Help with Craig Anderton Metronome circuit

[Rodgre]

Hello all.

I have been looking for a simple 555-based metronome circuit to use as a pulse tone generator to test audio systems. I decided to build the old faithful metronome out of Electronic Projects For Musicians. The circuit works great and does what I need it to do perfectly. My only issue is that when I first power it up and when I power it down, the output goes crazy with the pulsing going crazy fast, sounding like it's broken. When powered up, after about a second, it settles down and puts out a steady beat. At power down, it goes crazy for about a second again and then shuts off.

The schematic is within this book. https://msu.edu/~dougl126/Electronic%20Projects%20for%20Musicians.pdf

I am wondering if anyone can suggest a way to eliminate this issue. Is there someplace in the circuit where a capacitor could delay the start up and soften the power down so it doesn't do this?

If not, is there another simple 555 (or other timer?) based circuit that I could build to do the same thing, but without these artifacts?

Thank you!

Roger

[ElectricDruid]

First, try taking out the 100uF cap and see if the circuit still works ok. If it does, it should have fixed the start-up/slow-down problem.

The 555 famously crowbars the power rails, so the big 100uF across +V and Ground (pins 1 and 8 ) is probably to help prevent that. The CMOS version of the chip, the 7555, doesn't have this problem so much, so you could use a 7555 and live without the big cap.

One other thing is a bit odd with that circuit. He's used 2 x 2uF electrolytic to make a 1uF non-polarised cap. But the 555 doesn't have to have a non-polarised cap as its timing capacitor. You could instead use a simple 1uF electrolytic for C1 and leave C2 out.

Edit: Fixed to 8 ), a more standard pinout - thanks, Antonis!

[antonis]

(pins 1 and

Is  a standard pin-out configuration..?? 

[cab42]

I have built this, and I do not recall that it had the issues you describe. I don't have it anymore, so I can't check.



From here: https://www.electronics-tutorials.ws/waveforms/555_oscillator.html (app. halfway down)

[ElectricDruid]

I have built this, and I do not recall that it had the issues you describe. I don't have it anymore, so I can't check.



From here: https://www.electronics-tutorials.ws/waveforms/555_oscillator.html (app. halfway down)

Yeah, that one avoids the big cap across the power rails, so it won't have a slow start-up and power-down. It'll just go and then just stop.

Plus it uses a bigger timing cap (10u instead of 1u) so then the variable resistor can be a smaller value. It makes more sense generally.

[PRR]

> The schematic is within this book.

Whoh, that's a big file, 45MB. Here's *just* the metronome plan:


I'm not a fan of the '555 but I do not recall any remarks about it "stuttering" at turn-on/off. The way a '555 works, it really "shouldn't". It could be that big cap. But it could also be a bad joint, not really connected.

[Rob Strand]

I don't remember NE555's powering up and down like that.

It could be doing something weird because there is no cap from pin 5 to ground, try 10n to 100n.

If you are switching the supply to the unit so only the 100uF is holding the rails up, and you have the protection diode D1 in place, then one thing that might work is to connect the positive side of the timing resistor (R2) to a second diode (1N4148) then connect the diode anode to the "+" terminal.   What that does is remove the timing circuit as soon as the input power is removed and regardless of the 100uF cap voltage.

If you are powering down the mains off an external supply like a wall-wart which is permanently connected to the unit then the diode trick won't work.   To solve the problem in this case you would need a PNP transistor and two resistors which switches out the positive rail when the supply is low.

[Rodgre]

Thanks everyone! I will try the other schematic on Wednesday and see if that takes care of the problem. Otherwise I will try to implement Rob's ideas into the circuit I've built.

Thank you!

Roger

[ElectricDruid]

Thanks everyone! I will try the other schematic on Wednesday and see if that takes care of the problem.

The only really significant difference is the lack of C3/100uF. Otherwise, the circuits are basically the same. The Anderton one adds a reverse-polarity protection diode on the power, which isn't a bad idea, but won't change anything much. The electronics-tutorials.ws one uses a different ratio of timing components to get the same result (cap goes up by x10, variable resistor goes down by x10) but again that won't change anything much. The two circuits are otherwise identical - a standard 555 astable.

Consequently, the first thing to try is to dump that 100uF!

[Rodgre]

I'll lift that cap for sure.

This may seem like a dumb question, but can I just swap the CMOS 555 for the standard 555?

Roger

[Rob Strand]

This may seem like a dumb question, but can I just swap the CMOS 555 for the standard 555?

Usually you can just swap it out.

FWIW, cab42's circuit needs a fixed resistor in series with the pot otherwise pin 7 will short across the supply rail.
   

[ElectricDruid]

FWIW, cab42's circuit needs a fixed resistor in series with the pot otherwise pin 7 will short across the supply rail.

That's true. 100K pot+47K resistor would be good start.

[cab42]

FWIW, cab42's circuit needs a fixed resistor in series with the pot otherwise pin 7 will short across the supply rail.

That's true. 100K pot+47K resistor would be good start.

I actually think, I did that! Mostly because I didn't have a 150K pot and I am not that fast anyway.

[ElectricDruid]

FWIW, cab42's circuit needs a fixed resistor in series with the pot otherwise pin 7 will short across the supply rail.

That's true. 100K pot+47K resistor would be good start.

I actually think, I did that! Mostly because I didn't have a 150K pot and I am not that fast anyway.

There's a "sort of" implication on the schematic that that's what you should do. Notice the little table top-left goes from 46.1K to 142.3K to give a comprehensive range of tempos? That's a neat 100K range, plus a 47K resistor.
They could have actually put it on the diagram though. It wouldn't have killed them, would it?!? What, we're supposed to use a virtually-unheard-of "150K pot", and then only use 2/3rds of the range? Yeah, right...