Hello, I am building an alarm for a project on stripboard. The problem I have got is that I have only got a 1 tone noise coming from it, when obviously I should have a two tone noise - like an alarm. I have tested it on an oscilloscope, and multimeter. I have got power going to both of the 555 timers, but only got one square wave coming from one of the 555 timers. I have tested the capacitor on the oscilloscope to see if it is charging and discharging which it is. What could the problem be? I could post a picture of my drawing design and picture of my actual circuit if that would help?

Anyyyyy help would be very much appreciated, Thanks.

 

You begin by posting a circuit diagram.

 

Ok. Hopefully you will be able to work out where I have gone wrong on my circuit diagram. Its not the best diagram in the world but please appreciate that this is my first ever attempt to convert a schematic diagram to a circuit diagram. Any feedback is appreciated,

Thanks.

 

I think there is a minor mistake in your PCB. You forget to link P2 to P6 of the second 555 timer.

See the red wire in the attachment.

Allen

 

I think there is a minor mistake in your PCB. You forget to link P2 to P6 of the second 555 timer.

See the red wire in the attachment.

Allen

Hello my friend. When I redone that diagram in sharpie to show you more clearer. I actually forgot to put that in. I do have that link already in my build. So still have an unknown problem?

 

I simulated your circuit on ISIS and it was working. It produces a 680Hz tone going up and down like an ambulance siren. The voltage at P5 of 2nd 555 is changing from 2.8V to 3.6V then back to 2.8V. The cycle repeats and that's what controls the tone.

The whole thing is quite simple. First get a voltmeter and measure P5 of the 2nd 555 timer. If you're not getting a varying voltage, then something is wrong with the first 555 circuit.

BTW, I am using 5V for the DC supply.

Allen

 

If you are getting a steady tone from the speaker, that would indicate that the U2 circuit is working. The variation (rise and fall) in the tone comes from the U1 circuit. You should see a rising and falling voltage on pin 5 of U2. If that is not there, then check to see why U1 is not oscillating.


ETA: It looks like the 2N3702 is wired wrong on the perfboard. The pinout on that transistor is emitter, collector, base as viewed from the front. It looks like on the perfboard, you have the base in the center.