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

  • \$\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


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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.