Choke for a NC motion sensor on a NO system

[jbelectric777]

Hi everyone, Ok This buddy of mine put a normally open contact alarm system in for himself. Just a basic basic basic system where he punches his code in the keypad, that puts V on all the door/window contacts one side and if you open the door/window it just energizes a relay which locks itself in (like a motor starter) and uses the other N.O contact on the relay for the siren. I know, I know damn electricians........ Anyway, he bought a motion sensor but he cant make it work because the trigger is normally closed and opens on movement. I drew this attachment and worked it up on a breadboard and seems to be working, I clamp pos and neg with a resistor in between, when the contact goes open the Vce will conduct again triggering a small relay which will trigger the master relay setting off the alarm. What do you think? Is it okay for now? Thanks, Jim

 

[Harald Kapp]

Sorry, I don't understand your schematic. What kind of transistor do you use (NPN vs. PNP)? Where's the emitter? What does the red circle mean? What does the symbol to the right of the resistor labeled 6V 5k mean?

You can achieve your goal with a simple additional normally closed (n.c.) relay. Connect the relay coil to the sensor such that when the sensor is not active (normally closed) the relay is turned on by the sensor and the contact is active, i.e.open.
When the sensor becomes active, it opens thereby opening the circuit to the additional relay's coil. The additional relay will turn off, closing its contact.

 

[Anon_LG]

I agree with Harald Kapp, you should label your schematics so that they can be understood by an observer. If your friend is doing this as a hobby he/she could get some circuit simulation software that would allow him/her to draw schematics neatly, or do it on paper. I found this useful (a link provided by krisblueNZ): http://electronics.stackexchange.co...es-and-guidelines-for-drawing-good-schematics. It looks as if it was done on paint with "pencil" tool.

 

[jbelectric777]

let me redraw it for em, maybe you'll understand better, or explain here now, all it is ..... is an npn controlling a red led. there is a normally closed swith across pos and neg with a resistor. Harold I don't want him using a mechanical relay through a normally closed contact because if the relay lost power and dropped out it would latch the relay sending power to the siren

 

[jbelectric777]

ok - I did another one. go easy on me, im a retired controls electrician, logic etc, semi-conductors / electronics are night and day except for switches, contacts, relays and the logic (the logic behind and or not nor nand etc im very fortunate!) its that I teach myself everything, and at 47 they put me out to pasture! I was an electrical inspector in nj and the stairs in the new homes (huge million dollar homes) even stopped me so I cant have surgery on my back and knees til im at least 50 they say so this keeps me really busy. I love it! but you guys are pro's, just try to see what im trying to convey ya know, I don't wanna know how to hook things up, I wanna know why........ and theres a huge difference, which also reflected in my work, if you know why you can do anything but if you just know the how, then your in trouble when the tough stuff comes, ya know. thanks, jim ( I just finished pos and neg electrons around an atom and movement through doping silicone, now I know it, even though I know to use electrons and holes I still wanted to know why)

 

[Harald Kapp]

This wil not work. The transistor will be on regardless of the state of the switch. Lacking more detaisl of the setup (operating voltage, load current etc.) I can only propose a principle:

I assumed an operating voltage of 24V and ignored load current (to get started).
Resistor R1 limits the current through the LED in the phtocoupler U1 to approx. 5mA. Adapt R1 for an operating voltage other than 24V.
The switch is normally closed and bypasses the current through R1 by short-circuiting the LED. The LED is therefore off and the transistor in the output of the photocoupler is off, too.
When the switch opens, the current through R1 then flows through the LED, turning on the output transistor of U1.

The photocoupler is just one i had available n my schematics tool. Which photocoupler is suited depends on the load volage and current. For high power loads (e.g.mains operated) you can use a slid state relay instead (also know by the trade name Photomos relay).

 

[KrisBlueNZ]

With R1 at 320Ω, from a 24V supply, I make it about 72 mA (allowing 1V for the optocoupler LED). For 5 mA LED current, R1 should be about 4k7. Everything else looks good

 

[Harald Kapp]

Right. Not yet fully awake at that time

 

[jbelectric777]

photomos is that a semi conductor? I really like those PCB relays with a 1k ohm coil. but my picture is close to what I wanted but I did remember after I sent it that the led is the load (the relay coil) and the short as you call it has a resistive load between it so normally I would not call that a short, I though maybe between the led connections. my first approach is to learn your definitions like in my trade we use line and load where you use collector emitter, I always thought a transmitter was like a relay and the base was the coil.....! WRONG, LOL I dove deeper in physics about silicon doping etc and realized how base current is utilized. so I have much to learn from you. so would this be an accurate definition below, or good enough for me for now? - Transistor: (noun)- a semi conducting device that comes in two types, P type and N type, N type being the most common. Each type will either push or pull valence (free) negative charged electrons to create current flow. The direction of the arrow indicates it's type of either pushing or pulling electrons, also referred to as negative electrons and positive charged holes. ya think that's good or just simplify somehow by push pull or like most refer to them as electrons and holes. Thanks, Jim

 

[jbelectric777]

I been drawing my own library but I know where to dl one for free

 

[jbelectric777]

and I didn't do any of the calculating yet, I just wanted to know if it would work first, as far as doing my ohms law on it I didn't get that far sizing resistors etc. I almost understand how you guys think but after 30 years in construction, draw, build electrical Im slowly getting it, like a guy told me once to refer to a dc source as high and low not pos/neg

 

[jbelectric777]

Im getting Yenka and if you and Kris Blue don't mind maybe I can pick up some stuff. - You will be MASTER and I your GRASSHOPPER (lol)

 

[KrisBlueNZ]

Sure Jim. There are lots of knowledgeable people here who would like to help you with your electronics understanding and projects

 

[KrisBlueNZ]

Harald's circuit in post #6 is a very good answer to your question. He is using an optocoupler to convert a normally closed switch to a normally open signal, and to provide electrical isolation, the way a relay does.

An optocoupler consists of an LED and a phototransistor. The phototransistor is like a normal transistor, but instead of conducting when base current is applied to it, it conducts when light falls on it. The light comes from the LED, which is in the same package but is electrically separate from the phototransistor, so they are isolated from each other.

The first part of the circuit uses the normally closed contact to control the current to the LED. The 24V power source is fed through R1 and this produces a current flow. When the switch is closed (short circuit), this current flows through the switch, and there is no voltage across the switch, so the LED cannot light up. (That is not a very thorough explanation, but that's what happens. Current takes the path of least resistance, which is through the switch, and the switch shorts out the LED so there is no voltage across it.)

When the switch is open (open circuit), current cannot flow through the switch, so it flows through the LED. This causes the phototransistor to conduct. You connect the phototransistor into the normally open alarm circuit.

So when the normally closed switch is activated (goes open circuit), current flows through the LED, and the phototransistor conducts and completes the circuit for the normally open alarm.

You could do the same thing with a relay, but optocouplers are smaller and cheaper.