I made an schematic to show my idea. Some people are understanding it wrong I think. The idea is to produce sound when the reed switch is open and to remain quiet when the switch is closed

 

hi G,
The Piezo will sound all the time the Trig is Low and when the switch closes the Mono will time out with C2/R2, is that what you want,?
E

 

Can you use the reed switch to hold the Reset pin low?

If closed for a long time, wouldn't the switch and whatever associated resistor constantly draw power from the battery and drain it?

 

If closed for a long time, wouldn't the switch and whatever associated resistor constantly draw power from the battery and drain it?

Certainly they will draw current. How long to drain the battery depends on both the resistor and the battery.

 

hi G,
The Piezo will sound all the time the Trig is Low and when the switch closes the Mono will time out with C2/R2, is that what you want,?
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Yes, thats the idea. Imagine you have a key (with a magnet glued to it) that you always forget to leave at the keyholder, so while the the key is away the buzzer will beep-stop-beep-stop-beep etc; like an alarm. When you place back the key to the keyholder (with the circuit) the beep would stop.

The 555 would help me produce a square signal to feed the piezo, which 50% of the time would be low and 50% would be high every one second; this would make the beep 1 second on, stop 1 second, beep 1 second, etc.

If the switch is closed for a long time the switch wont be draining the battery. Is just a closed switch. Or maybe I didnt understand well

 

hi G,
This is LTSpice sim of the basic circuit.
The circuit you posted is just a Monostable, works as this sim.
What you want is an Astable, which runs 2 Second Rate while the key out of the key holder.

Do you have a Piezo or a Beeper/Sounder?

The Piezo requires a 3kHz square wave drive , but a Beeper only a DC voltage.

If you have a Piezo, you will require two 555' one for the 3kHz and the other for the 2 Secs
E

 

If you want low power when the switch is closed, use it to control a P-MOSFET to apply power to the 555 when the switch is open (circuit below).

 

hi G,
This is LTSpice sim of the basic circuit.
The circuit you posted is just a Monostable, works as this sim.
What you want is an Astable, which runs 2 Second Rate while the key out of the key holder.

Do you have a Piezo or a Beeper/Sounder?

The Piezo requires a 3kHz square wave drive , but a Beeper only a DC voltage.

If you have a Piezo, you will require two 555' one for the 3kHz and the other for the 2 Secs
E

Indeed, you are right. Two 555 are neccesary to make it work as described. I was focused on how to make it work with the NO switch I didnt pay attention to that. I am planning to use a piezo, that is why I used the external drive transistor with the resistor. The initial 555 would be used to generate the square wave to drive the piezo

 

If you want low power when the switch is closed, use it to control a P-MOSFET to apply power to the 555 when the switch is open (circuit below).

View attachment 217002

That is a great idea but why so high resistor of 1M? Isnt 100K enough? Ive never liked using so high value resistors in general

 

hi G,
If this project is going to be battery powered, you should consider switching the supply voltage with a magnet/reed switch.
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Depending on what the application is there may be a different and better way to implement a beeping function. But so far we have no explanation. So we have a lot of guesses.

 

hi G,
If this project is going to be battery powered, you should consider switching the supply voltage with a magnet/reed switch.
E

I could also use the reed switch to power the LDO through a transistor (to reverse the NO contact), this way the battery wont be drained, as the LDO would just work when the contact is open therefore no battery would be drain. The littlefuse 59170-1-S-00-D can handle 500mA current and up to 200VDC

If the reed switch works as main switch for the circuit, the 555 could be triggered just when it powers up

Depending on what the application is there may be a different and better way to implement a beeping function. But so far we have no explanation. So we have a lot of guesses.

What explanation would you need? This is just an idea, I up to answer any question that you or any other would think is neccesary to be answered to develop this idea.

 

If the reed switch works as main switch for the circuit, the 555 could be triggered just when it powers up

hi G,
Post your circuit when ready, we can run thru it.
E

 

I could also use the reed switch to power the LDO through a transistor (to reverse the NO contact), this way the battery wont be drained, as the LDO would just work when the contact is open therefore no battery would be drain. The littlefuse 59170-1-S-00-D can handle 500mA current and up to 200VDC

If the reed switch works as main switch for the circuit, the 555 could be triggered just when it powers up



What explanation would you need? This is just an idea, I up to answer any question that you or any other would think is neccesary to be answered to develop this idea.

What would be the applicatiot of a noise-maker that triggers on when a magnet is removed? That seems like an anti-theft setup such as would be handy for a carry-on computer at an airport. I thwarted to one attempt at stealing my computer simply with my loud voice. The frustrating part was that the security person right there did not nab the guy when he did not have a computer that he said he mistook mine for. That proved to me that it was an attempted ripoff.

 

That is a great idea but why so high resistor of 1M? Isnt 100K enough? Ive never liked using so high value resistors in general

Sure you can use 100k if that makes you feel better.
Just trying to save a few microamps.

 

Lets see what do you think about it now.

The NO reed switch now controls power to the voltage regulator through the P-MOSFET. When the magnet is far, the switch is open and the transistors gate=gnd, then the transistor powers the LDO, powering the 555.

Also the 555 now works as astable oscillator

 

hi G,
A P MOSFET with a positive Drain supply.????
E

 

hi G,
A P MOSFET with a positive Drain supply.????
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You were too fast. I was changing it already before you post it

 

hi,
Look at this, 3.6kHz tone while the switch is closed.
Simplified version of your circuit.
E
Update MOSFET .

Try a FDS4072N3

 

Why such a low value resistor for R2?
It just wastes significant current when the circuit is off.