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  1. hevans1944

    Wi-Fi IoT using RF backscatter

    I think perhaps you miss the point. This "new" technology provides a way to implement low-power IoT devices passively, without the huge power overhead of a Wi-Fi transceiver. One possible goal is to harvest wireless power to provide the minuscule power requirements of a passive IoT device, but...
  2. hevans1944

    Optocoupler delay off MOSFET solenoid

    All of your circuit variations have a capacitor in parallel with the gate-to-source terminals of the MOSFET switch. The capacitor holds enough charge to keep the MOSFET conducting when the 4N35M opto-isolator turns off. One or more resistors in parallel with the capacitor (or in one instance...
  3. hevans1944

    Optocoupler delay off MOSFET solenoid

    A negative edge-triggered delay on the opto-isolator input would work. You would logically OR the input to the opto-isolator with the output of a negative edge-triggered one-shot multivibrator and then use the OR gate output to drive the opto-isolator. There will be a short "glitch" lasting a...
  4. hevans1944

    Optocoupler delay off MOSFET solenoid

    Your circuit will turn on the relay when the opto-coupler is turned on and charge up the capacitor, which will then discharge through the 500 kΩ resistor when the opto-coupler is turned off. The MOSFET will remain on for a short time until the capacitor discharges sufficiently to make the MOSFET...
  5. hevans1944

    Wi-Fi IoT using RF backscatter

    Recently someone posted a video showing an IoT (Internet of Things) device that did not require an RF transceiver to link into a local Wi-Fi network. Instead, it modulated the antenna impedance digitally with a MOSFET to utilize RF backscatter from a local, powered, wireless Wi-Fi transceiver to...
  6. hevans1944

    Info for 2N3904 and 2N3906

    The 2N3904 (NPN) and 2N3906 (PNP) are my favorite go-to small-signal transistors. They have good gain and frequency characteristics for most analog or digital projects, but most significant to me: they are dirt cheap. I keep a hundred or so of each type on hand to play with. If I accidentally...
  7. hevans1944

    RF Energy Harvesting Banking

    There is another thread here on this topic with an interesting self-published paper. You might want to give it look. The conclusions (at least for GHz frequencies from WiFi wireless router as a power source) are rather pessimistic.
  8. hevans1944

    detecting battery charge level

    I would build a voltage divider to reduce the analog input by 50%, because the battery voltage could be considerably larger than the Arduino's voltage-regulated 5 V. It isn't uncommon to power an Arduino from a 9 V battery so reducing this to 4.5 V before measuring it is desirable. The Arduino...
  9. hevans1944

    diy metal detector

    There might be a good reason for the ten thousand dollar price. Have you done any research on metal detectors? Principle of operation? Minimum size of objects detected? Depth of detection? Type of soil object can be detected in? Detection through water? If you want to build something, it helps...
  10. hevans1944

    detecting battery charge level

    No, the reference voltage is a circuit that derives its power from the battery you are trying to measure but provides a stable reference voltage as the battery voltage varies. You can purchase an IC that does this. And the multiple analog comparators suggested by @cjdelphi is also a viable...
  11. hevans1944

    Hook Up Wire for PSU

    I guess "New2Electronics" is a self-descriptive screen name. Well, we all have to start somewhere. It would be helpful to see exactly what kind of power supply the OP will tackle as a "first power supply". I can appreciate the desire to build something reliable. My first power supply was a...
  12. hevans1944

    This some kind lightbulb or...

    Well, at least I was able to eventually find the NE-77 part number. Probably unobtainable part now, but maybe those clever Chinese will start manufacturing them again. No exactly rocket science.
  13. hevans1944

    This some kind lightbulb or...

    Great! Can you provide a part number or a source of supply? Here is a datasheet for a Russian version. And here is an explanation for a touch-sensor circuit that uses the NE-77 (scroll down to TOUCH SWITCH):
  14. hevans1944

    Converting Switch/Case statement in C to ASM?

    Back in the day, I usually wrote 8085 assembly code and linked it in relocatable mode just because it made it easier to link modules that were more or less independent from each other. IIRC, it did complicate troubleshooting at the hardware level because the location of various parts of each...
  15. hevans1944

    Interfacing joystick

    I am "bumping" this thread for further comment. I offered to design the circuit for @SGW back in July 2015 but have dropped the ball on getting a parts list together. This was supposed to finished by last September! We have been in communication privately, both here and via e-mail, but now it is...
  16. hevans1944

    RaspBerry Pi Zero

    Is this another fine example of German over-engineering? :D Love it, @Hellmut1956! Looks like the zero stock has been gobbled up by those who want to sell you a complete kit for ten to twenty times the price of the Raspi 0 alone.
  17. hevans1944

    Problem on Radio Energy Harvesting circuit- Is Cockcroft Walton circuit fail?

    @allhksar You are correct that the forward voltage drop of a diode is a function of current. If you look at the V-I curve for any diode the forward voltage goes through zero at the same time the current goes through zero. The reverse current is small (but never zero) as the reverse voltage...
  18. hevans1944

    Impact accelerometer

    It would help a lot if you described what you are trying to DO so we could propose a solution. This sounds like it could be a carnival game, where you strike a steel plate with a sledge hammer and light up an LED. For that type of application you can place the impulse sensor in the hammer head...
  19. hevans1944

    diy metal detector

    If you are genetically qualified, you could learn dowsing. If you want to do it electronically, there are several different approaches that work reasonably well, depending on distance and metal size. There are plenty of DIY versions of metal detectors available in the Internet. Be prepared for a...
  20. hevans1944

    Help needed to replace a relay

    Yeah, four Form A contacts, two normally open, two normally closed are an unusual configuration. I've seen this in large contactors though. They could be wired as two Form C sets of contacts, but that may not be how these relays are used. @dorke's suggestion to get a quote is probably going to...
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