Golborne Vintage Radio

I wonder if anyone might like to comment on the attached circuit please.

In looking at the circuit, I'm unsure about the role of the 1N4148 diode - 'D1'.

I'm puzzled by the polarity of that diode as to my simple mind it seems to be back to front, in that as shown, it will surely block the output from pin 3 of IC1 from getting to IC2.

From my limited understanding on 555s, in an astable multivibrator configuration, (using the circuit below as an example), the combination of R1, R2 and C1 (10k, 75k, 10uF)  set the frequency of IC1, which (using an online calculator I must stress!) I believe will be just under 1 Hz. Maybe that is intended to be the repetition rate of the 'red alert' sound? The combination of R4, R5 & C3 (10k, 100k, 0.01uF) should I think produce a frequency of 687Hz from IC2 - the audible 'alarm sound'.

I might of course be quite mistaken in my understanding of the operation of the circuit. If so, no surprises there then!  

I haven't built it - someone else has (on strip-board)  and it doesn't work!

Maybe he's made an error or maybe there's an error in the circuit?

I've designed a PCB for it, (which - if the circuit is wrong will only replicate such errors) so might built it to see what sound (if anything!) it produces.

PCB & circuit attached.

Any thoughts please anyone?

I hate 555s, always have, but ....

This is a guess because I haven't tried to analyse it. Where I used to work we used a similar Circuit to generate what was called an 'Alarm Signal'. IC1 generates a low frequency which modulates the frequency of IC2 generating a 'Whoop, whoop' sound. IC1 discharges the C via the Diode but doesn't charge it up again. Unfortunately I can't get at the old Circuit any more.

Does IC2 oscillate ob its own?

Alan

When the output of IC1 is low, it "silences" IC2. That's because C3 pulled down to ground and is unable to charge, hence IC2 isn't able to oscillate.

When the output of IC1 is high, IC2 does oscillate. The diode is needed to ensure that IC1 is not interfering with the operation of IC2 when in this state - otherwise, with D1 replaced by a short circuit, C3 will be charged rapidly via the 1k resistor R3. With the diode fitted as shown, it becomes open-circuit when IC1 goes high, meaning that IC1 plays no part in the behaviour of IC2.

So the output is a gated square wave. Change the time constants and you'd almost have the BBC "pips"

An obvious question is why didn't they simply connect pin 3 of IC1 to the top of R4? Thus saving the diode and 1k resistor. With the top of R4 being fed by pin 3 rather than the 12V rail, then oscillation can only take place when pin 3 of IC1 is high - which is the same behaviour, but saving 2 components.

Why doesn't it work? Without bread-boarding it, I can't speculate, but I would start by building both oscillators as separate "lumps", and I would omit the power transistor at this stage (just connect to the 'scope). Once both are seen to be oscillating at the expected frequencies, then I'd add the diode and 1k resistor. Then, I'd test my suggestion of dropping the diode and 1k resistor, then return R4 to IC pin 3 rather than +V. Finally, I'd add in the transistor and loudspeaker (and earplugs!) and see what happens.

0.1" plug-in bread-board is really useful - do you have one?

This circuit is almost (but not quite!) fine

The first 555 astable has a period of about 1.5 seconds - 1 second on, 0.5 seconds off - its used to "gate" the second astable - whilst the first 555 output is high, the second one runs at about 680Hz; when the output of the first 555 goes low, the second astable is stopped from running.

One thing I HATE about this design is that the output of the second astable goes HIGH during its supposed off period, i.e. for 0.5 seconds in every 1.5 seconds, 12V is across the speaker, i.e. 18W dissipated for 0.5 seconds out of 1.5 seconds, or 6W RMS of wasted energy - the speaker is dissipating 12.2W RMS on average as the setup stands.

As Mark has also said, you can drop D1 and R3 and take the output of U1 (pin 3) directly to the top of R4 instead of connecting the top of R4 to Vcc - this drops two completely unnecessary components.

To solve the "cook the speaker" issue you need to invert the output from U2 pin3 - a simple small signal NPN and resistor will do this happily - see attached schematic.

This drops the power dissipated in the speaker to about 5W RMS, and stops it cooking whilst maintaining exactly the same audible power output.

Net effect of these changes is that you have the same number of components, use half the power, save the planet and don't kill the speaker

Yes, I was planning to come back to the loudspeaker driver section later.

I was going to suggest using the transistor as an emitter follower to get around some of the problem. But I was planning to take that a stage further, using a PNP transistor in addition to the NPN, to make a push-pull stage. Obviously no need for biasing into Class B, as it's being fed with a square wave; a class C job with no biasing would work just fine.

That's because this simple stage has a big problem - the cone is only being moved in one direction from its rest position. Or, to put that another way, the signal has a DC offset. That's a very wasteful way to use a loudspeaker.

Obviously, the junction of the two output transistors would connect to the loudspeaker via a capacitor, so it doesn't matter what "state" IC2 assumes when silenced. (I could knock up a diagram later, but I'm about to go out)

I was also going to say that although they are cheap and plentiful, the 2N3055 is OTT for this. Bearing in mind that the highest current delivered to an 8 ohm load will be comfortably less than 2A, I'd consider a TIP31 or similar. Smaller package, easier to deal with. The TIP32 is the PNP compliment. Of course, there are countless alternatives. In operation, the voltage drop across these will be nice and low, so they won't get hot, but I'd provide for a small heatsink on the PCB, just in case.

