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  1. AnalogKid

    circuit breaker alarm

    Increase the 10 K bias resistor to 100 K or 1 M. That still might not be enough. Do you have a DMM or scope? ak
  2. AnalogKid

    circuit breaker alarm

    First guess, the photodiode is not passing enough current to pull up the non0inverting input above the pot setpoint. Pin 5 is low >> pin 7 is low >> buzzer is on. Link to the original circuit page - ? ak
  3. AnalogKid

    .5 delay with a 555 timer

    The circuit, complete with component values, is in the datasheet. If you can read, you can cook. ak
  4. AnalogKid

    Auto shut off circuit

    My schematic software does not show power and GND connections to logic devices. I should have pointed that out to the TS. oops. C2 is the power supply decoupling capacitor. It connects to the device Vcc and GND pins with the shortest possible leads. Most ICs require power supply impedance...
  5. AnalogKid

    Auto shut off circuit

    Here is the same circuit using a 555, with the timing capacitor in its traditional location. Note that while a bipolar 555 output stage is extra-beefy, it is not super efficient. At your load current and a 12 V operating supply, the voltage across the siren will be around 10 V, not 12. Still...
  6. AnalogKid

    Auto shut off circuit

    While you were typing, I updated the post with an improved schematic. In almost all R-C timer circuits, a capacitor is charged "up" through a resistor. Over a time period, the voltage across the cap steadily (but not linearly) increases until it equals the supply voltage. Actually, the...
  7. AnalogKid

    Auto shut off circuit

    Turns out I already answered this two years ago on another forum. Here is that schematic, re-worked for your application. This circuit powers the load for approx. 44 seconds. R1and C1 set the delay. R2 and D1 provide a rapid discharge path for C1 when power is removed, so it doesn't take the...
  8. AnalogKid

    Auto shut off circuit

    The siren draws 120 mA at 12 V, which is within the capabilities of a standard 555 without an external driver transistor. Based on your responses, a single 555 should do what you want. Schematic to follow. However, this also can be done with just 1 transistor, a small-signal MOSFET like the...
  9. AnalogKid

    Auto shut off circuit

    Please clarify your question. Do you mean that you want the siren to come on immediately when power is applied, and they turn off after 30-60 seconds? Then what? Does the siren remain silent until power is removed and reapplied? What is the voltage for the timer circuit? What is the...
  10. AnalogKid

    Building a tilt sensor with an audible alarm...

    What is the power source? What is the alarm/beeper? The loudness and amount of power determines part of the circuit. How long is the delay before it starts beeping? How long does it beep before it shuts itself off? Or does it beep forever until the sensor is un-tilted? Do you want to build...
  11. AnalogKid

    Simple Hall sensor alarm circuit?

    Usually, what's inside the switch module is a hermetically sealed reed switch. In that application, good for decades - depending on quality. In a security application, the switch is usually closed 99.99% of the time. If the reeds have even a tiny bit of ferromagnetic content they can become...
  12. AnalogKid

    Question about DC current flow

    I prefer electron flow. ak
  13. AnalogKid

    Simple Hall sensor alarm circuit?

    That is a piezo beeper, not a buzzer. There are two types. One is just the bare piezo element, and requires an AC driving signal. The other has an oscillator circuit built-in, and runs on DC. This is what you have. It is a very low-current beeper, so just about any small-signal transistor...
  14. AnalogKid

    Simple Hall sensor alarm circuit?

    Just for fun, here is the MOSFET-based circuit. We're running out of parts to cut. Adjusting the input network impedance for a larger resistor reduces the static current draw through R1 when the door is closed. In this circuit it is only 9 microamps, pretty close to the battery's...
  15. AnalogKid

    Simple Hall sensor alarm circuit?

    Here is an updated schematic. Note that the transistor pinout might differ from what you have. One drawback of the Darlington transistor arrangement is that the "output" transistor (Q2 in your circuit) cannot saturate. The collector-emitter voltage (Vce) will be around 0.8 V when it is "on"...
  16. AnalogKid

    Simple Hall sensor alarm circuit?

    The question is how much current does the buzzer draw at 9 V? The answer might eliminate one transistor. ak
  17. AnalogKid

    Simple Hall sensor alarm circuit?

    Datasheet or required current for the buzzer - ? ak
  18. AnalogKid

    Simple Hall sensor alarm circuit?

    The capacitor does two things when the door is open. a) it acts as a noise filter, completely undoing my comment about noise susceptibility in this case. oops. b) it adds a short time delay between when the door opens and when the alarm sounds. My guess is that the delay is just a...
  19. AnalogKid

    Simple Hall sensor alarm circuit?

    Nothing in this circuit converts DC to AC or AC to DC. Let's call the left transistor Q1 and the right transistor Q2. When the door is open, the switch is open, and current through the 100K resistor turns on Q1. But that base current is very low. The only impedance at the Q1 base is a 100K...
  20. AnalogKid

    Simple Hall sensor alarm circuit?

    The two transistors are connected as a Darlington pair for increased gain. ak
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