Converting a normally open pushbutton to normally closed - Page 1
Forum section: electronics / beginners

author: fubar.gr
author_rank: Unknown
timestamp: August 17, 2016, 06:47:49 am
content: I've found a nice looking pushbutton that I would like to use in a project, but it comes in NO only and my application calls for a NC switch.So I thougt of using a transistor to convert it to NC like this:Two questions:1) Is directly shorting the transistor base to ground going to damage the transistor? Should I add a small value resistor in series with the switch?2) Is there a simpler way to convert a NO switch to NC?

author: EPTech
author_rank: Regular Contributor
timestamp: August 17, 2016, 09:49:09 am
content: Hi there,To my knowledge that would be the most basic inverter.If it Rload is the actual load you are switching, I would use a FET instead of a transistor. FET's are more suited for switching applications. they have a lot less voltage drop and hence they will waste a lot less power into heat. With FET's you can easily use a gate resistor that is 10 times bigger (even bigger 100k) than your base resistor to keep it turned on. Shorting gate to source in case of a FET should not cause you problems. Shorting base to emitter is also fine.If you are planning to use (long) wires from the switch to the circuit, it is also not a bad idea to put a small cap (1nF) from gate to GND, as close a possible to the FET to improve noise immunity. Better even to use a standard tranzil of about 15V. It will protect te gate and it has sufficient capacity, so two for the price of one.

author: firewalker
author_rank: Super Contributor
timestamp: August 17, 2016, 09:56:03 am
content: I would use a relay is the current for the load is in the order of amps.Alexander.

author: fubar.gr
author_rank: Unknown
timestamp: August 17, 2016, 10:59:52 am
content: The load is actually the coil of a relay!I am trying to make a self latching relay like this:http://www.electronics-micros.com/img/electronics/self-latching-circuit1.jpgIt is a tiny relay and the coil uses just 16 mA.

author: Kilrah
author_rank: Unknown
timestamp: August 17, 2016, 11:01:37 am
content: Don't forget the freewheeling diode then!

author: MosherIV
author_rank: Super Contributor
timestamp: August 17, 2016, 11:58:53 am
content: QuoteNo, the transistor will be fine. What the switch will do is divert the current that would flow into the base of the transistor to ground and turn off the transistor.No need for another resistor, the 2K resistor should limit the flow of current to 6mA

author: DTJ
author_rank: Super Contributor
timestamp: August 17, 2016, 12:12:17 pm
content: Can you use the n/o contacts of the relay and dispense with the inverter circuit?

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 17, 2016, 12:22:53 pm
content: to add values to the existing replies... i know most love electronically inclined, thats why i'm the minority love mechanical... 0mA leakage (base) current, 0V Vce (sat) @ normally 10A (bidirectional!), hFe Inf (sat), parallelable without any thermal runaway issue, and free clicking sound that otherwise the electronically inclined has to source an mcu, an eeprom, an bjt, an buzzer, an smd resistors bla3x to simulate so...Quote from: fubar.gr on August 17, 2016, 06:47:49 am1) yes if i have the switch in front of me...2) yes, buy another NC switch...

author: fubar.gr
author_rank: Unknown
timestamp: August 18, 2016, 07:27:15 am
content: Quote from: DTJ on August 17, 2016, 12:12:17 pmNo, you need the two buttons to be independent from the relay contacts. The idea behind a self-latching relay circuit like that is that you use one of the relay contacts to feed its own coil.So you momentarily energize the coil by pushing the NO button. This makes the relay contact to close. When you release the NO button the coil is kept energized by current flowing through the relay contact and the relay is kept latched.To turn it off you need a NC button that is connected in series with the relay coil. Pushing that button cuts the current to the coil, the relay contact opens and the relay unlatches.It is very similar in operation to a RS flip-flop.I will make a video demonstration when I finally build the board.

author: Watth
author_rank: Frequent Contributor
timestamp: August 20, 2016, 01:40:51 pm
content: Noob here:Wouldn't a PNP do the trick?

author: Ian.M
author_rank: Super Contributor
timestamp: August 20, 2016, 01:56:14 pm
content: The whole point of a latching relay circuit is to avoid using any active devices (except possibly diodes), otherwise you might as well use a NE555 wired as a S-R flipflop!Use *DOUBLE* the relay's nominal coil voltage as your supply voltage. Add a resistor equal to the coil resistance in series with the coil. Wire the N.O. Reset switch directly across the coil. The N.C Reset switch in the schematics you linked to can then be replaced with a simple wire link.