I need to turn on an LED when switch is open

[Neil Ives]

Hi folks, first post here.
My knowledge of electronics is fairly shallow so sorry I know this question is simple.
I have a magnetically operated switch on my house side gate. The switch is Normally Open so when the gate is opened the switch is also open circuit. Please will you point me to a simple circuit that will allow an LED, and possibly a buzzer to come on when the gate is opened. I've found many examples of turning on a transistor when the signal goes high but I need the opposite situation.
Many thanks for any replies.
Neil Ives

 

[Harald Kapp]

Welcome to our forum, Neil.

Have a look at this thread, Specifically posts #13 ff.

 

[Neil Ives]

Thanks for that. I had already thought of wiring the reed switch in parallel so shorting the LED. This is ok if you have a continuous power supply but if you were running from a battery you would not want current in the circuit all the time.

 

[Harald Kapp]

but if you were running from a battery you would not want current in the circuit all the time.

Right, this was not obvious from your post. You will not be anle to eliminate the idle current completely, as the contact is closed in idle mode, but you can reduce the idle current drastically using e.g. this circuit

When the switch is closed, the base-emitter junction of Q1 has zero bias and the transistor is off. When the contact is open, the resistor R2 biases Q1 with a current that is sufficient to turn Q1 on. Idle currrent is reduced by the gain (beta) of Q1. Use a darlington transistor for very high gain aka very small idle current. Use a MOSFET instead of Q1 to make the transistor voltage controlled (no bias current) and you can increase R1 even further.

 

[Neil Ives]

Thank you for that. I kind of understand the circuit. I wouldn't know what components to buy. I think a mains PSU with my switch in parallel with the LED is going to be a lot easier for me to implement.

 

[AnalogKid]

There are magnetic alarm switches available that have both normally open (form A) and normally closed (form C) contacts, like a SPDT switch. Under $3 with free shipping. One of those in series with the LED and its resistor will work.

http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=alarm+switch+no+contacts&_sop=15

ak

 

[Neil Ives]

Thanks, yes I thought there must be but I've bought a switch already and it happens to be a NO pattern. It suits my purposes because it's a cylinder of about 4mm diameter; this means I can bury it in the brick wall with just the circular end facing the magnet making it very hard to spot and less prone to damage. This is the switch http://goo.gl/hXUUEI it states it is NO/NC which is confusing as it only has two wires and states lower down that it is N/O

 

[GPG]

with just the circular end facing the magnet

Check that it operates in that configuration. I think you will find that operation is more reliable with the magnet and switch parallel to each other.

 

[cjdelphi]

or go for an old style 4000/7HC series not gate ic...

Sorry it refused to copy url!

 

[CDRIVE]

If that cylindrical microswitch is similar to what I've used many times in the past they're designed to work best on their end. They're handy for woodworkers that want to add lighting to cabinetry while hiding the magnet and switch. When I used them the magnets were also round and both the switch and the magnet were buried in a hole drilled into the cabinet door and frame.

Chris

 

[Neil Ives]

Check that it operates in that configuration.

Checked; works fine.