Panic Switch

[Guitartech101]

I have a unique situation and I am trying to find a cicuit that will do the following. A momentary switch that triggers a small buzzer, from radio shack, and turns on a pair of LED's. Now the light and buzzer have to stay on even after the momentary switch has been let go. There has to be a switch of some sort to turn it off. Also the buzzer must have the option to be disabled by a toggle switch. This circuit must be able to fit any a small project box that is 4.5" x 2.5" x 1.5" and the buzzer will sit on top of the box, not inside. Any kind of Diagram or explanation would greatly be appreciated or even paid for.

[Chris Smith]

A SCR will do the trick, once initiated using DC current, it holds till another switch that is always closed, is opened in the main power feed. The momentary switch you mention triggers the gate only, which causes the SCR to operate, it holds all by it self in a "latching" manner and any and all other switches you just place in line depending on what and why you need them.

[chessman]

Just a question...why does it have to be a momentary switch?

[russlk]

I posted a circuit, using a Radio Shack relay, at: http://www.geocities.com/russlk/ltckt.jpg<p>NOTE: you have to copy & paste the URL.<p>[ March 14, 2003: Message edited by: Russ Kincaid ]</p>

[Chris Foley]

I believe you can do the whole thing from Radio Shack. Get a 4 AA battery holder which will fit in your enclosure, giving you a 6VDC supply. While you're there, get the LEDs with panel mounting hardware, some switches, a logic-level SCR, and some 220 ohm and 10 K ohm 1/4W resistors. From the red lead (+) of the 6VDC supply, hook up your ON/OFF switch. The other side of the switch will be called +. The black lead of the battery holder will be -. Connect one end of the momentary pushbutton to the +, and connect the other end to the gate lead of the logic level SCR through a 10 K ohm resistor. Connect the cathode of the SCR to -, and another 10K resistor from the gate of the SCR to the cathode, and now you can connect up the loads to the anode of the SCR. Your three loads (the 5VDC buzzer, and the two LEDs with 220 ohm series resistors) can be connected between the + and the anode of the SCR. If you want, put another switch in series with the buzzer to take it in and out of the circuit separate from the LEDs.<p>Turn on the power to arm the circuit. Press the pushbutton to turn on the SCR. It should stay on once you let go of the pushbutton. To turn off the SCR, just toggle the power switch off and on.<p>An SCR, once triggered, will stay on until either the load current goes below the required minimum, or the voltage is removed or reversed. Like a diode, it only forward-conducts. With two LEDs and a buzzer, you should be in excess of 55 mA, which is way above the minimum 5 mA required by the SCR.<p>273-054 -- Piezo Buzzer
910-5041 -- Enclosure
2N5062 TO-92 Logic Level SCR -- I think you have to special-order this from the Shack-- it usually takes a couple of days to a week
LEDs and switches to taste.<p>Data sheet on the logic level SCR :<p>http://www.nteinc.com/specs/5400to5499/ ... 400_06.pdf<p>Happy hunting -- no charge.

[Guitartech101]

Thanks to everyone who relied on this little project that I have. Any questions related to quitars feel free to E-mail me at [email protected]

[chessman]

Guitartech:<p>Still, i'm just curious, why does it have to be a momentary switch and not a latch push-button?<p>Just wonderin,
Kyle

[gadgeteer]

An SCR is a good choice. I gutted the power supply for my Bachman "N" guage 4-4-0 "General", Central Pacific model train. Idgets had used a multi-tap transformer, so the train speed varied by jerky levels (and nowhere in between). A simple linear regulator, with a zener reference driving a darlington output made an excellent supply. THEN I added an SCR,tied to a current-sense-resistor, which, once ON, shorted the npn darlington drive to ground (shutting off the output). And lighting a very large (½ - inch) red LED. If the train wrecks and shorts (or if you throw a penny onto the track), the power snaps off and the red LED lights; this in forward AND reverse. The power remains off (and the LED on) until the speed control is turned to zero.<p>REVERSE is implimented with a simple cam and a DPDT relay. I suppose I coulda used a DPDT microswitch...<p>You could also realize your alarm control with a quad NAND, with two gates wired into an SR latch. Buy a 4011 (cmos), and it will work for any supply voltage from 3 to 15 (I think). Imagine two of the NAND gates connected with output GATE #ONE tied to input GATE #TWO, and output GATE #TWO tied to input GATE #ONE. The two unused inputs tied HIGH (say, 100k resistors). A "LOW" at either resistored input will store a LOW on the OPPOSITE GATE'S output (and conversely a HIGH on the gate receiving the LOW). Was that clear as mud?<p>Slap a 0.01 µF cap between one of the resistored inputs and ground, to force the latch into a known state on power-up --- for a few microseconds at power-up, the capacitor will short that input to ground and set your latch (until the capacitor charges through the 10k resistor).<p>An "SR Latch" is a good foundation for an alarm, anyway...