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I have built the following circuit but it doesn't seem to work properly! The sound from the speaker should be maintained until the power is cut off but it is only maintained as long as the reed switch is ON. I have tried to built it again and again based on different equivalent circuits but it doesn't work! What is the problem?

The original schematics is the following:

schematic

simulate this circuit – Schematic created using CircuitLab

The equivalent schematic and the circuit I built on the breadboard are the following:

schematic

simulate this circuit

enter image description here

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    \$\begingroup\$ What is supposed to hold it ON when SW1 is released? \$\endgroup\$ Dec 11, 2015 at 20:55
  • \$\begingroup\$ @BrianDrummond What do you mean? \$\endgroup\$
    – Adam
    Dec 11, 2015 at 21:10
  • \$\begingroup\$ some long breadboards have the power busses divided in the middle, are you sure that power and ground are being delivered to the other end of the breadboard? \$\endgroup\$
    – Wesley Lee
    Dec 11, 2015 at 21:11
  • \$\begingroup\$ @WesleyLee yes they are both delivered exactly as they should, I am 100% sure. \$\endgroup\$
    – Adam
    Dec 11, 2015 at 21:13
  • \$\begingroup\$ The circuit is not self latching, it will only be powered while Switch 1 is closed/pressed. \$\endgroup\$
    – Passerby
    Dec 11, 2015 at 21:18

2 Answers 2

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TL/DR : the circuit seems to be working as intended.

I think you misunderstood the (bad) explanation on the linked page:

Diode D2 which is linked with the switch S2 begin its conduction and offers power supply the transistor T1 and T2, which is in the waking state and as a result sound comes from the speaker attached to it. But in this instance a high frequency tone comes out which is a sign that there is some intruder present around the locker. The sound that came from the speaker can only be stopped by cut off the power supply.

"... switch S2 ... offers power supply the transistor T1..."

actually S2 provides base current to T1, the "power supply" is permanently connected to T2 and other components. That being so, it's easier to understand "stopped by cut off the power supply" to mean, stopped by cutting off T1's base current - i.e. releasing the switch.

There would need to be some other arrangement to supply T1 base current, perhaps driven from the oscillation, or a separate latch also triggered from the switch, for the buzzer to continue until power was removed.

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  • \$\begingroup\$ Thank you for your answer. Do you by any chance happen to have any links to schematics that do the job I was initially trying to do? :P \$\endgroup\$
    – Adam
    Dec 11, 2015 at 23:16
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Reading between the lines of the description that Brian Drummond found, I expect that they mean that the thing will sound as long as the alarm switch is closed. The sound can only be stopped by removing power, or by resetting (opening) the alarm switch.

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  • \$\begingroup\$ Thank you, the website had a very bad explanation of the circuit... \$\endgroup\$
    – Adam
    Dec 11, 2015 at 23:17

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