Need help with this circuit

[windozeuser]

I've built this circuit about 5 times, checked all components but the SCR doesn't seem to be latching. The Alarm works but when you close the loop after it has been disconnected the alarm stops. It's suppose to stay on until you remove the battery.

Please Help

Thanks!

 

[Roff]

The holding current on the 2N5060 can be as high as 5 ma. Your instantaneous current when the 2N3906 is off is less than that. Try adding 1.5k from the anode of the SCR to the positive side of the battery.
A 2N3906 is pretty wimpy for driving a speaker. What is the speaker's resistance? Even a 32 ohm speaker is too much of a load. You need something like a 2N2907/PN2907, or even beefier.
I think some people drive speakers directly from a 555, but at 9 volts, I think it's too heavy a load.

 

[windozeuser]

Hey thanks Ron , It works perfect! May I ask for a more detailed explanation on why it works like that. I don't like doing something and not knowing why it now works.

Thanks A lot!

 

[Roff]

Hey thanks Ron , It works perfect! May I ask for a more detailed explanation on why it works like that. I don't like doing something and not knowing why it now works.

Thanks A lot!

An SCR, as you probably know, turns off when the current through it drops below a value called the Holding Current. The SCR you are using has a maximum holding current of 5 ma. This means that if you turn it on and then remove the gate drive (which your loop does when it is closed), and the current ever drops below the holding current, the SCR will turn off.
The quiescent current of the LM555 at VCC=9V is around 5 ma nominal, but it could be lower, and if you are using a CMOS unit, it most certainly be lower than 5 ma. The 2N3906/speaker combo draws nothing on one half of the cycle (when pin 3 is high).
The added 1.5k resistor adds a constant load of (9V/1.5k)=6ma when the SCR turns on, giving it enough current to stay on when the gate drive is removed.

 

[windozeuser]

AH Thanks Ron, Very Nice explanation!