Siren schematic

Thread Starter

Wendy

Joined Mar 24, 2008
22,596
When I was in my mid teens I copied this schematic from one in 50 projects kit from RadioShack. That little toy got me started in electronics, I would recommend something like that for any parent whose kids seems to be leaning that way.

Here is the schematic( I know the speaker capacitor is backwards, like I said I was in my teens when I drew it):

Siren.jpg

For reasons i don't quite understand Q1 is critical, I have built this circuit several times. So other than the capacitor this schematic is verified. The only thing I can figure that is wrong is the gain of Q1( the components listed have been used and work). In an attempt to understand this circuit better I have redrawn it as follows:

Test.png

I started this thread to open speculation on what it is about Q1 that is so critical. The next time I wire this on a protoboard, I will probably try a Sziklai pair and see if it makes a difference. On the next layout I will leave off the switch.
 
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Thread Starter

Wendy

Joined Mar 24, 2008
22,596
Saw that just haven't updated breadboard drawing yet. Real life has a way of slowing things down.
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,596
Still doesn't work dang it. Because some of the devices were slightly different from the original drawing I tweaked some wires. I'm stopping work on this until someone else has another idea:
 
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bertus

Joined Apr 5, 2008
21,605
Hello,

Is the power line over the hole breadboard?
There are breadboards with a break ik the powerline:
breadboard-connections.jpg
Bertus
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,596
I understand this protoboard fairly well, and that is not the case. It is possible that stray capacitance is killing the design put on the RadioShack layout it is pretty crude (springs with stripped tinned wire going between them) so I doubt it. I would like to understand why Q1 is so critical? Almost anything works for Q2.
 
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Thread Starter

Wendy

Joined Mar 24, 2008
22,596
Push to sound siren. It's late I will take another look at it tomorrow morning, thanks Bertus. In a Szulkia Pair that is the collector.

Szuklai Pair.jpg
 
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bertus

Joined Apr 5, 2008
21,605
Hello,

I see, Then the collector of Q2 is not correct.
It is now to the minus.
The emittor is open and should be to the minus.

Bertus
 

AnalogKid

Joined Aug 1, 2013
9,408
What function does the switch perform?
Push to sound siren.
OK, two things.

1. That is a strange location for an on/off switch. When the switch is open, there is a constant current draw from the battery through both transistors.

2. With the word "siren" in the title, I assumed there would be some type of change in pitch; either the sweeping American or the two-tone European style. Is the circuit in fact a constant-frequency oscillator?

Also, given a 9 V power source, I wouldn't think the difference in Vbe between Ge and Si parts would matter. However, a Ge part will have much higher leakage, and this might be necessary to start oscillation.

ak
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,596
OK, two things.

1. That is a strange location for an on/off switch. When the switch is open, there is a constant current draw from the battery through both transistors.

2. With the word "siren" in the title, I assumed there would be some type of change in pitch; either the sweeping American or the two-tone European style. Is the circuit in fact a constant-frequency oscillator?

Also, given a 9 V power source, I wouldn't think the difference in Vbe between Ge and Si parts would matter. However, a Ge part will have much higher leakage, and this might be necessary to start oscillation.

ak
it is quiet until pressed, then starts from a low frequency and ramps up to a higher frequency. Think old cop shows like Adam 12. When it works it is quite good. Do you have any changes you would suggest? I do not think I have any germanium transistors in stock.

As I said earlier, I did not design this circuit. It is a 17 year old kids attempt to save a circuit he liked.
 
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