Relacement for SCR

forgotten. Thanks for all the good answers guys.

now stick that circuit in a freezer box and see if it works when it gets below freezing,,

Jamie

At this point I have gotten what I needed out of it. The final alarm design does not use the SCR. I used two 555 timers to perform the functions I nee ded. Have proto'd it and it works great. Has approx 4 minute alarm with 26 second hold off for entry. 12 vdc 26 ah battery powered. 2. 30 watt horn ex ternal sirens plus 6 internal siren annunciators. If they do happen to gain entry it will be so annoying hopefully they will not want to hang around. Has home/away mode as well. Remote controlled using key fob and receiver. S everal traps as well that look like they would disable alarm which in fact set it off.

Now there's an idea. Put all the sirens and s*it inside the protected area. This will hurt their ears, and maybe cause hearing damage. What's more they won't hear you creeping up on them with your gun.

Know what ? How about strobe lights in the area as well. They are usually not going to be all that familiar with the floor plan and s*it, so this will trip them up. If they are familiar with the floor plan, somoene has the wrong friends.

Not sure whether you were being sarcastic or not. I do in fact have an stro be external to alert police. The main idea of an alarm is to deter would be thieves. If they aren't deterred sufficiently I figured it would be pruden t to annoy the s*it out of them and maybe change their minds by making it u ncomfortable to remain. I figured 6 of the 105 db sirens in unison inside t he house would be enough. The location is out in the country and has been b urglarized 3 times. They have also hit numerous other homes in the area rec ently. It is family and they are frankly fed up with it. Just got info that the police have identified persons of interest. Too late for my brother bu t at least he is taking steps to thwart future attempts.

OK. One more question for all who lurk. I am using 555 monostable. While tr iggered I have found that a new trigger has no effect. It will not accept a new trigger until it has timed out. I need to extend the timing period upo n receipt of additional triggers. I was thinking of using a transistor to d ump the timing cap charge so it will continue timing. Would it work and if so would an NPN work or should I use MOSFET? Thanks in advance.

Yes that would work. Your timing may be different than normal 555 ops because you are taking the charge down to whatever VCEsat is for the transistor you choose to use versus the 1/3 V+ the 555 normally works with.

Either should work. I've used 2N2222s and 2N7000's (N channel mosfet)

Great. I thought I remembered seeing it done somewhere previously. I happen to have a few 2222s. Will give it a try today and report back findings. Th anks for the response. You guys are really a great resource. Very nice to f ind a group with the same passions for electronics as I.

I think NPNs hate me. Used a 2N2222 and it screws up the timing. Never time s out. Tried diode isolation and several different permutations with no suc cess. Will try Mosfet next. After that I am thinking of just using small re lay to short the timing cap. At least I know that will work.

have you got the pins the right way round?

I hearya on that one. Once you get used to ECB all the time...

Look up the data sheet for the actual manufacturers part, not all use the common EBC pinout. You shouldn't have to resort to mosfets if you don't have any handy.

AND I recommend one of those multitesters that also checks transistor gain. Most are useless for mosfets as a rule, but it tells you something about the quality of the parts you have, and if the pinout you are using is the pinout of the part.

Any multimeter with a diode test (nearly all) function will also tell you the pinout.

I checked and the leads were correctly oriented. Will continue working on t hat problem. You mentioned the 555 output producing momentary high when res et is taken low. I am using the reset line to control enable/disable. Seems to work rather well. Using +5vdc out from wireless via level shift to cont rol reset on 3 555's. One for PIR input. One for delay entry and last for alarm output. The closed loop triggers entry and alarm. Entry trigger gnd i s sourced from output of alarm timer so once they are triggered entry is no t allowed to retrigger until alarm times out. This along with alarm cap cha rge deletion is what keeps it alarming with continuing triggers. That is th e Away mode. In the Home mode the entry delay and PIR are disabled with jus t the perimeter loop active. Any intrusion produces instant alarm giving oc cupant more time to respond to threat. Any way I have gone much deeper into its operation than needed but figured you guys would tell me if I have it wrong. Will continue to work on the thing cap issue.

Timing cap issue. Sorry for the typo.