Simple alarm circuit help!!

[Electronics48]

Hi,

I am new to electronics and i am making a simple burglar alarm on a breadboard. It consists of a pull up button, a monostable and 2 astables and a piezo transducer. However i cannot get my piezo transducer to work properly as it will beep all the time. Could you please look at the picture attached to see if you can spot any wrong connections or anything you would recommend changing in order for it to work?

 

[(*steve*)]

Do you have a circuit diagram? Your wiring looks neat and tidy, but without more information we can't do much.

 

[Electronics48]

I've uploaded the circuit diagram, although its not the clearest.

 

[(*steve*)]

Here's my guess at what it's supposed to do. You tell me if I'm right.

Let's call the 555's U1, U2, and U3 from left to right.

U3 is an astable oscillator which is supposed to make the piezo sensor go BEEEEEEEEEEEP

U2 is an astable designed to stop U3 oscillating when U2 is held in reset, When U2 is not held in reset it alternately enabled U3 allowing it to go BEEP BEEP BEEP...

U1 extends a press (or release?) of the switch. Normally U1 holds U2 reset which also holds U3 reset (so all is quiet) When U1 is triggered, it stays on for some time. This allows U2 to make U3 allow the buzzer to go BEEP BEEP BEEP...

If the piezo beeps at all then U3 is working.

If the beeping is continuous, then U2 is not gating U3 on and off.

If the button does nothing then U1 is your problem.

I would start by disconnecting the connection from pin 3 of U2 to pin 4 of U3. If you wire pin 4 of U3 to V+ the piezo should sound. If you wire it to ground it should stop. If that works OK, reconnect pin 3 of U2 to pin 4 of U3. If this fails, the problem is around U3.

Next disconnect the connection from pin 3 of U1 to pin 4 of U2. If you wire pin 4 of U2 to V+ the piezo should sound (on, off, on, off...). If you wire it to ground it should stop. If that works OK, reconnect pin 3 of U1 to pin 4 of U2. If this fails, the problem is around U2.

If both of these work, the problem is around U1

 

[KrisBlueNZ]

There are quite a few visible errors in your breadboard layout.

You have pin 2 of the first 555 connected to the wrong side of the pushbutton. It is shorted to the 0V rail. This will keep the monostable permanently triggered; the button has no effect.

The wire from pin 3 of the first 555 doesn't seem to connect to anything. It looks like it should connect to pin 4 of the second 555, but there's a gap, with a brown link in between that doesn't connect to either wire.

The trimpot connected to the first 555 isn't connected properly. When a pot or trimpot is drawn as a variable resistor on a schematic, i.e. only two connection points, you need to use one (either one) of the end pins and the wiper pin. (The other end pin may be connected to the wiper as well; this makes no difference.) You have three separate connections to the three terminals of the trimpot. The capacitor needs to be connected to the wiper, not the other end, i.e. it needs to be connected to pins 6 and 7 of the 555.

The trimpot on the second 555 may be wrong as well. You have the top right pin connected to pin 7, which is right, and the wiper connected to pins 2 and 6, which is right, but you have a black link wire connected to the top left pin. I can't see where it goes - it's obscured by the electrolytic, which (I hope) connects to the wiper of the trimpot. That black link shouldn't go anywhere.

The third trimpot is wrong as well. The top right pin is OK, it's connected to pin 7 of the third 555. The wiper is connected to pins 2 and 6, which is right too. But the 10 nF capacitor appears to be connected to the top left end of the trimpot (which shouldn't be connected to anything) instead of to the wiper.

Also make sure you have the right kind of piezo device. There are two kinds. You need the simplest kind, which contains just a piezoelectric transducer element and must be driven with an audio-frequency AC voltage (this is generated by the third 555 in your circuit). For example: http://www.digikey.com/product-detail/en/CEB-20D64/102-1126-ND/412385 (this one has no plastic enclosure) or http://www.digikey.com/product-detail/en/PT-2207/433-1058-ND/1245312 (this one has a plastic enclosure and two pins).

The other kind includes an oscillator, and must be powered from a DC supply. Its frequency is fixed. For example: http://www.digikey.com/product-detail/en/PK-12N40PQ/458-1066-ND/969793