# Producing a momentary low volt DC pulse from a phone charger

#### tmetford

Joined Apr 19, 2013
7
What would be a circuit design that would produce a momentary (low voltage DC) pulse, rather than a continuous low voltage current, from a phone charger (already plugged into a 240 volt mains power socket) when the mains socket switch turns the phone charger on?

I’m a novice when it comes to designing even a simple electronic circuit so I need your help to solve this puzzle which, hopefully, is all too obvious to your experienced eye!

When the mains switch is turned on the phone charger will, of course, produce a constant 5 volt direct current, but I want only a momentary 5 volt pulse. This is pulse would trigger the operation of a simple 555 timer delay circuit so that it can do its job of turning off a lamp after a set period of time: a continuous 5 volt current from the phone charger will, of course, defeat the efforts of the timer circuit to switch the lamp off. The low voltage DC pulse is intended to replace the action of the “push-to-make” button switch that currently triggers the timer circuit and the power for the timer circuit is supplied only when the mains AC power socket is turned on and left in the “on” state.

I hope this gives enough information to clarify my objective and your comments and ideas will be appreciated.

Many thanks.

#### Dodgydave

Joined Jun 22, 2012
11,350
If you're going to use a 555 , you can set it to auto trigger on power up, by putting pin 2 between a resistor and capacitor across the supply.
For long delays a CD4060 is best.

#### Ya’akov

Joined Jan 27, 2019
9,265
It sounds like you need a one-shot, also called a monostable multivibrator. A 555 can be wired to produce a pulse of the desired length and then turn off until it is triggered again.

It would be helpful to have a schematic of what you intend to do, or at least a wiring diagram. Your narrative is somewhat ambiguous.

#### crutschow

Joined Mar 14, 2008
34,842
Conceptually a capacitor in series with the phone charger output with a resistor to ground, will give a positive pulse with an exponential decay to 0V when the charger is turned on.

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#### tmetford

Joined Apr 19, 2013
7
If you're going to use a 555 , you can set it to auto trigger on power up, by putting pin 2 between a resistor and capacitor across the supply.
For long delays a CD4060 is best.
Thank you!! I replaced the push button switch with a 1uF capacitor and it worked! I did as you suggested, putting pin 2 between a resistor and the 1uF capacitor across the supply (where the switch had been connected) and hey presto!
I'm really chuffed to get this to work and delighted to learn that there was a simple solution. I am enjoying the learning curve here too!
I've attached a circuit diagram of the modification you suggested which is a bit late now, of course, but thank you for being able to decipher what I was going on about without a more complete explanation!
Lastly, thanks for the idea of the 4060 IC which, if I need longer delays this would be useful, provided my circuit assembly skills are up to scratch!
Many thanks, Tim

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#### tmetford

Joined Apr 19, 2013
7
It sounds like you need a one-shot, also called a monostable multivibrator. A 555 can be wired to produce a pulse of the desired length and then turn off until it is triggered again.
It would be helpful to have a schematic of what you intend to do, or at least a wiring diagram. Your narrative is somewhat ambiguous.
Thank you for your help here and, your're right, my narrative is ambiguous - my apologies. I hope to give a clearer explanation next time.

#### tmetford

Joined Apr 19, 2013
7
Conceptually a capacitor in series with the phone charger output with a resistor to ground, will give a positive pulse with an exponential decay to 0V when the charger is turned on.
Thank you for your help and I shall study the electronic theory so I understand why this solution works!

#### Ramussons

Joined May 3, 2013
1,418
What would be a circuit design that would produce a momentary (low voltage DC) pulse, rather than a continuous low voltage current, from a phone charger (already plugged into a 240 volt mains power socket) when the mains socket switch turns the phone charger on?

I’m a novice when it comes to designing even a simple electronic circuit so I need your help to solve this puzzle which, hopefully, is all too obvious to your experienced eye!

When the mains switch is turned on the phone charger will, of course, produce a constant 5 volt direct current, but I want only a momentary 5 volt pulse. This is pulse would trigger the operation of a simple 555 timer delay circuit so that it can do its job of turning off a lamp after a set period of time: a continuous 5 volt current from the phone charger will, of course, defeat the efforts of the timer circuit to switch the lamp off. The low voltage DC pulse is intended to replace the action of the “push-to-make” button switch that currently triggers the timer circuit and the power for the timer circuit is supplied only when the mains AC power socket is turned on and left in the “on” state.

I hope this gives enough information to clarify my objective and your comments and ideas will be appreciated.

Many thanks.
Use a UJT - Uni Junction Transistor 2N2646