Instructables

Will this Latching Laser Alarm circuit work?

Hello everyone ^^
I'm here to ask about this circuit. I've been searching for a laser alarm circuit for a long time which locks itself when the beam is broke and keeps the alarm running unless if I want to press the reset button without the use of a relay. Unluckily, I wasn't able to find that kind of circuit in google... AND IT FELT HORRIBLE!!! =O Ahem ahem! Anyway, I've come up with an idea of combining the latching push ON and OFF circuit and the Laser alarm circuit. Will it work? please reply =(      WILL THIS WORK-- http://www.instructables.com/id/DIY-Latching-Laser-Alarm/

jduffy541 year ago
If you want transistors, yes, that circuit will work. If you wanted a simpler one, seveastrouk's answer is better. A thyristor is another type of semiconductor that does what you're asking with very little other circuitry, and is similar to an SCR. They're generally a little harder to find (I don't believe radioshack stocks them), but are very easy and cheap online. I believe the same place that you got the latching also has an SCR based circuit nearby.
Charles Mendoza (author)  jduffy541 year ago
so... it will work? are the resistor values correct? And more importantly, are the resistors placed correctly?
Yes, I believe so. If I'm not mistaken, I've used this circuit before for a similar purporse.
SCRs and thyristors are, to all intents and purposes, the same thing
I was saying that they're both hard to come by, and very similar, but I guess I did word it funkily.
They're similar, but correct me if I'm wrong, don't thyristors allow AC wheras SCRs only allow DC. Basically a thyristor is two SCRs hooked up anode-cathode, cathode-anode, and gate-gate. At least that's how I learned it.
No, you're thinking of a TRIAC,
Just checked, and I didn't know that there's two basic definitions of thyristor, one being any SCR or TRIAC like device, one being just SCRs.
Sorry for any confusion I may have caused, but the point is that any or at least most types of thyristors latch, and could be used in this manner.
Charles Mendoza (author) 1 year ago
thanks everyone. To sum it all up... will this circuit work? I don't want any recommendations of any kind of transistor. I know SCR's are much simpler to use but buying one are just balls. LOL
bottom line: yes.
afterthough line: What kind of load are you running? Anything more than a few hundred miliamps will burn out the transistor very quickly. If you want to switch mains or anything above 150mA, put a relay where "load" goes, and use that to switch the power.
Charles Mendoza (author)  jduffy541 year ago
I'm using a siren for the load. it's voltage rating is 3V and it's current draw is something that i don't know but I've used it in many circuits which includes transistors. Anyway, I'm sure that it won't fry the transistor. Thanks by the way ^^
A circuit that just "includes" transistors may not actually be switching it, and if it is, if it's only for short beeps you may not fry them quickly, but extended running may draw too much. I would just measure the current going to it without the transistor before you connect it, just to be safe.
iceng1 year ago
There are also GTOs [GateTurn Off] SCRs...
These 5 layer devices turn on like an SCR .... But
can be turned off at the gate ( no need to interrupt the current )
( also no need to reduce the current below the holding value ! )

A...
Just use a thyristor. Connect your reset switch across the device anode and cathode.