Reed switchs

[Noodles141]

I've read up about reed switch and they seem ideal for the task i need. I know its old technology but its the only solution i can think of, I'll describe this problem after my question about reed switchs though. What i want the reed switch to do is act as a switch for my circuit so i will put it right before the supply, before any other components meaning if its open no current through the entire circuit. I donno if this is seems they can only hand low currents or am i mistaken and will i be able to do this. i haven't figured out my current reading yet but my supply voltage will be 9v using a simple 9v battery. That's the question. The problem is i want to power a circuit that needs to run until its function is complete and shut off completely drawing 0 current from the battery, this function is activated by a one of those click buttons that have a spring so they are only in contact when they are being pressed(don't know formal name). My solution was a flip-flop to keep the reed switch on while the circuit is running and reset it when the function has finished. is there a newer or simpler solution?

 

[davenn]

OK firstly.... you do understand that a reed switch is operated by a magnet either permanent or electro.. dont you ??
You make no mention of a magnet and how it will activate or deactivate the switch when it comes into proximity with the reed switch

There are plenty of reed switches available with wil handle the several 100 mA thats available from a 9V battery


Dave

 

[Joe G]

Its called a momentary switch. Also if you know how much current/volts you will be using, you might be able to just use the "momentary switch".

 

[Noodles141]

Sorry I was ment to add I was gonna use a small electromagnet connected to the flip-flop. This will help me keep the circuit on and then off when reset. The idae is to use a simple momentary switch in parrallel to set the flip-flop which will activate the magnet and trigger the circuit while reed switch keeps the circuit on for the functions time. I would use a different button like one that holds its position but I want it to be a one push button, no holding it down either, to start the whole procces from start to finish. And thanks this has helped

 

[KeepItSimpleStupid]

Linear Technology recently introduced this part https://www.linear.com/product/LTC2955 which might help you.

 

[ChrisP58]

Can you put up a schematic or other drawing of what you are trying to do?

I suspect that your idea of using the reed switch and an electromagnet (which together make what is called a relay) are adding complexity to what may be a very simple function.

 

[MikeMl]

You can also use a series FET as a power switch. If set up correctly, the leakage through the FET after it turns off would be so low that the shelf life of the battery would not be affected... The power required to operate your reed relay will be kill your 9V battery in a couple of hours run time...

 

[Noodles141]

View attachment 67497ok did away with the relay cause Mike you said it kill the battey life and that's important and came up with a simple solution, donno how i didnt see it before, with a transistor. Sorry if the 555 timer chip isnt setup right doing this quickly before lectures. So instead of the relay i used a transistor. couldnt think of it cause i've never used a transistor like that. the 555 is ment to be setup as a rs flip flop with the 2 being reset and 6 being set. putting a high voltage across the wire connecting the 2 and 6 pins will set it on and give an output powering the base of the transistor. grounding the wire connecting the 2 and 6 pins will reset the 555 and no power will be provided to the base of the transistor meaning no power through the circuit. also this shows how a momentary switch could be used to get a circuit on for period of the function. Once the momentary switch has been pressed it will let current flow to the rest of the circuit, which will set the rs flip flop this specific connection isnt shown but imagine that the wire leading of the wire connecting 2 and 6 is connected to the Vs rail for now, the momentary switch can be released and the circuit will remain on. Just wanna check if this is a valid solution or if i'm making this up.

 

[MikeMl]

I didn't trace the logic of using the 555 as a flip flop. If battery life is important, I would use the CMOS version of the 555.
Can you stand the Vce drop of the series-pass transistor, which will subtract from the battery voltage? If not, then use a FET as I proposed.

 

[alec_t]

If your circuit only draws a few tens of mA at the most and operates from ~ the same voltage as the 555 you could probably use the pin 3 output voltage of the 555 as the +V supply voltage for your circuit, thus avoiding the need for the transistor.

 

[Noodles141]

mike; I've never used that part before plus this is a project for university and they are only supplying us with certain components so this seems like the easiest way with what we are given.
alec; i thought about doing that but i want the circuit to be completely off when no function is being operated. I dont want and on/off switch i just want one button that will do everything in a timed sequence. Having the pin 3 supply the rest of the circuit would mean i need that on/off switch for say turning it off when one goes to sleep and its pointless to keep it on.

 

[kinarfi]

Once you start the circuit, what turns it off? does it time out or does the current quit flowing, will an SCR do the job? Once triggered, they conduct until the current stops flowing.

 

[Noodles141]

I have a trigger to turn it off later on. Sorry I didn't add everything but that would take me forever cause there is a lot more to the circuit. I went to my lecturer and he said this should work but I should rather make the flip flop out of transistors and use a pnp for the relay