Hi,
I'm new to this forum and need a little help with a simple alarm circuit. I have a set of PE beams (Aleph XA100) across my driveway
when the beam is tripped it sounds a sonalert and lights a led. The sonalert is supposed to latch on until a momentary switch is opened and then stay off until the PE beams are cleared, then reset and be ready to go off again, and the led is supposed to follow the status of the PE beams no matter what state the sonalert is in. I also have a manual switch to silence the sonalert if we leave the house, but the Led is supposed to still work on and off with status of the PE beams.

This circuit is almost 30 years old and when I opened the box it was a total birds nest and I had to cut the wires. Recently I've had to replace the wires going outside and now it's not working right. it trips ok and latches on, but the sonalert won't stay off when I hit the momentary reset if the PE beams are still tripped.
here's a little sketch of what I have, I tried to do it with a latching relay but that didn't work either. the max current I can put through the PE beam contacts is 300ma and I want to try to stay a little below that.

Thanks for any help you can provide.

John Kruse

 

Your thyristor gate is permanently connected to the positive supply, and so will always be latched, the gate needs to be wired to the sonalert output, then it will trigger, to switch off the thyristor you need to break the Anode/Cathode supply with a NC button.

 

Actually it latches on and off fine, the thing I want to do is to silence the sonalert even if the gate is still triggered and the LED is on, it used to work like that, but that was a lot of years ago.

the PE beam switch closes and the sonalert latches on, if I break the current through the SCR it silences the sonalert but if the current comes back it comes on again.

I guess I can't get there from here

 

Dodgydave is correct with respect to your posted schematic and the way it would behave.

You might have the cathode and anode swapped in your schematic, in that case the latching works fine because the gate is not always getting current from +12. (Edit: it would the that the 1k resistor connects to the cathode instead of the anode, not that both electrodes were swapped wholesale.)The schematic below shows the way I would re-arrange your parts to obtain the function you described.

 

Thanks!!
I'll try that as soon as I get home today

 

Here is a datasheet for an equivalent part, with the pinout.
http://www.nteinc.com/specs/5400to5499/pdf/nte5400_06.pdf

ak

 

Thanks, that will help alot
I guess there aren't any SK around anymore
I feel like a dinosaur, you can't even walk into a radio shack and get discrete components anymore, and there is only one place here in
New Orleans that sells resistors and SCRs

 

well it doesn't work the sonalert is on faintly all the time, I added a manual switch in series with the wireless reset and the sonalert is still on faintly

 

I also tried changing the resistor values and it didn't work, this should be the simplest circuit in the world -- I'll bet something like this is off the shelf available and I'm beating my head against a wall for nothing...... I should be able to do this with a relay ...heads off to research...

NO switch closes momentarily and latches (light and sound)
press a NC or NO button and silence the noisemaker
led stays on until alarm is cleared
alarm cleared -- system resets

 

Dodgydave is correct with respect to your posted schematic and the way it would behave.

You might have the cathode and anode swapped in your schematic, in that case the latching works fine because the gate is not always getting current from +12. (Edit: it would the that the 1k resistor connects to the cathode instead of the anode, not that both electrodes were swapped wholesale.)The schematic below shows the way I would re-arrange your parts to obtain the function you described.

View attachment 132892

Thanks for the circuit, as it is it doesn't work right, the sonalert is on faintly all the time, very low like if you connected it directly to power with a large resistor in series with it, could be some leakage from somewhere -- I'm ordering a decade setup and a bread board and I'm going to play with the resistor values a little

 

Sounds like the SCR has enough leakage to power the Sonalert. If that is the case putting a resistor across the Sonalert such that the IR drop is less than what the Sonalert needs should make it quiet.

 

Try putting a 1K resistor in parallel across the sonalert, to bypass the leaking current, and try using a different thyristor like a C106d type.

 

Try putting a 1K resistor in parallel across the sonalert, to bypass the leaking current, and try using a different thyristor like a C106d type.

Thanks -- this one?

http://www.picaxe.com/docs/trt004.pdf

 

Sounds like the SCR has enough leakage to power the Sonalert. If that is the case putting a resistor across the Sonalert such that the IR drop is less than what the Sonalert needs should make it quiet.

been doing a lot of reading on this forum and I think this is what you are talking about?

https://forum.allaboutcircuits.com/threads/thyristor-leakage.119374/

going to try some of this this weekend

Thanks Guys!!

 

If I followed the thread they are discussing leakage to the gate. It appears that what your Sonalert is seeing is off leakage from anode to cathode. SCRs rated a lower currents tend to have less leakage from anode to cathode.

Let us know what you find out!

 

Thanks -- this one?

http://www.picaxe.com/docs/trt004.pdf

Yes, i use them for most applications, use a 1K resistor across the G/K terminals to suppress false gate triggers,