Assuming this is for a siren that you want to be as loud as possible, I'd carefully investigate the loudspeaker in question. Many have strong peaks at points in their frequency response range, and it makes sense to tune the oscillator so that you are on one of these peaks. If the peak is 10dB up relative to another, nearby frequency - which is not unlikely with a cheap loudspeaker - then that's a subjective doubling in loudness for no extra power. Along the same lines, it makes sense to be where the human hearing system is most sensitive - which is somewhere in the 3 to 4kHz region. So, I'd allow for a pre-set resistor to determine the frequency of IC2.

You might also want to investigate piezo sounders. These generally come with a tuned acoustic system that produces a strong peak at a certain frequency. And, they take almost no current in operation - you might even be able to do away with the driver transistors. They are very, very efficient compared to moving coil loudspeakers - you just need to have plenty of voltage available.

But ultimately, I guess it all depends on what you're trying to achieve. If the goal is to make a really loud siren for a particular application, then hopefully the above will help (although frankly, I'd probably just buy something ready-made). But if the real objective is to have some fun messing about with some electronics - and that's a highly worthwhile goal in my opinion - then I'd drop the transistor and just drive the loudspeaker from the 555, with a resistor in series to limit the sound level (say, 100 ohms), and a series capacitor (say, 10 to 100uF), and also use a high impedance loudspeaker - the sort that are sold for exactly this sort of fun

Finally, I took another look at the original post. The circuit is lacking a smoothing capacitor. The 555s are decoupled, but the feed to the loudspeaker is not. This means that the gated square wave will be modulated by 100Hz ripple. This may or may not be intentional, or indeed desirable (good reason to breadboard it first, of course).

PS: "Watts RMS". Tsk Tsk

(06-06-2015, 08:29 AM)Nick Wrote: [ -> ]This drops the power dissipated in the speaker to about 5W RMS

Still, 5W isn't far off. The peak voltage might be 6V, depending on the rail, and depending on the losses in the transistors. The EF configuration will drop a minimum of 0.6 to 0.7V, and a bipolar 555 isn't quite "rail to rail".

But of course, the exact power will depend on the duty cycle of the second 555, with a maximum at 50% (which they obviously don't achieve in the default astable configuration). And I suppose that this should be multiplied by the duty cycle of the first so that we can derive a "continuous average power" rating.

OT ramble:

As a philosophical point, it's easy to hung up about the voltage drops in an EF output stage. In a high quality circuit, they are normally higher because, amongst other things, you use Darlington pairs. Many years back, I designed an amplifier for my first car, and I decided that I needed to squeeze every available millivolt from the 13.8V I had available. The circuit used a topology that - looking back - was really very innovative to drive a common-emitter output stage that could get to within half a volt of each rail - it was extremely "voltage-efficient". There were 4 of these, as the output was bridged, so I needed to keep the component count down. But sadly, as a result of that, it did have a major flaw - the quiescent current varied (very!) dramatically with supply voltage! I started making progress with a solution, but then did a "reality check". Exactly how much was my complicated and troublesome circuit gaining me? Let's say I was getting 12.8V pk-pk from mine, while a TDA2005 would get 9.8V (can't remember exact numbers - this was 20+ years back). Basically, it's a 2dB difference. Can we hear a 2dB difference? Probably, depending on the circumstances. But perhaps not... So, I went for the TDA2005 solution. And guess what? There was space in the box for 4 of these chips - each of which was bridged - so I was able to have a four channel amplifier, and that more than compensated for the 2dB loss in each. I made a PCB at work, and it did sterling service until I got a new "head unit" with a 4 channel bridged amp built within. Next time I'm in the attic I must dig it out for old times sake

End ramble. Really must go out now...

Thanks for reading the thread guys, and for your really helpful and erudite input, in explaining how the circuit functions, what it's shortcomings are, and how to overcome them.

Much food for thought - another of those 'simple' projects which soon takes on a life of its own!!

(06-06-2015, 11:45 AM)Mark Hennessy Wrote: [ -> ]PS: "Watts RMS". Tsk Tsk

(06-06-2015, 08:29 AM)Nick Wrote: [ -> ]This drops the power dissipated in the speaker to about 5W RMS

Still, 5W isn't far off. The peak voltage might be 6V, depending on the rail, and depending on the losses in the transistors. The EF configuration will drop a minimum of 0.6 to 0.7V, and a bipolar 555 isn't quite "rail to rail".

But of course, the exact power will depend on the duty cycle of the second 555, with a maximum at 50% (which they obviously don't achieve in the default astable configuration). And I suppose that this should be multiplied by the duty cycle of the first so that we can derive a "continuous average power" rating.

The RMS reading was calculated over a 5 second window, taking into account the duty cycles of both 555s and losses in the output stage (2N3055) for both the original and corrected versions - it wasn't a straight V=IR calculation - it is indeed a "continuous average power" rating...

Actually, do away with the second 555, keep the driver transistor (you only need the original one, not the inverting one too) and use a piezo sounder. A burglar alarm one is typically only a few quid and will shatter any nearby eardrums... (even Argos & B&Q etc. sell them).

Even smaller, cheaper, more efficient...

Thanks again for the additional input guys.

The alarm has been made on the PCB I posted above, and I'm pleased to say that it works!

Piezo sounders on order from e-bay - (£1.79 each, plus 79p post - "buy two get one free"!).

http://www.ebay.co.uk/itm/Piezo-Buzzer-Sounder-12V-DC-85dB-Buy-Two-Get-One-FREE-/121654890049?pt=LH_DefaultDomain_3&hash=item1c53324e41

85dB at 30 CMs.

Not quite as loud as  police car, but as loud as a passing diesel truck or snow blower:

http://www.noisehelp.com/noise-level-chart.html

Loud enough innit?