555 delay between restart

Thread Starter

TheDag

Joined Jan 1, 2010
94
Hi,
I Have 555 monostable circuit, when 12v input, its pulse for 5 seconds....
now,
If im unplug the voltage source and plug it again quick, its not work...
I have to wait 1-2 seconds before i plug the voltage source in again....
I Want it to be without delay, i think its something with the capacitors.....

Please Help, Thanks.
 

Attachments

jimkeith

Joined Oct 26, 2011
540
Try this:
The 470Ω discharges the power supply quickly.
The diode discharges the timing capacitor quickly.
Beyond that, reduce the value of the timing capacitor while increasing the timing resistor.
 

Attachments

Wendy

Joined Mar 24, 2008
23,415
If you were to put a back biased diode across R3 it would discharge the capacitor it the power supply were shorted when turned off. This would do what you want.
 

Wendy

Joined Mar 24, 2008
23,415
Pin 7 is a open collector.

The reason you have to wait a while is the capacitor has a charge on it. You need a way of discharging it, which posts #2 and #3 explained how to accomplish.

Try Jim's circuit, it will work if wired correctly.

This is a power on delay circuit, it resembles a monostable, but you have tied the trigger (pin 2) to pin 6 instead of the classic 555 monostable circuit.

There is another way to do it, if you are interested.
 

Thread Starter

TheDag

Joined Jan 1, 2010
94
in jim's circuit,
i can connect the resistor just to battery + and battery - ?
or its must be beside the capacitor and potentiometer ?
 

Wendy

Joined Mar 24, 2008
23,415
Yes. It is the same thing. Schematics are not about physical locations.

Do be aware of a switch if you have one though, you do not want the resistor on the wrong side of the switch.
 

jimkeith

Joined Oct 26, 2011
540
Now to reduce battery current, you can start increasing the 470Ω resistor until it malfunctions in order to determine maximum value.
 

Thread Starter

TheDag

Joined Jan 1, 2010
94
if before the 555 i have 12v to 9v.
Can i put the resistor and the diode in the 12v from the beggining and its will still work ?
 

Thread Starter

TheDag

Joined Jan 1, 2010
94
for fast discharging lm317 circuit
which capacitor i should discharge?
470ohm resistor between vOut and GND, and where the diode goes to ?
 

Wendy

Joined Mar 24, 2008
23,415
Jim was referring you to the fact that a 470Ω resistor is a drain on your battery. It is assumed you want your battery to last as long as possible.

In all circuits like this there is compromise, you need the 470Ω resistor to drain the capacitor when there is no power, but that same resistor will drain your battery when the power is on.

If you had a power switch that connected the two power rails (+ and -) together when the power was off you could eliminate the 470Ω resistor and the power drain.
 
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