasn't to my liking, so I redrew it:
upload_2018-9-30_7-31-6.png

I put the reset pin on the top next to V+ because it's usually connected to V+.
 
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