As a kid I had one of those electronic project kits that Radio Shack would sell. One particular circuit I really liked was the "Electronic Siren" and I kind of want to recreate it to see if it's sounds as good as I remember it. It was basically a sweep oscillator(?) that when you pressed the key, the siren would start off and wind up to a peak frequency, then wind back down and eventually stop when the key was released.

I have a good inkling of the components needed but I'm stuck on what transformer to use. Based on the parts list in the manual, it's center-tapped with a 900Ω input and an 8Ω output (the manual describes it as "900 CT, 8 ohm"). I'm trying to find one online but I keep finding large power transformers. Would a 4.5v source be enough to drive one of those?

See attachment...

 

Hi RA,
Search for Audio output centre tapped transformers, not power.
E

https://www.google.com/search?client=firefox-b-d&q=900+ohm+centre+tapping+audio+transformer

 

Try looking for something like these parts.
https://www.digikey.com/en/products/detail/prem-magnetics-inc/SPT-403/17827605
https://www.premmagnetics.com/content/SPT-400 Series & SPT-129 Data Sheet.pdf
A 1.0k 1.2k CT audio transformer might be more common.

I'm sure you could run it with a common 5vdc wall-wart supply too.

 

One source maybe to find one of the older portable/small transistor radios,

 

Thanks for your responses everyone. This is this the first time I built a circuit where I'd needed a transformer (or at least a transformer that wasn't in a presupplied kit) so I'm treading in unfamiliar territory.

 

Also known as Eagle LT700 and commonly spotted on Ebay!

 

This is this the first time I built a circuit where I'd needed a transformer

Because audio circuits have not used a transformer for 60 years.

 

Because audio circuits have not used a transformer for 60 years.

and their use in sirens ceased the moment the 555 arrived.

 

As a kid I had one of those electronic project kits that Radio Shack would sell. One particular circuit I really liked was the "Electronic Siren" and I kind of want to recreate it to see if it's sounds as good as I remember it. It was basically a sweep oscillator(?) that when you pressed the key, the siren would start off and wind up to a peak frequency, then wind back down and eventually stop when the key was released.

I have a good inkling of the components needed but I'm stuck on what transformer to use. Based on the parts list in the manual, it's center-tapped with a 900Ω input and an 8Ω output (the manual describes it as "900 CT, 8 ohm"). I'm trying to find one online but I keep finding large power transformers. Would a 4.5v source be enough to drive one of those?

See attachment...

You can replicate this with a 555 timer chip, and an output transistor..

 

This is this the first time I built a circuit where I'd needed a transformer (or at least a transformer that wasn't in a presupplied kit) so I'm treading in unfamiliar territory.

In my younger days when transistor radio's first appeared on the scene, a Company soon got into the business of putting together a kit, I would pick them up and put together for family and friends, they got a cheap radio, cheaper than off the shelf, and I got beer money!

 

You can replicate this with a 555 timer chip, and an output transistor..

If you had the same 555 circuits book that I did, they all needed a 64Ω speaker to run it off a 9V battery. With the transformer you would be able to use an 8Ω soeaker.

 

Because audio circuits have not used a transformer for 60 years.

Now they're used in digital circuits to match, transfer energy/signals and galvanic isolation. Nothing really changes at the EM level.

 

With the transformer you would be able to use an 8Ω soeaker.

DD mentioned an output transistor, probably as a speaker driver. But I don't think you can get a continuously repeating frequency ramp with only one 555.

ak

 

But I don't think you can get a continuously repeating frequency ramp with only one 555.

Nor with the above circuit.

 

Based on the parts list in the manual, it's center-tapped with a 900Ω input and an 8Ω output (the manual describes it as "900 CT, 8 ohm").

You probably can get away with a 1K primary, which is much more common.

Mouser is a good source for low cost audio transformers. They have a 1K CT in three different power levels.

https://www.mouser.com/c/passive-co...mers-signal-transformers|~Secondary Impedance

ak

 

The 555 circuit was not a siren, instead it was a buzzer with the frequency manually adjusted.

 

Try looking here.
https://www.google.com/search?client=firefox-b-1-d&q=556+siren+circuit#imgrc=X3tanBHc6rv_gM

 

My thought was that you might be able to substitute power audio amplifier LM386 for the transformer. But I see that the supply voltage is connected to the center tap 51 of the transformer's primary, while the other two taps of the primary connect to the base and collector of the 2nd transistor. Knowing very little about transistor circuits, I can't say what is accomplished by these connections to the primary winding.

Except for LM386N-4, the minimum supply voltage for the amp is 4V.

-Pete

 

My thought was that you might be able to substitute power audio amplifier LM386 for the transformer. But I see that the supply voltage is connected to the center tap 51 of the transformer's primary, while the other two taps of the primary connect to the base and collector of the 2nd transistor. Knowing very little about transistor circuits, I can't say what is accomplished by these connections to the primary winding.

Except for LM386N-4, the minimum supply voltage for the amp is 4V.

-Pete

The base connect side of the CT primary provides a 180 phase shift, that combined with the common emitter transformer driver transistor 180 phase shift, it gives the positive feedback needed for circuit oscillation.

 

My thought was that you might be able to substitute power audio amplifier LM386 for the transformer. But I see that the supply voltage is connected to the center tap 51 of the transformer's primary, while the other two taps of the primary connect to the base and collector of the 2nd transistor. Knowing very little about transistor circuits, I can't say what is accomplished by these connections to the primary winding.

Except for LM386N-4, the minimum supply voltage for the amp is 4V.

-Pete

The transformer is a very much integral part of the circuit. It provides the feedback to make the circuit oscillate. So there is no way the posted circuit can possibly use a 555 or am LM386 as substitutes.
ALSO, transformers are still used, but not usually for audio amplifier outputs. So a low-power transformer, 200 to 500 ohms primary impedance, center tapped, to 8 ohms, or 10 ohms secondary impedance, should work.

A NOTE TO ALL: the TS did not ask for a redesign that would be entirely different!!