Reverse a NO switch to use a 555

Thread Starter

Goxeman

Joined Feb 28, 2017
107
Hello everyone,

I have a circuit regulated with a LDO 9VDC that uses a 555 timer and I want it to work when I activate the circuit with a magnet. Willing to do that I chose a reed switch and as I could read, most reed switches have a NO contact.

That said, the normal application would make the 555 work when you approach a magnet to the switch and the contact is closed; but I want to reverse it, I want to make the 555 timer work when the magnet is far from the swtich and stop working when the magnet gets close and the switch closes.

What do you think is the most efficient way to do this? I want to use a NO switch, so using a NC switch is not an option.
 

Thread Starter

Goxeman

Joined Feb 28, 2017
107
That is a complex solution, I mean I have tried things like that and the results are not always very accurate. For that you need to place phisically two magnets
 

Thread Starter

Goxeman

Joined Feb 28, 2017
107
You are right, I wasn’t clear about that

My initial intention was to use the reed switch as trigger for the 555 but it could also be used to power the it
 

Tonyr1084

Joined Sep 24, 2015
4,889
A normal reed switch is open until in the presence of a magnetic field. As a magnet approaches it will switch on. When pulled away it will switch off. You're asking for the opposite? If so, there are reed switches that have both NO and NC contacts. Google should turn something up.

An alternative is to use a buffer inverter. But then you're constantly drawing current from a battery (if you're going battery operated).
 

Tonyr1084

Joined Sep 24, 2015
4,889
@OBW0549 If he's going battery powered then that will drain the battery fairly quickly when working AND when sitting idle. A reed switch in the open position (C to NC in the presence of a magnet) uses no power at all.

@Goxeman Is this a battery powered project?
 

Thread Starter

Goxeman

Joined Feb 28, 2017
107
@OBW0549 If he's going battery powered then that will drain the battery fairly quickly when working AND when sitting idle. A reed switch in the open position (C to NC in the presence of a magnet) uses no power at all.

@Goxeman Is this a battery powered project?
This is a battery powered project but it will be using large batteries

In this case idle means that the switch is closed and the 555 is not working, in this state the circuit should drain as less power as possible. Working means that the switch is open and the 555 is working

I don’t have any schematic as this is just an idea in my head for the moment
 

Tonyr1084

Joined Sep 24, 2015
4,889
I tried such an arrangement using a reed switch to close an oscillator. The oscillator could not oscillate until the magnet was pulled away. I used a 9V transistor battery (OK, y'all - don't blast me for not knowing the battery number) and a VERY large resistor. I don't recall the configuration but it worked. For a few weeks. Then the battery was dead. I didn't want to, but I eventually went to a mechanical switch. When the button was pressed the oscillator did nothing. When let go (NC closed) - the oscillator would oscillate (as they will do). That worked well.
 

Thread Starter

Goxeman

Joined Feb 28, 2017
107
Ummm the 555 triggers on LO so it typically has a pullup resistor to hold it HI. So closing a switch to ground would do it.
I didn’t about that. Indeed that is great option, in any case an external pullup can be used but holding the trigger low for long term would affect the output? I would need to add additional components to guarantee a fast recovery I think as the trigger is expected just to receive a pulse and not to be held low for a long period. I don’t know if I am missing something
 

Reloadron

Joined Jan 15, 2015
5,604
I didn’t about that. Indeed that is great option, in any case an external pullup can be used but holding the trigger low for long term would affect the output? I would need to add additional components to guarantee a fast recovery I think as the trigger is expected just to receive a pulse and not to be held low for a long period. I don’t know if I am missing something
How are you configuring the 555 timer chip? Bi-Stable, Mono-Stable multi-vibrator? Guarantee a fast recovery from what? You really need to better address the application.

Ron
 

Thread Starter

Goxeman

Joined Feb 28, 2017
107
I was thinking about the proposition to close the switch to ground and it should be the other way. It should close the switch to VCC and have a pull down resistor, as I want the 555 to work when the switch is open (meaning pulled low).

What I don’t really know is when does start the 555 working? When the low pulse is finished or since the low pulse starts?

I can give more details about the end application. I’m making this to always remember to leave a key on its place. I glue a magnet to the key and when I take the key away from the reed switch a buzzer controlled by the 555 would start beeping.
 

MisterBill2

Joined Jan 23, 2018
6,685
"Biased" reed switches are used a whole lot in security systems which use an open loop. So they do exist and it is just a matter of locating them.
Experimenting with multiple magnets and a reed switch can also work, but certainly the arrangement will be very dependent on the magnet polarity.
If the TS is willing to provide more information about the application then probably a better solution can be provided. With no information all we see is guesses, some OK and many quite useless.
 

Reloadron

Joined Jan 15, 2015
5,604
This is a 555 Data Sheet. You trigger a 555 by placing a logic low on pin two the trigger pin. I can connect a switch including a magnetic reed switch between ground and pin two and a 10K or 100K resistor between pin two and VCC. When the NO switch closes it triggers the 555. Take a look at Application and Implementation section 8 of the data sheet (page 12). There is a classic example. Remove the PB switch and insert a NO magnetic reed switch.

You can also get a SPDT magnetic reed switch giving you a Common, Normally Open and Normally Closed set of terminals.

Ron
 
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