My comment regarding the LED was whether it would/should light up if there was a dodgy part, ie, if I invert the transistor it makes the sounds, but the LED still lights anyway, so maybe it's simply the speaker that's the wrong impedance for some of these circuits.

The LED is handy to show current is indeed flowing.

 

Remove the LED because it is giving you false negative, i.e. your circuit is not working even though the LED is lit.
What you need as proof of it working is the sound from the loudspeaker.

 

Do you have an actual schematic that you are working from, so we can check it against the assembly photos?

ak

 

No the assembly photos are the only thing in the booklet.

 

Removing the LED produces an extremely loud screech, no way is that intended, it would scare most 8 year olds.

 

Removing the LED produces an extremely loud screech, no way is that intended, it would scare most 8 year olds.

That is the sign that the circuit is working.
Try adjusting the value of the resistor to give a different frequency.

 

Sliding the variable resistor does change the tone, but there's also no higher static resistor that 100K for the one in series with the variable resistor.

I'm intrigued how the LED makes such a difference?

 

Hello,

Change the piezo number 11 to a real capacitor.

Bertus

 

What difference is that expected to make?

I'm more interested in finding out what the issue is with this kids experiment, rather than just replace all the parts for what I have in the garage to make it work

 

Pull out the PNP module (i.e. don't connect at all). Does it work? Same for NPN?

 

Disconnecting either transistor doesn't work no (with or without the LED).

 

With both transistors installed per the picture, did you remove LED and replace with wire (3)? What was result.
I think you said it was a loud screech? Were you using 8ohm or 32ohm cpeaker? Which one comes with kit?
Did screech change tone with variable resistor?

 

yes, sorry that was in one of my previous replies.

A very high pitch screaming tone, clearly not as intended for a childs toy.

 

Did screech change tone with variable resistor?

 

Which speaker 8 or 32 ohm and which come with kit?

 

32 ohm speaker came with the kit.

32 ohm speaker with LED = no sound.
32 ohm speaker with no LED = very high pitched screaming sound.

8ohm speaker with LED = sensible tone.
8ohm speaker with no LED = very high pitch screaming.

 

The screech as you saw is a high frequency sound and that indicates that the circuit might be working albeit at a high freq. Try replacing the piezo buffer with a 1uf or 10uf cap that is in your kit and see if frequency is lowered.

 

8ohm speaker with LED = sensible tone. Is this with transistors installed per diagram?
and to confirm we are on Experiment 24, not 23.

 

Swapping to a 0.1uF cap instead of the piezo buzzer gives a nicer tone, without the LED and the 32 ohm speaker.

We're on the Variable Tone Generator (I think the experiment number is slightly different in our booklet), but it is the one after the Morse Code one.

8 ohm speaker, with both transistors installed as normal, with the LED, sounds sensible, including using the piezo buzzer as a capacitor, which leads me to think it's basically just the speaker that's a bad fit.

I'd love to know why, or why the impedance makes such a difference, and why inverting the transistors works at all.

 

Play with the values of both the resistor and capacitor. Going to higher values lowers the tone. You could lower one and raise the other.