Power "off" delay timer

Thread Starter

Manfred Von Steinborn

Joined Feb 3, 2009
38
I'd like to build a 12V one shot delay timer which would arm when the ignition key is turned "on" and start countdown (3-4 min) when key is turned OFF. So, the key OFF is a triggering event.

I found this circuit at http://home.cogeco.ca/~rpaisley4/LM555.html
and need help in determining values of C1,R1 to achieve 3-4 min delay or making it adjustable. Do I have the right circuit?

Power OFF Delay Circuits

These circuits will delay the removal of power to an external circuit by using mechanical relays or transistors. Other output control devices could also be used.



Thank you for your help

Manny
 
Last edited:

Dodgydave

Joined Jun 22, 2012
9,651
I'd like to build a 12V one shot delay timer which would arm when the ignition key is turned "on" and start countdown (3-4 min) when key is turned OFF. So, the key OFF is a triggering event.

I found this circuit at http://home.cogeco.ca/~rpaisley4/LM555.html
and need help in determining values of C1,R1 to achieve 3-4 min delay or making it adjustable. Do I have the right circuit?

Power OFF Delay Circuits

These circuits will delay the removal of power to an external circuit by using mechanical relays or transistors. Other output control devices could also be used.



Thank you for your help

Manny
No pin 4 needs to be connected to pin 8, then it should work, i take it S1 is the ignition key ?
 

Wendy

Joined Mar 24, 2008
22,170
Pin 4 of a 555 must be connected to Vcc for the chip to work correctly. Pin 4 at low shuts the 555 down.

This appears to be an automotive thread. This is not allowed at AAC, but since I am not quite certain I have refrained from closing it.

If it is please let us know, and then move on to a different subject. Automotive is a firm restriction on this site.
 

Thread Starter

Manfred Von Steinborn

Joined Feb 3, 2009
38
My calculations show for C1=100uF and R1=1 M ohm, timing cycle = 110 sec
?
Should I follow pin 4&5 typical connection recomendations, per notes below?




<H4> </H4><H4> </H4><H4>LM555 Timer Internal Circuit Block Diagram</H4>




RESET And CONTROL Terminal Notes

Most of the circuits at this web site that use the LM555 and LM556 timer chips do not show connections for the RESET and CONTROL inputs. This was done in order to keep the schematics as simple as possible.
If the RESET terminal of a 555 or 556 timer is not going to be used, it is normal practice to connect this input to the supply voltage. If the RESET terminal is left unconnected the operation of the timer will not be affected, however, the RESET of CMOS version of these timers should not be left unconnected as the inputs of these devices are more sensitive and this may cause problems.
In many cases the CONTROL input does not require a bypass capacitor if a well regulated power supply is used. However, it is good practice to place a 0.1 microfarad (C2) capacitor at this terminal to minimize voltage spikes during transitions of the timer's output transistors.
It is also good practice to place a 0.1uF bypass capacitor (C1) across the power supply and located as close to the IC as possible. This will also reduce voltage spikes when the output transistors of the timer change states.
Typical Pin 4 And 5 Connections

Note - If the period of the power supply variations is short when compared to the period of the timer, the overall effect of C2 is reduced.
For example; If the power supply - ripple voltage is 120 Hz and the oscillator frequency is 1000 Hz then C2 will have greater benefit than if the oscillator frequency is 10 Hz.
Therefore, at low astable frequencies or long monostable times the effectiveness of a capacitor at the CONTROL input is less than at higher frequencies and short pulse times.
 
Last edited:

wayneh

Joined Sep 9, 2010
16,400
It's a small quibble, but I'd use a larger capacitor, maybe 1000µF or larger, so that I could get more flexibility on the resistor and timing. You could use a 500K linear pot to dial in the timing you like and then, optionally, replace it with a fixed resistor. Make sure you put this circuit where it will be safe from condensation. The components won't likely be rated for marine service unless you coat them.
 

bertus

Joined Apr 5, 2008
20,876
Hello,

You have replied to an old thread.
Manfred Von Steinborn had his last activity on September 28 2012.
He was not seen after that.

Bertus
 
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