I am preparing some circuits for children to be soldered in thumbtack technique. I found some that seem to fit my purpose. One of them is a siren. In the wiring diagram, I see that there's a line through two transistors that potentially connects plus and minus without any resistor (highlighted in red).


Here's my redraw of it:


simulate this circuit – Schematic created using CircuitLab

In that circuit, is there the danger of a short-circuit?

  • \$\begingroup\$ Wow, those are really obnoxiously fat junction dots! And, they appear in places that shouldn't have junction dots. There are lots of bad schematics out there. That's not your fault. However, when you post one here, it becomes your bad schematic. Never dump slop on us. Clean up or redraw as needed. It hurts too much to look at that mess, so -1 for that, and closing since without reading I don't what is being asked. \$\endgroup\$ – Olin Lathrop Feb 17 '18 at 14:25
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    \$\begingroup\$ They're not circuit dots, they're thumbtacks. It's not a schematic, it's a wiring diagram \$\endgroup\$ – Transistor Feb 17 '18 at 15:04
  • \$\begingroup\$ @Transistor: No better, since wiring diagrams aren't substitutes for schematics. \$\endgroup\$ – Olin Lathrop Feb 18 '18 at 18:33
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    \$\begingroup\$ mmmm C3 looks pretty strange in that position \$\endgroup\$ – carloc Feb 18 '18 at 19:14

You should really include a proper schematic of this circuit, as it is now it is difficult to see how it works. I have my doubts about this circuit as well as there is no base resistor present for the BC547 transistor.

Regarding short circuiting:

There is always a danger of a short circuit, especially with children as you cannot predict what they will do.

Instead of trying to prevent a short circuit, I would make the design such that a short-circuit doesn't cause any damage.

The problem with a short circuit is that too much current can flow. If you simply prevent that from happening, by limiting the amount of current the battery can deliver, a short circuit should not cause any problems.

It all depends on what type of battery you intend to use.

Since this is a low-power circuit you don't need high power batteries. I would choose Zink-Carbon batteries, so not the Alkaline type. These Zink-Carbon batteries also happen to be the cheapest.

Zink-Carbon have a reasonably high internal resistance and that limits how much current they can deliver. If you would use a 9V block or a couple of AAA batteries in series, hardly any damage can be done.

  • \$\begingroup\$ I have drawn a schematic with the built-in tool. Regarding the children: it's supposed that the circuit is operated as expected, so the children are instructed to press the switch only. A 9V block is indeed used. \$\endgroup\$ – Thomas Weller Feb 17 '18 at 14:50

In this circuit:

You want do know how the path thru E-C of Q1 and B-E of Q2 isn't shorting the supply.

The answer is that Q1 needs to be on for this current path to exist. The feedback keeps Q1 on for only short periods of time.

Still, I agree, this is not a great design. The current into the base of Q2 will go unnecessarily high when Q2 is being driven on. The initial positive feedback makes this worse.

When Q2 is first turned on a little thru R3, it draws current into its collector. This lowers the collector voltage, which draws current out of the base of Q1 thru C2. That dumps more current into the base of Q2, turning it on more, etc.

Note that Q2 is driving a 8 Ω load. With the 9 V source, that requires over 1 A to drive the low side of the speaker low. I didn't look up the minimum guaranteed gain of a BC547 at 1 A collector current, so let's say it is 30 just to pick something for use as example. To get 1.1 A of collector current therefore needs 37 mA of base current.

I would put a resistor between C of Q1 and B of Q2 that limits the current to a bit over this value. Let's say Q1 saturates to 200 mV and the B-E drop of Q2 is 750 mV at full load. That leaves about 8 V across the resistor. (8 V)/(37 mA) = 216 Ω, so 200 Ω would be a good value with this example gain of Q2.

  • \$\begingroup\$ Downvoter: Please explain what you think is wrong. \$\endgroup\$ – Olin Lathrop Feb 19 '18 at 12:51

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