Power supply failure alarm

[kong]

Hey forum,

I tried to built this circuit but the buzzer just turns on and won't turn off when I adjust the pot.

I've built this circuit before and it worked, but I can't seem to get it this time.
The only thing I'm doing differently is using the BC558 equivalent which is the NTE159....although I'm using the NTE159M...not sure if that matters or not.

Any help would be appreciated.

 

[davenn]

Hi Kong
welcome to the forums

thats a bit better for people to view
if the circuit has worked for you before, then its possible you have a wiring error this time around. How about posting a pic of your construction for us guys to have a look at.... make sure its nice and sharp and resized to ~ 800x600 max

cheers
Dave

 

[kong]

Ok, when I get a chance I'll throw one up...but do you think it could be because I'm using the NTE159M instead of the NTE159? That's the only thing I've done differently

 

[CocaCola]

I'm using the NTE159M instead of the NTE159? That's the only thing I've done differently

Are you sure it's installed correctly and you did not swap the collector and emitter locations?

 

[kong]

Installed correctly. I tested it and it works, by taking the base low. But when I use it in the above circuit, when I apply power....the buzzer stays even when I adjust the 100k pot. I'm assuming this is because R2 is holding Q1 low. But this should work, it worked for me last time I tried it. Confused. lol

 

[KrisBlueNZ]

That circuit isn't going to work. The voltage divider that feeds the base will always turn the transistor ON, even when the circuit is receiving power. The 100K trimpot needs to be in series with R2, not with R1, and even then it needs to have a higher resistance.
This circuit is not very well designed. I will draw something better for you and upload it soon.

 

[weird_dave]

R1 and VR1 can't bring the base high enough to stop the transistor conducting.

 

[kong]

Oh! Well thank you! I swear I got it to work...must have been another circuit. Thanks again!

 

[CocaCola]

Installed correctly. I tested it and it works, by taking the base low.l

Forcing the base low doesn't confirm the orientation of the collector and emitter it will work regardless of it's orientation if forced, just not as well when backwards...

Confirm the orientation with the proper datasheet for your transistor...

 

[john monks]

R2 needs to be removed.

 

[KrisBlueNZ]

Here's a simpler circuit based on your one that doesn't need an adjustment.
For a longer warning time, increase the 4700 uF capacitor and/or use a piezo buzzer with a lower current consumption.
The 22K base resistor can be changed as well. It needs to provide enough base current to keep Q1 saturated for the useful part of the C1 discharge curve. The transistor has a gain of several hundred, but to saturate it, aim for a base current about 1/100 of the collector current. If the piezo buzzer draws, say, 20 mA, a base current of about 0.2 mA is good, and you want it to work down to about 5V so calculate R1 as R=V/I which is 5/0.0002 which is 25K. Check the current drain of the piezo buzzer; if it draws more than 20 mA you should probably reduce R1.
Good luck

 

[kong]

Wow! Thank you! Is it ok do you think for me to use a 27k on the base? I don't have any 22s

 

[kong]

hmm found a 22 tried it, but nothing. I have the collector output and the - side connected to the power supply pins of a 555 astable......

 

[KrisBlueNZ]

That circuit will work. It does exactly what john monks suggested earlier in the thread, it removes the resistor from the base to ground, which was what was holding the transistor ON before.
What is your "beeper" circuit? It's a 555 oscillator, but what is it driving? It may need a lot more current than a piezo beeper. If so, you should reduce the base resistor, or replace Q1 with two PNP transistors connected in a Darlington configuration (look up Darlington on Wikipedia). Even if this is the problem, I would expect you would hear something from it.
Check your circuit and replace the transistor (just in case it was damage by your earlier experiments).
You need a load on the power supply, so that the voltage across the input of this circuit drops to zero quickly when the power supply fails or is switched off.
Does the voltage across C1 drop quickly after you turn off the power supply? You know this circuit will only make a noise for a short time after the power supply fails. Maybe you should use a battery-powered circuit instead.

 

[kong]

Once I connect power to the circuit, and then disconnect it, there is no output from the piezo.

I'm just using a 555 osc that drives a small piezo beeper. Current shouldn't be more than 20mA

I can't get any sound out with this configuration -I might need the NTE159 instead of what I'm using, which is the NTE159M.

I tried dropping R1 to 10k. Still nothing.

The 555 circuit is hooked up so that once the circuit senses power has been disconnected, the capacitor will discharge into the 555 circuit and sound the beeper momentarily.

But for some reason I still cannot achieve that. I know the transistor is still good because I momentarily test it by taking R1 and connecting it to the negative rail.

When I do this, the beeper continues to sound until I remove it, and then it sharply cuts off.

So for some reason the capacitor is not discharging into the 555.

Thanks for the circuit and your help, I'm sure it's just something stupid and obvious that I'm missing

 

[KrisBlueNZ]

OK.
I just checked out the NTE159 data sheet. It has a fairly low gain. You would be better off using a BC557/8/9 with a B or C suffix.
Are you testing it by turning the power supply off, to simulate power supply failure?
Does the power supply have a load on it? If it doesn't, the output voltage may stay up for a while, then decay slowly, even though the power supply is switched off.
What happens to the voltage across the 4700 uF capacitor when the power supply is switched off?

 

[kong]

Yeah, testing it by turning power supply off.
I connected a lamp to a battery, then spliced in the + and - to the circuit you gave me.

I disconnected the power supply, (keep in mind just one battery)
and didn't get anything.

When the power is switched off, the lamp slowly starts to fade-the capacitor discharges into the load.

I can't acquire any B or C suffix transistors, but I do have some 2n2907, 2n3906 2n4403 etc. Could I connect these in a darlington pair configuration to increase gain?

 

[KrisBlueNZ]

So you tested it using a battery as your power supply, and a light bulb? That will work, but you need to connect the circuit across the light bulb and disconnect just the battery. The light bulb has to stay connected across the input to the circuit.
I don't understand what you mean about the lamp fading. You mean the lamp fades as the capacitance inside the power supply discharges? That makes sense. If you're talking about the 4700 uF capacitor in the circuit, it doesn't discharge into the load. That current path is blocked by the diode. Are you sure the diode is present and not damaged?
Yes, the 2N2907 looks pretty good. You can hook two of them up as a Darlington pair and see if that makes any difference.

 

[KrisBlueNZ]

That circuit will beep when the voltage across its inputs falls. It won't beep if you just disconnect it from the power source. To test it, you have to make the voltage across the input connections of the circuit go to zero. You can do that by shorting the input connections together.
If you want it to beep when you disconnect it, add a 10K resistor across the input terminals of the circuit.

 

[kong]

Doesn't the line on the diode indicate which way the current will flow? That would make sense because then, according to your schematic, it would be blocking capacitor discharge.

I took a battery and connected its terminals to a lamp. Where the lamp and the battery connect, I took some alligator clips and tapped in and ran those to the inputs to the power failure circuit. So, in a sense, the power failure circuit is monitoring the lamp circuit, but also using its power to charge the capacitor. Is this correct?

I added a 10k resistor across the + and - inputs to the power failure circuit,

and build a darlington to replace the 159M.

Still not working. I should probably make clear my requirements for the circuit just to be sure we're on the same page, I don't want to lead anyone down the wrong path

I would like a circuit that monitors another circuit, (only one battery mind you, that powers the primary circuit) The secondary circuit, this circuit, would monitor the primary, let's say it's a battery powering a lamp. When the lamp loses power, so does the secondary (this circuit) and that triggers the secondary to sound a buzzer. The secondary, if possible, should rely on power from a large capacitor rather than a secondary battery. We're so close I think, but I just don't know what to fix here.