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I was helping my kids with this circuit in their Elenco Snap Circuits kit. It doesn't work. U2 is a simple audio IC, and I verified that it is connected correctly to the speaker and batteries (when I use a jumper to bypass Q3 to the input, it turns on as expected). The resistor is 10k. The kit describes Q3 as an SCR, which I think is a silicon rectifier diode. The circuit is supposed to...

  1. Begin off.
  2. Turn on as soon as the jumper wire from point A to B is removed.

But when we try it, nothing happens.

How would I test the SCR, and/or is there a reason that the circuit would not operate as intended?. I do have a digital multimeter that I could use to test it. enter image description here

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  • \$\begingroup\$ Would you mind measuring the voltage between the battery (-) and the C3 point (between Q3 and U2), when initially powered on, after the jumper is removed, and after the jumper is reapplied? (same for point B1) \$\endgroup\$
    – nanofarad
    Commented Dec 22, 2021 at 21:00
  • \$\begingroup\$ Check the polarity of everything and make sure it matches \$\endgroup\$
    – Voltage Spike
    Commented Dec 22, 2021 at 21:17

1 Answer 1

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Q3 must be defective.

Try this:

  • Insert jumper wire A to B
  • Remove U2 alarm IC
  • Remove SP speaker
  • Add new resistor from C3 to E3 (1k to 10k is fine)
  • Place DMM probes on C3 and E3 in voltage measurement mode
  • Verify zero volts on readout

Now remove jumper wire and check voltage from DMM again. If it stays at zero volts, then the Q3 part is defective.

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