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  1. 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...
  2. 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"...
  3. 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
  4. AnalogKid

    Simple Hall sensor alarm circuit?

    Datasheet or required current for the buzzer - ? ak
  5. 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...
  6. 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...
  7. AnalogKid

    Simple Hall sensor alarm circuit?

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

    Simple Hall sensor alarm circuit?

    Without a datasheet or pinout for the BC547, or a schematic, it's hard to say. I can note that it *appears* that you are applying 9 V to a 5 V buzzer. Also, 100K is a very large value for such a high gain circuit, and enables false tripping due to noise. Reduce to 10K. Also, when the door is...
  9. AnalogKid

    Simple Hall sensor alarm circuit?

    Or a different Hall sensor. IIRC, don't the same people make one that has an open output in the presence of a field? Not at my computer; can't search. ak
  10. AnalogKid

    Simple Hall sensor alarm circuit?

    Yes, you probably have damaged the sensor. With pin 3 connected directly to 9 V, when the sensor is tripped it attempts to put a dead short across the battery. This probably has blown its output transistor. To correct this, break the connection between pins 1 and 3, and connect the buzzer...
  11. AnalogKid

    24v to 12v converter

    Agree. There are a lot of fins, but not enough for a linear regulator. But wait - that statement assumes western science design values. If we assume a cheaper-than-dirt ethic, it *is* a linear regulator, and assumes direct exposure to a 40 mph air stream. ak
  12. AnalogKid

    Latching switch

    Actually, if the switch is self-latching (called an alternate-action switch), you don't need a circuit. If the switch is a simple momentary pushbutton, then a circuit can transform this into alternate-action performance. Which is it? ak
  13. AnalogKid

    Circuit not working like intended...help please!

    What is the piezo beeper you are using? Part number / web page / datasheet? Specifically, does it beep continuously when powered by DC, or does it require an oscillating signal? Is the 555 supposed to be a monostable timer, making the piezo beep for a short period of time, or is it an astable...
  14. AnalogKid

    constant current source with transistor

    I'm trying to follow along, but it is difficult. Please post a schematic of the oscillator. Be sure to use the correct symbol for each component, and include a reference designator and part value. ak
  15. AnalogKid

    How to reduce IR sensitivity for can trash

    What is the question?
  16. AnalogKid

    Help wiring vintage record player speaker

    It is high voltage DC with AC ripple superimposed on top of it. The DC value depends on the AC mains voltage and the rectifier configuration. The AC ripple is what causes the background hum out of the speaker. There are several rectifier/filter capacitor configurations. We can't get into...
  17. AnalogKid

    Help wiring vintage record player speaker

    Your image has the speaker magnet assembly (large steel rectangular shape lower left), the amplifier output transformer (transformer-looking thing upper left) and a connector (round thing upper right). The yellow and yellow-stripe wires from the connector go directly to the magnet. Those are...
  18. AnalogKid

    Veeder Root 6 Digit electric counter . Does it work ?

    The front panel does not look right for an elapsed-time indicator, so it probably is an impulse counter. A small 115Vac solenoid ratchets the wheel mechanism. Sometimes these units do not like continuous activation, so drive with a pulsed signal. You do not need any external current limiting. ak
  19. AnalogKid

    Silent Switch for MIDI Surface Controller

    Conductive rubber key switches are very quiet, with a slightly tactile but almost completely inaudible little thump. The tradeoff is that the contact resistance can be tens of ohms. ak
  20. AnalogKid

    Silent Switch for MIDI Surface Controller

    Photos of the current device so we can see what kind of space we have to work with? PCB or wired switches? ak
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